Thermographic recording elements

Abstract: A thermographic recording element having an image forming layer contains an organic silver salt, a reducing agent, an optional photosensitive silver halide, and a specific nucleating agent. The element has high Dmax, high sensitivity, satisfactory contrast, low fog, and minimal dependency of photographic properties on developing temperature.

Description: This invention relates to thermographic recording elements and more particularly, to photothermographic recording elements suitable for the manufacture of printing plates. BACKGROUND OF THE INVENTION Photothermographic materials which are processed by a thermographic process to form photographic images are disclosed, for example, in USP 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tone of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80°C or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming images. Such photothermographic materials have been used as microphotographic and medical photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation. With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the graphic printing field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic materials for use in the graphic printing field which can be effectively exposed by means of laser image setters and produce clear black images having a high resolution and sharpness. These photothermographic materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment. USP 3,667,958 discloses that a photothermographic element comprising a polyhydroxybenzene combined with a hydroxylamine, reductone or hydrazine has high image quality discrimination and resolution. This combination of reducing agents, however, was found to incur an increase of fog. For producing a thermographic recording element having high Dmax and high contrast, it is effective to add to the element the hydrazine derivatives described in US? 5,496,695. Although this results in a thermographic recording element having high Dmax and high contrast, all of sensitivity, contrast, Dmax, Dmin, and storage stability of compounds are not fully satisfied. Improvements in contrast and storage stability of compounds are achieved by using the hydrazine derivatives described in EP 762196A1, but the fully satisfactory level has not been reached. Further, USP 5,545,515 and 5,635,339 disclose the use of acrylonitriles as the co-developer. With these acrylonitrile compounds, a fully satisfactory high contrast is not achieved, fog rises, and the photographic properties largely depend on the developing time. EP-A-0 803 764 relates to a photothermographic material comprising a support, a photosensitive layer disposed on at least one surface of the support and containing a photosensitive silver halide and a binder, and a non-photosensitive silver salt and a reducing agent therefore. In this document, under the heading "preparation of sample number 101" a backlayer coating solution is described, which contains among other dyestuffs so-called "dyestuff (b)" which is exempt from formula (B) defined in present claim 1. WO 97/34195 discloses a black and white photothermographic element comprising a support bearing at least one photosensitive, image-forming, photothermographic emulsion layer comprising (a) photosensitive silver halide, (b) a non-photosensitive, reducible silver source, (c) a reducing agent system for silver ion, and (d) a binder, wherein the reducing agent system comprises (i) at least one hindered phenol, and (ii) at least one 2-substituted malondialdehyde compound of the following formula wherein R represents an aryl group or an electron withdrawing group. SUMMARY OF THE INVENTION An object of the present invention is to provide a thermographic recording element having low fog, high sensitivity, high Dmax, and minimized developing-temperature dependency. Another object of the present invention is to provide a recording element for use in the manufacture of graphic printing plates which forms an image of quality and can be processed in a fully dry basis without a need for wet processing. According to the invention, there is provided a thermographic recording element having at least one image forming layer. The element contains an organic silver salt, a reducing agent, and at least one nucleating agent of the following formulas (A) and (B), with the proviso defined in claim 1. Herein, Z 1 and Z 2 each are a group of non-metallic atoms capable of forming a 5- to 7-membered ring structure with the carbon atoms; Y 1 and Y 2 each are -C(=O)- or -SO 2 -; X 1 and X 2 each are a hydroxy or salt thereof, alkoxy, aryloxy, heterocyclic oxy, mercapto or salt thereof, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, arylamino, heterocyclic amino, acylamino, sulfonamide or heterocyclic group; and Y 3 is hydrogen or a substituent. Preferably, the compound of formula (A) has at least 6 carbon atoms in total, and the compound of formula (B) has at least 12 carbon atoms in total. More preferably, in formula (A), the total number of carbon atoms in Z 1 is at least 3, and in formula (B), the total number of carbon atoms in Z 2 and Y 3 is at least 8. In one preferred embodiment wherein a photosensitive silver halide is further contained, a photothermographic recording element is provided. BRIEF DESCRIPTION OF THE DRAWING The only figure, FIG. 1 is a schematic view of one exemplary heat developing apparatus for use in the processing of the thermographic element according to the invention. DETAILED DESCRIPTION OF THE INVENTION The thermographic recording element of the invention has at least one image forming layer and contains an organic silver salt and a reducing agent. Preferably it further contains a photosensitive silver halide, providing a photothermographic recording element. More preferably, it is a high contrast photothermographic recording element suitable as a printing plate. According to the invention, a compound of formula (A) or (B) is contained as a nucleating agent in the thermographic recording element for achieving a fully satisfactory high contrast and low fog and minimizing the dependency of photographic properties on developing temperature. The containment of the specific compound is also effective for achieving a high Dmax and high sensitivity. In contrast, the use of different compounds outside the scope of formulas (A) and (B), for example, acrylonitrile compounds fail to achieve both the effects of achieving high contrast and low fog and restraining developing-temperature dependency. Increasing the amount of such compounds for contrast enhancement tends to increase the fog and developing-temperature dependency. Now the compounds of formulas (A) and (B) are described in detail. In formula (A), Z 1 is a group of non-metallic atoms capable of forming a 5- to 7-membered ring structure with -Y 1 -C(=CH-X 1 )-C(=O)-. Z 1 is preferably a group of atoms selected from carbon, oxygen, sulfur, nitrogen, and hydrogen atoms wherein plural atoms selected from these atoms bond to each other through a single bond or double bond and form a 5- to 7-membered ring structure with -Y 1 -C(=CH-X 1 )-C(=O)-. Z 1 may have a substituent. Also Z 1 itself may be a part of an aromatic or non-aromatic carbocycle or an aromatic or non-aromatic heterocycle, and in this case, the 5- to 7-membered ring structure that Z 1 forms with -Y 1 -C(=CH-X 1 )-C(=O)- becomes a fused ring structure. In formula (B), Z 2 is a group of non-metallic atoms capable of forming a 5- to 7-membered ring structure with -Y 2 -C(=CH-X 2 )-C(Y 3 )=N-. Z 2 is preferably a group of atoms selected from carbon, oxygen, sulfur, nitrogen, and hydrogen atoms wherein plural atoms selected from these atoms bond to each other through a single bond or double bond and form a 5- to 7-membered ring structure with -Y 2 -C(=CH-X 2 )-C(Y 3 )=N-. Z 2 may have a substituent. Also Z 2 itself may be a part of an aromatic or non-aromatic carbocycle or an aromatic or non-aromatic heterocycle, and in this case, the 5- to 7-membered ring structure that Z 2 forms with -Y 2 -C(=CH-X 2 )-C(Y 3 )=N- becomes a fused ring structure. Where Z 1 and Z 2 have substituents, the substituents are selected from the following examples. Typical substituents include halogen atoms (e.g., fluorine, chlorine, bromine and iodine atoms), alkyl groups (including aralkyl, cycloalkyl and active methine groups), alkenyl groups, alkynyl groups, aryl groups, heterocyclic groups, quaternized nitrogen atom-containing heterocyclic groups (e.g., pyridinio), acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, carboxy groups or salts thereof, sulfonylcarbamoyl groups, acylcarbamoyl groups, sulfamoylcarbamoyl groups, carbazoyl groups, oxalyl groups, oxamoyl groups, cyano groups, thiocarbamoyl groups, hydroxy groups, alkoxy groups (including groups containing recurring ethylenoxy or propylenoxy units), aryloxy groups, heterocyclic oxy groups, acyloxy groups, (alkoxy or aryloxy)carbonyloxy groups, carbamoyloxy groups, sulfonyloxy groups, amino groups, (alkyl, aryl or heterocyclic) amino groups, N-substituted nitrogenous heterocyclic groups, acylamino groups, sulfonamide groups, ureido groups, thioureido groups, imide groups, (alkoxy or aryloxy)-carbonylamino groups, sulfamoylamino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino groups, quaternary ammonio groups, oxamoylamino groups, (alkyl or aryl)sulfonylureido groups, acylureido groups, acylsulfamoylamino groups, nitro groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, (alkyl or aryl)sulfonyl groups, (alkyl or aryl)sulfinyl groups, sulfo groups or salts thereof, sulfamoyl groups, acylsulfamoyl groups, sulfonylsulfamoyl groups or salts thereof, phosphoramide or phosphate ester structure-bearing groups, silyl groups, and stannyl groups. These substituents may be further replaced by other substituents selected from the foregoing examples. In formula (B), Y 3 is a hydrogen atom or substituent. When Y 3 represents a substituent, it is selected, for example, from alkyl, aryl, heterocyclic, cyano, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, amino, (alkyl, aryl or heterocyclic) amino, acylamino, sulfonamide, ureido, thioureido, imide, alkoxy, aryloxy, and (alkyl, aryl or heterocyclic) thio groups. These substituents may have substituents thereon, for example, those exemplified for Z 1 and Z 2 . In formulas (A) and (B), X 1 and X 2 independently represent hydroxy groups or salts thereof, alkoxy groups (e.g., methoxy, ethoxy, propoxy, isopropoxy, octyloxy, dodecyloxy, cetyloxy and t-butoxy), aryloxy groups (e.g., phenoxy, p-t-pentylphenoxy and p-t-octylphenoxy), heterocyclic oxy groups (e.g., benzotriazolyl-5-oxy and pyridinyl-3-oxy), mercapto groups or salts thereof, alkylthio groups (e.g., methylthio, ethylthio, butylthio and dodecylthio), arylthio groups (phenylthio and p-dodecylphenylthio), heterocyclic thio groups (e.g., 1-phenyltetrazoyl-5-thio, 2-methyl-1-phenyltriazolyl-5-thio, and mercaptothiadiazolylthio), amino groups, alkylamino groups (e.g., methylamino, propylamino, octylamino and dimethylamino), arylamino groups (e.g., anilino, naphthylamino and o-methoxyanilino), heterocyclic amino groups (e.g., pyridylamino and benzotriazol-5-ylamino), acylamino groups (acetamide, octanoylamino, and benzoylamino), sulfonamide groups (e.g., methanesulfonamide, benzenesulfonamide, and dodecylsulfonamide) or heterocyclic groups. The heterocyclic groups mentioned above are aromatic or non-aromatic, saturated or unsaturated, monocyclic or fused ring, substituted or unsubstituted heterocyclic groups, for example, N-methylhydantoin, N-phenylhydantoin, succinimide, phthalimide, N,N'-dimethylurazolyl, imidazolyl, benzotriazolyl, indazolyl, morpholino, and 4,4-dimethyl-2,5-dioxo-oxazolyl groups. The salts mentioned above are salts of alkali metals (e.g., sodium, potassium and lithium) and alkaline earth metals (e.g., magnesium and calcium), silver salts, quaternary ammonium salts (e.g., tetraethylammonium and dimethylcetylbenzylammonium salts), and quaternary phosphonium salts. In formulas (A) and (B), Y 1 and Y 2 represent -C(=O)- or -SO 2 -. Of the compounds of formulas (A) and (B), preferred ones are now described. In formulas (A) and (B), Y 1 and Y 2 preferably represent -C(=O)-. In formulas (A) and (B), X 1 and X 2 preferably represent hydroxy or salt thereof, alkoxy, mercapto or salt thereof, alkylthio, arylthio, heterocyclic thio, amino, sulfonamide or heterocyclic groups; more preferably hydroxy or salt thereof, alkoxy, mercapto or salt thereof, alkylthio, amino or heterocyclic groups; further preferably hydroxy or salt thereof, alkoxy, mercapto or salt thereof, amino or heterocyclic groups; most preferably hydroxy or salt thereof, alkoxy, mercapto or salt thereof amino or heterocyclic groups. When X 1 and X 2 in formulas (A) and (B) represent alkoxy groups, the total number of carbon atoms in the alkoxy group is preferably 1 to 18, more preferably 1 to 12, most preferably 1 to 5. When X 1 and X 2 in formulas (A) and (B) represent heterocyclic groups, the total number of carbon atoms in the heterocyclic group is preferably 2 to 20, more preferably 2 to 16. In formula (A), Z 1 is preferably a group of atoms forming a 5 or 6-membered ring structure. Exemplary are groups of atoms selected from nitrogen, carbon atoms, sulfur atoms, and oxygen atoms, such as -N-N-, -N-C-, -O-C-, -C-C-, -C=C-, -S-C-, -C=C-N-, -C=C-O-, -N-C-N-, -N=C-N-, -C-C-C-, -C=C-C-, and -O-C-O- linkages, which may further have hydrogen atoms or substituents. More preferably, Z 1 represents -N-N-, -N-C-, -O-C-, -C-C-, -C=C-, -S-C-, -N-C-N-, and -C=C-N- linkages which further have hydrogen atoms or substituents. Most preferably, Z 1 represents -N-N-, -N-C-, and -C=C- linkages which further have hydrogen atoms or substituents. It is also preferred that Z 1 itself be a part of an aromatic or non-aromatic carbocycle or an aromatic or non-aromatic heterocycle so that the 5- to 7-membered ring structure that Z 1 forms with -Y 1 -C(=CH-X 1 )-C(=O)- becomes a fused ring structure. Examples of the aromatic or non-aromatic carbocycle or aromatic or non-aromatic heterocycle include benzene, naphthalene, pyridine, cyclohexane, piperidine, pyrazolidine, pyrrolidine, 1,2-piperazine, 1,4-piperazine, oxane, oxolane, thiane, and thiolane rings. In formula (B), Z 2 is preferably a group of atoms forming a 5 or 6-membered ring structure. Exemplary are groups of atoms selected from nitrogen, carbon atoms, sulfur atoms, and oxygen atoms, such as -N-, -O-, -S-, -C-, -C=C-, -C-C-, -N-C-, -N=C-, -O-C-, and -S-C- linkages, which may further have hydrogen atoms or substituents if possible. It is also preferred that Z 2 itself be a part of an aromatic or non-aromatic carbocycle or an aromatic or non-aromatic heterocycle so that the 5- to 7-membered ring structure that Z 2 forms with -Y 2 -C(=CH-X 2 )-C(Y 3 )=N- becomes a fused ring structure. Examples of the aromatic or non-aromatic carbocycle or aromatic or non-aromatic heterocycle include benzene, naphthalene, pyridine, cyclohexane, piperidine, pyrazolidine, pyrrolidine, 1,2-piperazine, 1,4-piperazine, oxane, oxolane, thiane, and thiolane rings. More preferably, Z 2 in formula (B) represents -N-, -O-, -S-, -C-, or -C=C- linkages which may further have hydrogen atoms or substituents if possible. Most preferably, Z 2 represents -N- or -O- which may further have hydrogen atoms or substituents if possible. In one preferred embodiment of the nucleating agents of formula (A) and (B) the ring structures formed by Z 1 and Z 2 are 5-membered rings and both Y 1 and Y 2 are -C(=O)-. In formulas (A) and (B), preferred examples of the substituents that Z 1 or Z 2 can have include alkyl, aryl, halogen, heterocyclic, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, carboxy or salt thereof, sulfonylcarbamoyl, cyano, hydroxy, acyloxy, alkoxy, amino, (alkyl, aryl or heterocyclic) amino, acylamino, sulfonamide, ureido, thioureido, imide, (alkoxy or aryloxy)carbonylamino, sulfamoylamino, nitro, mercapto, (alkyl, aryl or heterocyclic) thio, (alkyl or aryl) sulfonyl, sulfo or salt thereof, and sulfamoyl groups. Where Z 1 or Z 2 itself becomes a part of an aromatic or non-aromatic carbocycle or an aromatic or non-aromatic heterocycle, to form a fused ring structure, the aromatic or non-aromatic carbocycle or aromatic or non-aromatic heterocycle may have substituents which are preferably selected from the same range as described above. In formula (B), Y 3 is preferably a hydrogen atom or a substituent selected from alkyl, aryl (especially phenyl and naphthyl), heterocyclic, cyano, acyl, alkoxycarbonyl, carbamoyl, (alkyl, aryl or heterocyclic) amino, acylamino, sulfonamide, ureido, imide, alkoxy, aryloxy, and (alkyl, aryl or heterocyclic) thio groups. More preferably, Y 3 in formula (B) is a substituent selected, for example, from alkyl, phenyl, amino, anilino, acylamino, alkoxy, aryloxy, and carbamoyl groups. These substituents may have substituents thereon. The total number of carbon atoms in Y 3 is preferably 1 to 25, more preferably 1 to 21. Preferably, the compound of formula (A) has at least 6 carbon atoms in total, and the compound of formula (B) has at least 12 carbon atoms in total. No particular upper limit is imposed on the total number of carbon atoms although it is preferred that the total number of carbon atoms be up to 40, more preferably up to 30, for the compound of formula (A) and up to 40, more preferably up to 32, for the compound of formula (B). Preferably in formula (A), the total number of carbon atoms in Z 1 , inclusive of the substituents thereon if any, is at least 2, more preferably at least 3. Also in formula (B), the total number of carbon atoms in Z 2 and Y 3 , inclusive of the substituents thereon if any, is preferably at least 8. It is further preferred in formula (A) that the total number of carbon atoms in Z 1 , inclusive of the substituents thereon if any, be 3 to 30, especially 6 to 25. It is also preferred in formula (B) that the total number of carbon atoms in Z 2 and Y 3 , inclusive of the substituents thereon if any, be 8 to 30, especially 8 to 27. The compounds of formulas (A) and (B) each may have incorporated therein a group capable of adsorbing to silver halides. Such adsorptive groups include alkylthio, arylthio, thiourea, thioamide, mercapto heterocyclic and triazole groups as described in USP 4,385,108 and 4,459,347, JP-A 195233/1984, 200231/1984, 201045/1984, 201046/1984, 201047/1984, 201048/1984, 201049/1984, 170733/1986, 270744/1986, 948/1987, 234244/1988, 234245/1988, and 234246/1988. These adsorptive groups to silver halides may take the form of precursors. Such precursors are exemplified by the groups described in JP-A 285344/1990. The compounds of formulas (A) and (B) each may have incorporated therein a ballast group or polymer commonly used in immobile photographic additives such as couplers. In particular, compounds having ballast groups incorporated therein are preferred in the practice of the invention. The ballast group is a group having at least 8 carbon atoms and relatively inert with respect to photographic properties. It may be selected from, for example, alkyl, aralkyl, alkoxy, phenyl, alkylphenyl, phenoxy, and alkylphenoxy groups. The polymer is exemplified in JP-A 100530/1989, for example. The compounds of formulas (A) and (B) each may contain a cationic group (e.g., a group containing a quaternary ammonio group and a nitrogenous heterocyclic group containing a quaternized nitrogen atom), a group containing recurring ethylenoxy or propylenoxy units, an (alkyl, aryl or heterocyclic) thio group, or a group which is dissociable with a base (e.g., carboxy, sulfo, acylsulfamoyl, and carbamoylsulfamoyl). In particular, compounds containing a group containing recurring ethylenoxy or propylenoxy units or an (alkyl, aryl or heterocyclic) thio group are preferred in the practice of the invention. Illustrative examples of these groups are described in, for example, in JP-A 234471/1995, 333466/1993, 19032/1994, 19031/1994, 45761/1993, 259240/1991, 5610/1995, and 244348/1995, USP 4,994,365 and 4,988,604, and German Patent No. 4006032. Illustrative, non-limiting, examples of the compounds represented by formulas (A) and (B) are given below. The compounds of formulas (A) and (B) according to the invention can be synthesized by various well-known methods. Some typical synthesis examples are described below. Synthesis Example 1: Synthesis of Compound 22 A mixture of 5 g of 1,2-diphenyl-3,5-pyrazolidinedione, 3.3 ml of triethyl o-formate, and 3.7 ml of acetic anhydride was heated and stirred at 70°C for one hour. The precipitated solid was filtered off and the filtrate was worked up by column chromatography, obtaining 1 g of the end compound, Compound 22. Synthesis Example 2: Synthesis of Compound 33 Compound 33 was synthesized according to Scheme 1. Synthesis of Intermediate 1 A solution of 23 g of imidazole in 150 ml of acetonitrile was ice cooled, and 35 ml of decanoic acid chloride was slowly added dropwise. At the end of addition, 200 ml of dimethylacetamide was added. The resulting solution was added dropwise to 200 ml of an acetonitrile/dimethylacetamide solution containing 25 g of N-(4-aminophenyl)-N'-formylhydrazine. After the mixture was stirred for 3 hours at room temperature, 1 liter of dilute hydrochloric acid was added thereto. The precipitated solid was collected by filtration and recrystallized from methanol, obtaining 33 g of Intermediate 1. Synthesis of Intermediate 2 To 600 ml of a methanol/acetonitrile solution containing 30 g of Intermediate 1 was added 20 g of 1,5-naphthalene disulfonic acid. The mixture was heated and stirred at 50°C for 4 hours. The reaction solution was ice cooled. The precipitated solid was collected by filtration, obtaining 41 g of Intermediate 2. Synthesis of Intermediate 3 A solution of 13 g of imidazole in 70 ml of acetonitrile was ice cooled, and 12 ml of ethylmalonyl chloride was slowly added dropwise. At the end of addition, 50 ml of dimethylacetamide was added. The resulting solution was added dropwise to 200 ml of an acetonitrile/dimethylacetamide solution containing 37 g of Intermediate 2 and 12 ml of triethylamine. After the mixture was stirred for 3 hours at room temperature, 1 liter of dilute hydrochloric acid was added thereto. The precipitated solid was collected by filtration and recrystallized from methanol, obtaining 30 g of Intermediate 3. Synthesis of Intermediate 4 To a solution of 10 g of Intermediate 3 in 40 ml of methanol was added 21 ml of a 28% methanol solution of sodium methoxide. After the mixture was stirred for one hour at room temperature, 15 ml of conc. hydrochloric acid was added thereto. The precipitated solid was collected by filtration and recrystallized from ethanol, obtaining 5 g of Intermediate 4. Synthesis of Compound 33 The synthesis procedure of Compound 22 was substantially followed except that Intermediate 4 was used instead of 1,2-diphenyl-3,5-pyrazolidinedione, obtaining Compound 33. Synthesis Example 3: Synthesis of Compound 54 In 100 ml of dimethylformamide (DMF) was dissolved 17.4 g (0.1 mol) of 3-methyl-1-phenyl-5-pyrazolone. Phosphorus oxychloride, 15.3 g, was added dropwise to the solution at room temperature, which was stirred at 80°C for one hour. To the reaction solution was added 500 ml of water. The resulting crystals were filtered, washed with water, and dissolved in a 1N sodium hydroxide solution, which was stirred for 30 minutes. The solution was made acidic with 3N hydrochloric acid. The resulting crystals were filtered, washed with water, and dried, obtaining 14.6 g (yield 72%) of the end product. Synthesis Example 4: Synthesis of Compound 55 In 100 ml of DMF was dissolved 27.7 ml (0.1 mol) of 3-methyl-1-(2',4',6'-trichlorophenyl) -5-pyrazolone. Phosphorus oxychloride, 15.3 g, was added dropwise to the solution at room temperature, which was stirred at 80°C for one hour. To the reaction solution was added 500 ml of water. The resulting crystals were filtered, washed with water, and dissolved in a 1N sodium hydroxide solution, which was stirred for 30 minutes. The solution was made acidic with 3N hydrochloric acid. The resulting crystals were filtered, washed with water, and dried, obtaining 23.2 g (yield 76%) of the end product. Synthesis Example 5: Synthesis of Compound 86 To 4.83 g (0.03 mol) of 3-phenyl-5-isooxazolone were added 7 ml of acetic anhydride and 7 ml (0.042 mol) of ethyl o-formate. The mixture was stirred at 80°C for 2 hours. The excess of acetic anhydride was distilled off in vacuum, obtaining a crude product. It was worked up by silica gel column chromatography, obtaining 2.4 g (yield 42%) of the end product. Synthesis Example 6: Synthesis of Compound 63 To 3.0 g of tetronic acid was added 5.9 ml of diethoxymethyl acetate. The mixture was heated and stirred at 50°C for 15 minutes. After cooling to room temperature, 20 ml of ether was added to the reaction mixture whereupon the resulting crystals were collected by filtration. The crystals were recrystallized from 20 ml of ethyl acetate, obtaining 1.0 g of the end product. Synthesis Example 7: Synthesis of Compound 1 To 4.4 g of indandione was added 5.9 ml of diethoxymethyl acetate. The mixture was heated and stirred at 50°C for one hour. After cooling to room temperature, 20 ml of ethyl acetate was added to the reaction mixture, which was filtered. To the filtrate was added 100 ml of hexane. The precipitated crystals were collected by filtration and recrystallized from a mixture of 20 ml of ethyl acetate and 100 ml of hexane, obtaining 2.6 g of the end product. Synthesis Example 8: Synthesis of Compound 72 To 4.2 g of dimedone was added 5.4 ml of diethoxymethyl acetate. The mixture was stirred at room temperature for one hour. Thereafter, 50 ml of hexane was added to the mixture, which was filtered. The filtrate was concentrated in vacuum, and 100 ml of hexane was added thereto for crystallization. The crystals were collected by filtration, obtaining 1.1 g of the end product. Synthesis Example 9: Synthesis of Compound 85 To 4.7 g of 1,3-dimethylbarbituric acid was added 5.4 ml of diethoxymethyl acetate. The mixture was stirred at room temperature for 30 minutes. Thereafter, 50 ml of ethyl acetate was added to the mixture whereupon crystals precipitated. The crystals were collected by filtration, and 30 ml of methanol was added thereto. After the insolubles were filtered off, 0.7 g of sodium hydroxide was added to the filtrate, which was stirred at room temperature for one hour. Thereafter, the solution was made acidic with 1N hydrochloric acid, and 100 ml of ethyl acetate was added. The organic layer was dried over magnesium sulfate and concentrated in vacuum, and 20 ml of hexane was added thereto for crystallization. The crystals were collected by filtration, obtaining 0.6 g of the end product. Synthesis Example 10: Synthesis of Compound 51 To 4.1 g of 1-phenyl-5,5-dimethylpyrrolidine-2,4-dione was added 3.6 ml of diethoxymethyl acetate. The mixture was stirred at 40°C for 30 minutes. After cooling to room temperature, 50 ml of ethyl acetate was added to the solution, which was filtered and concentrated in vacuum. Thereafter, 50 ml of methanol and 0.3 g of sodium hydroxide were added to the solution, which was stirred at room temperature for one hour. After the crystals were filtered, the filtrate was made acidic with 1N hydrochloric acid, and 100 ml of ethyl acetate was added. The organic layer was dried over magnesium sulfate, filtered, and concentrated in vacuum, and 20 ml of hexane was added thereto for crystallization. The crystals were collected by filtration, obtaining 0.3 g of the end product. In the practice of the invention, the compounds of formulas (A) and (B) according to the invention may be used as solution in water or suitable organic solvents. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethylsulfoxide and methyl cellosolve. A well-known emulsifying dispersion method is used for dissolving the compound of formula (A) or (B) with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the compounds in powder form in water or suitable solvents in a ball mill, colloidal mill or ultrasonic mixer. The compound of formula (A) or (B) or both may be added to an image forming layer or any other layer on the image forming layer side of a support, and preferably to the image forming layer or a layer disposed contiguous thereto. The amount of the compound of formula (A) or (B) or both added is preferably 1x10 -6 to 1 mol, more preferably 1x10 -5 to 5x10 -1 mol, and most preferably 2x10 -5 to 2x10 -1 mol per mol of silver. According to the invention, the compounds of formulas (A) and (B) may be used alone or in admixture of two or more. In the thermographic recording element according to one preferred embodiment of the invention, hydrazine derivatives are used in combination with the compounds of formula (A) or (B). Exemplary hydrazine derivatives which can be used herein include the compounds of the chemical formula [1] in JP-B 77138/1994, more specifically the compounds described on pages 3 and 4 of the same; the compounds of the general formula (I) in JP-B 93082/1994, more specifically compound Nos. 1 to 38 described on pages 8 to 18 of the same; the compounds of the general formulae (4), (5) and (6) in JP-A 230497/1994, more specifically compounds 4-1 to 4-10 described on pages 25 and 26, compounds 5-1 to 5-42 described on pages 28 to 36, and compounds 6-1 to 6-7 described on pages 39 and 40 of the same; the compounds of the general formulae (1) and (2) in JP-A 289520/1994, more specifically compounds 1-1 to 1-17 and 2-1 described on pages 5 to 7 of the same; the compounds of the chemical formulae [2] and [3] in JP-A 313936/1994, more specifically the compounds described on pages 6 to 19 of the same; the compounds of the chemical formula [1] in JP-A 313951/1994, more specifically the compounds described on pages 3 to 5 of the same; the compounds of the general formula (I) in JP-A 5610/1995, more specifically compounds I-1 to I-38 described on pages 5 to 10 of the same; the compounds of the general formula (II) in JP-A 77783/1995, more specifically compounds II-1 to II-102 described on pages 10 to 27 of the same; the compounds of the general formulae (H) and (Ha) in JP-A 104426/1995, more specifically compounds H-1 to H-44 described on pages 8 to 15 of the same; the compounds having an anionic group in proximity to a hydrazine group or a nonionic group capable of forming an intramolecular hydrogen bond with the hydrogen atom of hydrazine described in EP 713131A, especially compounds of the general formulae (A), (B), (C), (D), (E), and (F), more specifically compounds N-1 to N-30 described therein; and the compounds of the general formula (1) in EP 713131A, more specifically compounds D-1 to D-55 described therein. Also useful are the hydrazine derivatives described in "Known Technology," Aztech K.K., March 22, 1991, pages 25-34 and Compounds D-2 and D-39 described in JP-A 86354/1987, pages 6-7. Most often, the hydrazine nucleating agents are used as solution in water or suitable organic solvents. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethyl sulfoxide and methyl cellosolve. A well-known emulsifying dispersion method may be used for dissolving the hydrazine derivative with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the hydrazine derivative in powder form in a suitable solvent, typically water, in a ball mill, colloidal mill or ultrasonic mixer. The hydrazine derivative may be added to an image forming layer or any other layer on the image forming layer side of a support, and preferably to the image forming layer or a layer disposed contiguous thereto. The hydrazine derivative is preferably used in an amount of 1x10 -6 mol to 1 mol, more preferably 1x10 -5 mol to 5x10 -1 mol, and most preferably 2x10 -5 mol to 2x10 -1 mol per mol of silver halide. Organic silver salt The organic silver salt which can be used herein is relatively stable to light, but forms a silver image when heated at 80°C or higher in the presence of an exposed photocatalyst (as typified by a latent image of photosensitive silver halide) and a reducing agent. The organic silver salt may be of any desired organic compound containing a source capable of reducing silver ion. Preferred are silver salts of organic acids, typically long chain aliphatic carboxylic acids having 10 to 30 carbon atoms, especially 15 to 28 carbon atoms. Also preferred are complexes of organic or inorganic silver salts with ligands having a stability constant in the range of 4.0 to 10.0. A silver-providing substance is preferably used in an amount of 5 to 70% by weight of the image forming layer. Preferred organic silver salts include silver salts of organic compounds having a carboxyl group. Examples include silver salts of aliphatic carboxylic acids and silver salts of aromatic carboxylic acids though not limited thereto. Preferred examples of the silver salt of aliphatic carboxylic acid include silver behenate, silver arachidate, silver stearate, silver oleate, silver laurate, silver caproate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartrate, silver linolate, silver butyrate, silver camphorate and mixtures thereof. Silver salts of compounds having a mercapto or thion group and derivatives thereof are also useful. Preferred examples of these compounds include a silver salt of 3-mercapto-4-phenyl-1,2,4-triazole, a silver salt of 2-mercaptobenzimidazole, a silver salt of 2-mercapto-5-aminothiadiazole, a silver salt of 2-(ethylglycolamido)-benzothiazole, silver salts of thioglycolic acids such as silver salts of S-alkylthioglycolic acids wherein the alkyl group has 12 to 22 carbon atoms, silver salts of dithiocarboxylic acids such as a silver salt of dithioacetic acid, silver salts of thioamides, a silver salt of 5-carboxyl-1-methyl-2-phenyl-4-thiopyridine, silver salts of mercaptotriazines, a silver salt of 2-mercaptobenzoxazole as well as silver salts of 1,2,4-mercaptothiazole derivatives such as a silver salt of 3-amino-5-benzylthio-1,2,4-thiazole as described in USP 4,123,274 and silver salts of thion compounds such as a silver salt of 3-(3-carboxyethyl)-4-methyl-4-thiazoline-2-thione as described in USP 3,301,678. Compounds containing an imino group may also be used. Preferred examples of these compounds include silver salts of benzotriazole and derivatives thereof, for example, silver salts of benzotriazoles such as silver methylbenzotriazole, silver salts of halogenated benzotriazoles such as silver 5-chlorobenzotriazole as well as silver salts of 1,2,4-triazole and 1-H-tetrazole and silver salts of imidazole and imidazole derivatives as described in USP 4,220,709. Also useful are various silver acetylide compounds as described, for example, in USP 4,761,361 and 4,775,613. The organic silver salt which can be used herein may take any desired shape although needle crystals having a minor axis and a major axis are preferred. In the practice of the invention, grains should preferably have a minor axis of 0.01 µm to 0.20 µm and a major axis of 0.10 µm to 5.0 µm, more preferably a minor axis of 0.01 µm to 0.15 µm and a major axis of 0.10 µm to 4.0 µm. The grain size distribution of the organic silver salt is desirably monodisperse. The monodisperse distribution means that a standard deviation of the length of minor and major axes divided by the length, respectively, expressed in percent, is preferably up to 100%, more preferably up to 80%, most preferably up to 50%. It can be determined from the measurement of the shape of organic silver salt grains using an image obtained through a transmission electron microscope. Another method for determining a monodisperse distribution is to determine a standard deviation of a volume weighed mean diameter. The standard deviation divided by the volume weighed mean diameter, expressed in percent, which is a coefficient of variation, is preferably up to 100%, more preferably up to 80%, most preferably up to 50%. It may be determined by irradiating laser light, for example, to organic silver salt grains dispersed in liquid and determining the autocorrelation function of the fluctuation of scattering light relative to a time change, and obtaining the grain size (volume weighed mean diameter) therefrom. The organic silver salt used herein is preferably desalted. The desalting method is not critical. Any well-known method may be used although well-known filtration methods such as centrifugation, suction filtration, ultrafiltration, and flocculation/water washing are preferred. In the practice of the invention, the organic silver salt is prepared into a solid microparticulate dispersion using a dispersant, in order to provide fine particles of small size and free of flocculation. A solid microparticulate dispersion of the organic silver salt may be prepared by mechanically dispersing the salt in the presence of dispersing aids by well-known comminuting means such as ball mills, vibrating ball mills, planetary ball mills, sand mills, colloidal mills, jet mills, and roller mills. The dispersant used in the preparation of a solid microparticulate dispersion of the organic silver salt may be selected from synthetic anionic polymers such as polyacrylic acid, copolymers of acrylic acid, copolymers of maleic acid, copolymers of maleic acid monoester, and copolymers of acryloylmethylpropanesulfonic acid; semisynthetic anionic polymers such as carboxymethyl starch and carboxymethyl cellulose; anionic polymers such as alginic acid and pectic acid; anionic surfactants as described in JP-A 92716/1977 and WO 88/04794; the compounds described in Japanese Patent Application No. 350753/1995; well-known anionic, nonionic and cationic surfactants; and well-known polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, hydroxypropyl cellulose, and hydroxypropyl methyl cellulose, as well as naturally occurring high molecular weight compounds such as gelatin. In general, the dispersant is mixed with the organic silver salt in powder or wet cake form prior to dispersion. The resulting slurry is fed into a dispersing machine. Alternatively, a mixture of the dispersant with the organic silver salt is subject to heat treatment or solvent treatment to form a dispersant-bearing powder or wet cake of the organic silver salt. It is acceptable to effect pH control with a suitable pH adjusting agent before, during or after dispersion. Rather than mechanical dispersion, fine particles can be formed by roughly dispersing the organic silver salt in a solvent through pH control and thereafter, changing the pH in the presence of dispersing aids. An organic solvent can be used as the solvent for rough dispersion although the organic solvent is usually removed at the end of formation of fine particles. The thus prepared dispersion may be stored while continuously stirring for the purpose of preventing fine particles from settling during storage. Alternatively, the dispersion is stored after adding hydrophilic colloid to establish a highly viscous state (for example, in a jelly-like state using gelatin). An antiseptic agent may be added to the dispersion in order to prevent the growth of bacteria during storage. The organic silver salt is used in any desired amount, preferably 0.1 to 5 g/m 2 , more preferably 1 to 3 g/m 2 , as expressed by a silver coverage per square meter of the thermographic recording element. Silver halide When it is desired to use the thermographic recording element of the invention as a photothermographic recording element, a photosensitive silver halide can be used. A method for forming the photosensitive silver halide is well known in the art. Any of the methods disclosed in Research Disclosure No. 17029 (June 1978) and US? 3,700,458, for example, may be used. Illustrative methods which can be used herein are a method of preparing an organic silver salt and adding a halogen-containing compound to the organic silver salt to convert a part of silver of the organic silver salt into photosensitive silver halide and a method of adding a silver-providing compound and a halogen-providing compound to a solution of gelatin or another polymer to form photosensitive silver halide grains and mixing the grains with an organic silver salt. The latter method is preferred in the practice of the invention. The photosensitive silver halide should preferably have a smaller mean grain size for the purpose of minimizing white turbidity after image formation. Specifically, the grain size is preferably up to 0.20 µm, more preferably 0.01 µm to 0.16 µm, most preferably 0.02 µm to 0.14 µm. The term grain size designates the length of an edge of a silver halide grain where silver halide grains are regular grains of cubic or octahedral shape. Where silver halide grains are tabular, the grain size is the diameter of an equivalent circle having the same area as the projected area of a major surface of a tabular grain. Where silver halide grains are not regular, for example, in the case of spherical or rod-shaped grains, the grain size is the diameter of an equivalent sphere having the same volume as a grain. The shape of silver halide grains may be cubic, octahedral, tabular, spherical, rod-like and potato-like, with cubic and tabular grains being preferred in the practice of the invention. Where tabular silver halide grains are used, they should preferably have an average aspect ratio of from 100:1 to 2:1, more preferably from 50:1 to 3:1. Silver halide grains having rounded corners are also preferably used. No particular limit is imposed on the face indices (Miller indices) of an outer surface of photosensitive silver halide grains. Preferably silver halide grains have a high proportion of {100} face featuring high spectral sensitization efficiency upon adsorption of a spectral sensitizing dye. The proportion of {100} face is preferably at least 50%, more preferably at least 65%, most preferably at least 80%. Note that the proportion of Miller index {100} face can be determined by the method described in T. Tani, J. Imaging Sci., 29, 165 (1985), utilizing the adsorption dependency of {111} face and {100} face upon adsorption of a sensitizing dye. The halogen composition of photosensitive silver halide is not critical and may be any of silver chloride, silver chlorobromide, silver bromide, silver iodobromide, silver iodochlorobromide, and silver iodide. The halogen composition in grains may have a uniform distribution or a non-uniform distribution wherein the halogen concentration changes in a stepped or continuous manner. Preferred are silver iodobromide grains having a higher silver iodide content in the interior. Silver halide grains of the core/shell structure are also useful. Such core/shell grains preferably have a multilayer structure of 2 to 5 layers, more preferably 2 to 4 layers. Preferably the photosensitive silver halide grains used herein contain at least one complex of a metal selected from the group consisting of rhodium, rhenium, ruthenium, osmium, iridium, cobalt, mercury, and iron. The metal complexes may be used alone or in admixture of two or more complexes of a common metal or different metals. The metal complex is preferably contained in an amount of 1 nmol to 10 mmol, more preferably 10 nmol to 100 µmol per mol of silver. Illustrative metal complex structures are those described in JP-A 225449/1995. The cobalt and iron compounds are preferably hexacyano metal complexes while illustrative, non-limiting examples include ferricyanate, ferrocyanate, and hexacyanocobaltate ions. The distribution of the metal complex in silver halide grains is not critical. That is, the metal complex may be contained in silver halide grains to form a uniform phase or at a high concentration in either the core or the shell. Photosensitive silver halide grains may be desalted by any of well-known water washing methods such as noodle and flocculation methods although silver halide grains may be either desalted or not according to the invention. The photosensitive silver halide grains used herein should preferably be chemically sensitized. Preferred chemical sensitization methods are sulfur, selenium, and tellurium sensitization methods which are well known in the art. Also useful are a noble metal sensitization method using compounds of gold, platinum, palladium, and iridium and a reduction sensitization method. In the sulfur, selenium, and tellurium sensitization methods, any of compounds well known for the purpose may be used. For example, the compounds described in JP-A 128768/1995 are useful. Exemplary tellurium sensitizing agents include diacyltellurides, bis(oxycarbonyl)tellurides, bis-(carbamoyl)tellurides, bis(oxycarbonyl)ditellurides, bis(carbamoyl)ditellurides, compounds having a P=Te bond, tellurocarboxylic salts, Te-organyltellurocarboxylic esters, di (poly) tellurides, tellurides, telluroles, telluroacetals, tellurosulfonates, compounds having a P-Te bond, Te-containing heterocycles, tellurocarbonyl compounds, inorganic tellurium compounds, and colloidal tellurium. The preferred compounds used in the noble metal sensitization method include chloroauric acid, potassium chloroaurate, potassium aurithiocyanate, gold sulfide, and gold selenide as well as the compounds described in USP 2,448,060 and BP 618,061. Illustrative examples of the compound used in the reduction sensitization method include ascorbic acid, thiourea dioxide, stannous chloride, aminoiminomethane-sulfinic acid, hydrazine derivatives, borane compounds, silane compounds, and polyamine compounds. Reduction sensitization may also be accomplished by ripening the emulsion while maintaining it at pH 7 or higher or at pAg 8.3 or lower. Reduction sensitization may also be accomplished by introducing a single addition portion of silver ion during grain formation. According to the invention, the photosensitive silver halide is preferably used in an amount of 0.01 to 0.5 mol, more preferably 0.02 to 0.3 mol, most preferably 0.03 to 0.25 mol per mol of the organic silver salt. With respect to a method and conditions of admixing the separately prepared photosensitive silver halide and organic silver salt, there may be used a method of admixing the separately prepared photosensitive silver halide and organic silver salt in a high speed agitator, ball mill, sand mill, colloidal mill, vibrating mill or homogenizer or a method of preparing an organic silver salt by adding the already prepared photosensitive silver halide at any timing during preparation of an organic silver salt. Any desired mixing method may be used insofar as the benefits of the invention are fully achievable. One of the preferred methods for preparing the silver halide according to the invention is a so-called halidation method of partially halogenating the silver of an organic silver salt with an organic or inorganic halide. Any of organic halides which can react with organic silver salts to form silver halides may be used. Exemplary organic halides are N-halogenoimides (e.g., N-bromosuccinimide), halogenated quaternary nitrogen compounds (e.g., tetrabutylammonium bromide), and aggregates of a halogenated quaternary nitrogen salt and a molecular halogen (e.g., pyridinium bromide perbromide). Any of inorganic halides which can react with organic silver salts to form silver halides may be used. Exemplary inorganic halides are alkali metal and ammonium halides (e.g., sodium chloride, lithium bromide, potassium iodide, and ammonium bromide), alkaline earth metal halides (e.g., calcium bromide and magnesium chloride), transition metal halides (e.g., ferric chloride and cupric bromide), metal complexes having a halogen ligand (e.g., sodium iridate bromide and ammonium rhodate chloride), and molecular halogens (e.g., bromine, chlorine and iodine). A mixture of organic and inorganic halides may also be used. The amount of the halide added for the halidation purpose is preferably 1 mmol to 500 mmol, especially 10 mmol to 250 mmol of halogen atom per mol of the organic silver salt. Reducing agent The thermographic recording element of the invention contains a reducing agent for the organic silver salt. The reducing agent for the organic silver salt may be any of substances, preferably organic substances, that reduce silver ion into metallic silver. Conventional photographic developing agents such as Phenidone®, hydroquinone and catechol are useful although hindered phenols are preferred reducing agents. The reducing agent should preferably be contained in an amount of 5 to 50 mol%, more preferably 10 to 40 mol% per mol of silver on the image forming layer-bearing side. The reducing agent may be added to any layer on the image forming layer-bearing side. Where the reducing agent is added to a layer other than the image forming layer, the reducing agent should preferably be contained in a slightly greater amount of 10 to 50 mol% per mol of silver. The reducing agent may take the form of a precursor which is modified so as to exert its effective function only at the time of development. For thermographic recording elements using organic silver salts, a wide range of reducing agents are disclosed, for example, in JP-A 6074/1971, 1238/1972, 33621/1972, 46427/1974, 115540/1974, 14334/1975, 36110/1975, 147711/1975, 32632/1976, 1023721/1976, 32324/1976, 51933/1976, 84727/1977, 108654/1980, 146133/1981, 82828/1982, 82829/1982, 3793/1994, USP 3,667,958, 3,679,426, 3,751,252, 3,751,255, 3,761,270, 3,782,949, 3,839,048, 3,928,686, 5,464,738, German Patent No. 2321328, and EP 692732. Exemplary reducing agents include amidoximes such as phenylamidoxime, 2-thienylamidoxime, and p-phenoxyphenylamidoxime; azines such as 4-hydroxy-3,5-dimethoxybenzaldehydeazine; combinations of aliphatic carboxylic acid arylhydrazides with ascorbic acid such as a combination of 2,2-bis(hydroxymethyl)propionyl-β-phenylhydrazine with ascorbic acid; combinations of polyhydroxybenzenes with hydroxylamine, reductone and/or hydrazine, such as combinations of hydroquinone with bis(ethoxyethyl)hydroxylamine, piperidinohexosereductone or formyl-4-methylphenylhydrazine; hydroxamic acids such as phenylhydroxamic acid, p-hydroxyphenylhydroxamic acid, and β-anilinehydroxamic acid; combinations of azines with sulfonamidophenols such as a combination of phenothiazine with 2,6-dichloro-4-benzenesulfonamidephenol; α-cyanophenyl acetic acid derivatives such as ethyl-α-cyano-2-methylphenyl acetate and ethyl-α-cyanophenyl acetate; bis-β-naphthols such as 2,2'-dihydroxy-1,1'-binaphthyl, 6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, and bis(2-hydroxy-1-naphthyl)methane; combinations of bis-β-naphthols with 1,3-dihydroxybenzene derivatives such as 2,4-dihydroxybenzophenone and 2',4'-dihydroxyacetophenone; 5-pyrazolones such as 3-methyl-1-phenyl-5-pyrazolone; reductones such as dimethylaminohexosereductone, anhydrodihydroaminohexosereductone and anhydrodihydropiperidonehexosereductone; sulfonamidephenol reducing agents such as 2,6-dichloro-4-benzenesulfonamidephenol and p-benzenesulfonamidephenol; 2-phenylindane-1,3-dione; chromans such as 2,2-dimethyl-7-t-butyl-6-hydroxychroman; 1,4-dihydropyridines such as 2,6-dimethoxy-3,5-dicarboethoxy-1,4-dihydropyridine; bisphenols such as bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,4-ethylidene-bis(2-t-butyl-6-methylphenol), 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane, and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl )-propane; ascorbic acid derivatives such as 1-ascorbyl palmitate and ascorbyl stearate; aldehydes and ketones such as benzil and diacetyl; 3-pyrazolidones and certain indane-1,3-diones; and chromanols (tocopherols). Preferred reducing agents are bisphenols and chromanols. The reducing agent may be added in any desired form such as solution, powder or solid particle dispersion. The solid particle dispersion of the reducing agent may be prepared by well-known comminuting means such as ball mills, vibrating ball mills, sand mills, colloidal mills, jet mills, and roller mills. Dispersing aids may be used for facilitating dispersion. Toner A higher optical density is sometimes achieved when an additive known as a "toner" for improving images is contained. The toner is also sometimes advantageous in forming black silver images. The toner is preferably used in an amount of 0.1 to 50 mol%, especially 0.5 to 20 mol% per mol of silver on the image forming layer-bearing side. The toner may take the form of a precursor which is modified so as to exert its effective function only at the time of development. For thermographic recording elements using organic silver salts, a wide range of toners are disclosed, for example, in JP-A 6077/1971, 10282/1972, 5019/1974, 5020/1974, 91215/1974, 2524/1975, 32927/1975, 67132/1975, 67641/1975, 114217/1975, 3223/1976, 27923/1976, 14788/1977, 99813/1977, 1020/1978, 76020/1978, 156524/1979, 156525/1979, 183642/1986, and 56848/1992, JP-B 10727/1974 and 20333/1979, USP 3,080,254, 3,446,648, 3,782,941, 4,123,282, 4,510,236, BP 1,380,795, and Belgian Patent No. 841,910. Examples of the toner include phthalimide and N-hydroxyphthalimide; cyclic imides such as succinimide, pyrazolin-5-one, quinazolinone, 3-phenyl-2-pyrazolin-5-one, 1-phenylurazol, quinazoline and 2,4-thiazolidinedione; naphthalimides such as N-hydroxy-1,8-naphthalimide; cobalt complexes such as cobaltic hexamine trifluoroacetate; mercaptans as exemplified by 3-mercapto-1,2,4-triazole, 2,4-dimercaptopyrimidine, 3-mercapto-4,5-diphenyl-1,2,4-triazole, and 2,5-dimercapto-1,3,4-thiadiazole; N-(aminomethyl)aryldicarboxyimides such as (N,N-dimethylaminomethyl)phthalimide and N,N-(dimethylaminomethyl)-naphthalene-2,3-dicarboxyimide; blocked pyrazoles, isothiuronium derivatives and certain photo-bleach agents such as N,N'-hexamethylenebis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-diazaoctane)-bis(isothiuroniumtrifluoroacetate) and 2-tribromomethylsulfonyl-benzothiazole; 3-ethyl-5-{(3-ethyl-2-benzothiazolinylidene)-1-methylethylidene}-2-thio-2,4-oxazolidinedione; phthalazinone, phthalazinone derivatives or metal salts, or derivatives such as 4-(1-naphthyl)-phthalazinone, 6-chlorophthalazinone, 5,7-dimethoxyphthalazinone and 2,3-dihydro-1,4-phthalazinedione; combinations of phthalazinones with phthalic acid derivatives (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalic anhydride); phthalazine, phthalazine derivatives or metal salts such as 4-(1-naphthyl)phthalazine, 6-chlorophthalazine, 5,7-dimethoxyphthalazine and 2,3-dihydrophthalazine; combinations of phthalazine with phthalic acid derivatives (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid and tetrachlorophthalic anhydride); quinazolinedione, benzoxazine or naphthoxazine derivatives; rhodium complexes which function not only as a tone regulating agent, but also as a source of halide ion for generating silver halide in situ, for example, ammonium hexachlororhodinate (III), rhodium bromide, rhodium nitrate and potassium hexachlororhodinate (III); inorganic peroxides and persulfates such as ammonium peroxide disulfide and hydrogen peroxide; benzoxazine-2,4-diones such as 1,3-benzoxazine-2,4-dione, 8-methyl-1,3-benzoxazine-2,4-dione, and 6-nitro-1,3-benzoxazine-2,4-dione; pyrimidine and asym-triazines such as 2,4-dihydroxypyrimidine and 2-hydroxy-4-aminopyrimidine; azauracil and tetraazapentalene derivatives such as 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene, and 1,4-di(o-chlorophenyl)-3,6-dimercapto-1H,4H-2,3a,5,6a-tetraazapentalene. The toner may be added in any desired form, for example, as a solution, powder and solid particle dispersion. The solid particle dispersion of the toner is prepared by well-known finely dividing means such as ball mills, vibrating ball mills, sand mills, colloid mills, jet mills, and roller mills. Dispersing aids may be used in preparing the solid particle dispersion. Binder The image forming layer used herein is usually based on a binder. Exemplary binders are naturally occurring polymers and synthetic resins, for example, gelatin, polyvinyl acetal, polyvinyl chloride, polyvinyl acetate, cellulose acetate, polyolefins, polyesters, polystyrene, polyacrylonitrile, and polycarbonate. Of course, copolymers and terpolymers are included. Preferred polymers are polyvinyl butyral, butylethyl cellulose, methacrylate copolymers, maleic anhydride ester copolymers, polystyrene and butadiene-styrene copolymers. These polymers may be used alone or in admixture of two or more as desired. The polymer is used in such a range that it may effectively function as a binder to carry various components. The effective range may be properly determined by those skilled in the art without undue experimentation. Taken at least as a measure for carrying the organic silver salt in the film, the weight ratio of the binder to the organic silver salt is preferably in the range of from 15:1 to 1:2, more preferably from 8:1 to 1:1. At least one layer of the image-forming layers used herein may be an image forming layer wherein a polymer latex constitutes more than 50% by weight of the entire binder. This image forming layer is sometimes referred to as "image forming layer usable according to the invention" and the polymer latex used as the binder therefor is referred to as "polymer latex usable according to the invention" hereinafter. The term "polymer latex" used herein is a dispersion of a microparticulate water-insoluble hydrophobic polymer in a water-soluble dispersing medium. With respect to the dispersed state, a polymer emulsified in a dispersing medium, an emulsion polymerized polymer, a micelle dispersion, and a polymer having a hydrophilic structure in a part of its molecule so that the molecular chain itself is dispersed on a molecular basis are included. With respect to the polymer latex, reference is made to Okuda and Inagaki Ed., "Synthetic Resin Emulsion," Kobunshi Kankokai, 1978; Sugimura, Kataoka, Suzuki and Kasahara Ed., "Application of Synthetic Latex," Kobunshi Kankokai, 1993; and Muroi, "Chemistry of Synthetic Latex," Kobunshi Kankokai, 1970. Dispersed particles should preferably have a mean particle size of 1 to 50,000 nm, more preferably 5 to 1,000 nm. No particular limit is imposed on the particle size distribution of dispersed particles, and the dispersion may have either a wide particle size distribution or a monodisperse particle size distribution. The polymer latex usable according to the invention may be either a latex of the conventional uniform structure or a latex of the so-called core/shell type. In the latter case, better results are sometimes obtained when the core and the shell have different glass transition temperatures. The polymer latex should preferably have a minimum film-forming temperature (MFT) of -30°C to 90°C, more preferably 0°C to 70°C. A film-forming aid may be added in order to control the minimum film-forming temperature. The film-forming aid is also referred to as a plasticizer and includes organic compounds (typically organic solvents) for lowering the minimum film-forming temperature of a polymer latex. It is described in Muroi, "Chemistry of Synthetic Latex," Kobunshi Kankokai, 1970. Polymers used in the polymer latex include acrylic resins, vinyl acetate resins, polyester resins, polyurethane resins, rubbery resins, vinyl chloride resins, vinylidene chloride resins, polyolefin resins, and copolymers thereof. The polymer may be linear or branched or crosslinked. The polymer may be either a homopolymer or a copolymer having two or more monomers polymerized together. The copolymer may be either a random copolymer or a block copolymer. The polymer preferably has a number average molecule weight Mn of 5,000 to 1,000,000, more preferably 10,000 to 100,000. Polymers with a too lower molecular weight would generally provide a low film strength after coating whereas polymers with a too higher molecular weight are difficult to form films. The polymer of the polymer latex should preferably have an equilibrium moisture content at 25°C and RH 60% of up to 2% by weight, more preferably up to 1% by weight. The lower limit of equilibrium moisture content is not critical although it is preferably 0.01% by weight, more preferably 0.03% by weight. With respect to the definition and measurement of equilibrium moisture content, reference should be made to "Polymer Engineering Series No. 14, Polymer Material Test Methods," Edited by Japanese Polymer Society, Chijin Shokan Publishing K.K., for example. Illustrative examples of the polymer latex which can be used as the binder in the image-forming layer of the thermographic recording element of the invention include latexes of methyl methacrylate/ethyl acrylate/methacrylic acid copolymers, latexes of methyl methacrylate/2-ethylhexyl acrylate/styrene/acrylic acid copolymers, latexes of styrene/butadiene/acrylic acid copolymers, latexes of styrene/butadiene/divinyl benzene/methacrylic acid copolymers, latexes of methyl methacrylate/vinyl chloride/acrylic acid copolymers, and latexes of vinylidene chloride/ethyl acrylate/acrylonitrile/methacrylic acid copolymers. These polymers or polymer latexes are commercially available. Exemplary acrylic resins are Sebian A-4635, 46583 and 4601 (Daicell Chemical Industry K.K.) and Nipol LX811, 814, 820, 821 and 857 (Nippon Zeon K.K.). Exemplary polyester resins are FINETEX ES650, 611, 675, and 850 (Dainippon Ink & Chemicals K.K.) and WD-size and WMS (Eastman Chemical Products, Inc.). Exemplary polyurethane resins are HYDRAN AP10, 20, 30 and 40 (Dainippon Ink & Chemicals K.K.). Exemplary rubbery resins are LACSTAR 7310K, 3307B, 4700H and 7132C (Dainippon Ink & Chemicals K.K.) and Nipol LX416, 410, 438C and 2507 (Nippon Zeon K.K.). Exemplary vinyl chloride resins are G351 and G576 (Nippon Zeon K.K.). Exemplary vinylidene chloride resins are L502 and L513 (Asahi Chemicals K.K.). Exemplary olefin resins are Chemipearl S120 and SA100 (Mitsui Petro-Chemical K.K.). These polymers may be used alone or in admixture of two or more. In the image-forming layer usable according to the invention, the polymer latex described above is preferably used in an amount of at least 50% by weight, especially at least 70% by weight, of the entire binder. In the image-forming layer, a hydrophilic polymer may be added in an amount of less than 50% by weight of the entire binder. Such hydrophilic polymers are gelatin, polyvinyl alcohol, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and hydroxypropyl methyl cellulose. The amount of the hydrophilic polymer added is preferably less than 30% by weight of the entire binder in the image-forming layer. The image-forming layer is preferably formed by applying an aqueous coating solution followed by drying. By the term "aqueous", it is meant that water accounts for at least 30% by weight of the solvent or dispersing medium of the coating solution. The component other than water of the coating solution may be a water-miscible organic solvent such as methyl alcohol, ethyl alcohol, isopropyl alcohol, methyl cellosolve, ethyl cellosolve, dimethylformamide or ethyl acetate. Beside water, exemplary solvent compositions include a 90/10 mixture of water/methanol, a 70/30 mixture of water/methanol, a 90/10 mixture of water/ethanol, a 90/10 mixture of water/isopropanol, a 95/5 mixture of water/dimethylformamide, a 80/15/5 mixture of water/methanol/dimethylformamide, and a 90/5/5 mixture of water/methanol/dimethylformamide, all expressed in a weight ratio. The method described in USP 5,496,695 is also useful. In the image-forming layer, the total amount of binder is preferably 0.2 to 30 g/m 2 , more preferably 1 to 15 g/m 2 . To the image forming layer, crosslinking agents for crosslinking, surfactants for ease of application, and other addenda may be added. Sensitizing dye A sensitizing dye may be used in the practice of the invention. There may be used any of sensitizing dyes which can spectrally sensitize silver halide grains in a desired wavelength region when adsorbed to the silver halide grains. The sensitizing dyes used herein include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, styryl dyes, hemicyanine dyes, oxonol dyes, and hemioxonol dyes. Useful sensitizing dyes which can be used herein are described in Research Disclosure, Item 17643 IV-A (December 1978, page 23), ibid., Item 1831 X (August 1979, page 437) and the references cited therein. It is advantageous to select a sensitizing dye having appropriate spectral sensitivity to the spectral properties of a particular light source of various laser imagers, scanners, image setters and process cameras. Exemplary dyes for spectral sensitization to red light include compounds I-1 to I-38 described in JP-A 18726/1979, compounds I-1 to I-35 described in JP-A 75322/1994, compounds I-1 to I-34 described in JP-A 287338/1995, dyes 1 to 20 described in JP-B 39818/1980, compounds I-1 to I-37 described in JP-A 284343/1987, and compounds I-1 to I-34 described in JP-A 287338/1995 for red light sources such as He-Ne lasers, red semiconductor lasers and LED. For semiconductor laser light sources in the wavelength range of 750 to 1,400 nm, spectral sensitization may be advantageously done with various known dyes including cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol, and xanthene dyes. Useful cyanine dyes are cyanine dyes having a basic nucleus such as a thiazoline, oxazoline, pyrroline, pyridine, oxazole, thiazole, selenazole and imidazole nucleus. Preferred examples of the useful merocyanine dye contain an acidic nucleus such as a thiohydantoin, rhodanine, oxazolidinedione, thiazolinedione, barbituric acid, thiazolinone, malononitrile, or pyrazolone nucleus in addition to the above-mentioned basic nucleus. Among the above-mentioned cyanine and merocyanine dyes, those having an imino or carboxyl group are especially effective. A suitable choice may be made of well-known dyes as described, for example, in USP 3,761,279, 3,719,495, and 3,877,943, BP 1,466,201, 1,469,117, and 1,422,057, JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. Especially preferred dye structures are cyanine dyes having a thioether bond-containing substituent group, examples of which are the cyanine dyes described in JP-A 58239/1987, 138638/1991, 138642/1991, 255840/1992, 72659/1993, 72661/1993, 222491/1994, 230506/1990, 258757/1994, 317868/1994, and 324425/1994, Publication of International Patent Application No. 500926/1995, and USP 5,541,054; dyes having a carboxylic group, examples of which are the dyes described in JP-A 163440/1991, 301141/1994 and USP 5,441,899; and merocyanine dyes, polynuclear merocyanine dyes, and polynuclear cyanine dyes, examples of which are the dyes described in JP-A 6329/1972, 105524/1974, 127719/1976, 80829/1977, 61517/1979, 214846/1984, 6750/1985, 159841/1988, 35109/1994, 59381/1994, 146537/1995, Publication of International Patent Application No. 50111/1993, BP 1,467,638, and USP 5,281,515. Also useful in the practice of the invention are dyes capable of forming the J-band as disclosed in USP 5,510,236, 3,871,887 (Example 5), JP-A 96131/1990 and 48753/1984. These sensitizing dyes may be used alone or in admixture of two or more. A combination of sensitizing dyes is often used for the purpose of supersensitization. In addition to the sensitizing dye, the emulsion may contain a dye which itself has no spectral sensitization function or a compound which does not substantially absorb visible light, but is capable of supersensitization. Useful sensitizing dyes, combinations of dyes showing supersensitization, and compounds showing supersensitization are described in Research Disclosure, Vol. 176, 17643 (December 1978), page 23, IV J and JP-B 25500/1974 and 4933/1968, JP-A 19032/1984 and 192242/1984. The sensitizing dye may be added to a silver halide emulsion by directly dispersing the dye in the emulsion or by dissolving the dye in a solvent and adding the solution to the emulsion. The solvent used herein includes water, methanol, ethanol, propanol, acetone, methyl cellosolve, 2,2,3,3-tetrafluoropropanol, 2,2,2-trifluoroethanol, 3-methoxy-1-propanol, 3-methoxy-1-butanol, 1-methoxy-2-propanol, N,N-dimethylformamide and mixtures thereof. Also useful are a method of dissolving a dye in a volatile organic solvent, dispersing the solution in water or hydrophilic colloid and adding the dispersion to an emulsion as disclosed in USP 3,469,987, a method of dissolving a dye in an acid and adding the solution to an emulsion or forming an aqueous solution of a dye with the aid of an acid or base and adding it to an emulsion as disclosed in JP-B 23389/1969, 27555/1969 and 22091/1982, a method of forming an aqueous solution or colloidal dispersion of a dye with the aid of a surfactant and adding it to an emulsion as disclosed in USP 3,822,135 and 4,006,025, a method of directly dispersing a dye in hydrophilic colloid and adding the dispersion to an emulsion as disclosed in JP-A 102733/1978 and 105141/1983, and a method of dissolving a dye using a compound capable of red shift and adding the solution to an emulsion as disclosed in JP-A 74624/1976. It is also acceptable to apply ultrasonic waves to form a solution. The time when the sensitizing dye is added to the silver halide emulsion according to the invention is at any step of an emulsion preparing process which has been ascertained effective. The sensitizing dye may be added to the emulsion at any stage or step before the emulsion is coated, for example, at a stage prior to the silver halide grain forming step and/or desalting step, during the desalting step and/or a stage from desalting to the start of chemical ripening as disclosed in USP 2,735,766, 3,628,960, 4,183,756, and 4,225,666, JP-A 184142/1983 and 196749/1985, and a stage immediately before or during chemical ripening and a stage from chemical ripening to emulsion coating as disclosed in JP-A 113920/1983. Also as disclosed in USP 4,225,666 and JP-A 7629/1983, an identical compound may be added alone or in combination with a compound of different structure in divided portions, for example, in divided portions during a grain forming step and during a chemical ripening step or after the completion of chemical ripening, or before or during chemical ripening and after the completion thereof. The type of compound or the combination of compounds to be added in divided portions may be changed. The amount of the sensitizing dye used may be an appropriate amount complying with sensitivity and fog although the preferred amount is 10 -6 to 1 mol, more preferably 10 -4 to 10 -1 mol per mol of the silver halide in the image forming layer. Antifoggant With antifoggants, stabilizers and stabilizer precursors, the silver halide emulsion and/or organic silver salt according to the invention can be further protected against formation of additional fog and stabilized against lowering of sensitivity during shelf storage. Suitable antifoggants, stabilizers and stabilizer precursors which can be used alone or in combination include thiazonium salts as described in USP 2,131,038 and 2,694,716, azaindenes as described in USP 2,886,437 and 2,444,605, mercury salts as described in USP 2,728,663, urazoles as described in USP 3,287,135, sulfocatechols as described in USP 3,235,652, oximes, nitrons and nitroindazoles as described in BP 623,448, polyvalent metal salts as described in USP 2,839,405, thiuronium salts as described in USP 3,220,839, palladium, platinum and gold salts as described in USP 2,566,263 and 2,597,915, halogen-substituted organic compounds as described in USP 4,108,665 and 4,442,202, triazines as described in USP 4,128,557, 4,137,079, 4,138,365 and 4,459,350, and phosphorus compounds as described in USP 4,411,985. Preferred antifoggants are organic halides, for example, the compounds described in JP-A 119624/1975, 120328/1975, 121332/1976, 58022/1979, 70543/1981, 99335/1981, 90842/1984, 129642/1986, 129845/1987, 208191/1994, 5621/1995, 2781/1995, 15809/1996, USP 5,340,712, 5,369,000, and 5,464,737. The antifoggant may be added in any desired form such as solution, powder or solid particle dispersion. The solid particle dispersion of the antifoggant may be prepared by well-known comminuting means such as ball mills, vibrating ball mills, sand mills, colloidal mills, jet mills, and roller mills. Dispersing aids may be used for facilitating dispersion. It is sometimes advantageous to add a mercury (II) salt to an emulsion layer as an antifoggant though not necessary in the practice of the invention. Mercury (II) salts preferred to this end are mercury acetate and mercury bromide. The mercury (II) salt is preferably added in an amount of 1x10 -9 mol to 1x10 -3 mol, more preferably 1x10 -3 mol to 1x10 -4 mol per mol of silver coated. Still further, the thermographic recording element of the invention may contain a benzoic acid type compound for the purposes of increasing sensitivity and restraining fog. Any of benzoic acid type compounds may be used although examples of the preferred structure are described in USP 4,784,939 and 4,152,160, Japanese Patent Application Nos. 98051/1996, 151241/1996, and 151242/1996. The benzoic acid type compound may be added to any site in the recording element, preferably to a layer on the same side as the photosensitive layer serving as the image forming layer, and more preferably an organic silver salt-containing layer. The benzoic acid type compound may be added at any step in the preparation of a coating solution. Where it is contained in an organic silver salt-containing layer, it may be added at any step from the preparation of the organic silver salt to the preparation of a coating solution, preferably after the preparation of the organic silver salt and immediately before coating. The benzoic acid type compound may be added in any desired form including powder, solution and fine particle dispersion. Alternatively, it may be added in a solution form after mixing it with other additives such as a sensitizing dye, reducing agent and toner. The benzoic acid type compound may be added in any desired amount, preferably 1x10 -6 mol to 2 mol, more preferably 1x10 -3 mol to 0.5 mol per mol of silver. In the recording element of the invention, mercapto, disulfide and thion compounds may be added for the purposes of retarding or accelerating development to control development, improving spectral sensitization efficiency, and improving storage stability before and after development. Where mercapto compounds are used herein, any structure is acceptable. Preferred are structures represented by Ar-S-M and Ar-S-S-Ar wherein M is a hydrogen atom or alkali metal atom, and Ar is an aromatic ring or fused aromatic ring having at least one nitrogen, sulfur, oxygen, selenium or tellurium atom. Preferred hetero-aromatic rings are benzimidazole, naphthimidazole, benzothiazole, naphthothiazole, benzoxazole, naphthoxazole, benzoselenazole, benzotellurazole, imidazole, oxazole, pyrrazole, triazole, thiadiazole, tetrazole, triazine, pyrimidine, pyridazine, pyrazine, pyridine, purine, quinoline and quinazolinone rings. These hetero-aromatic rings may have a substituent selected from the group consisting of halogen (e.g., Br and Cl), hydroxy, amino, carboxy, alkyl groups (having at least 1 carbon atom, preferably 1 to 4 carbon atoms), and alkoxy groups (having at least 1 carbon atom, preferably 1 to 4 carbon atoms). Illustrative, non-limiting examples of the mercapto-substituted hetero-aromatic compound include 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, 2-mercapto-5-methylbenzimidazole, 6-ethoxy-2-mercaptobenzothiazole, 2/2'-dithiobis(benzothiazole), 3-mercapto-1,2,4-triazole, 4,5-diphenyl-2-imidazolethiol, 2-mercaptoimidazole, 1-ethyl-2-mercaptobenzimidazole, 2-mercaptoquinoline, 8-mercaptopurine, 2-mercapto-4 (3H)-quinazolinone, 7-trifluoromethyl-4-quinolinethiol, 2,3,5,6-tetrachloro-4-pyridinethiol, 4-amino-6-hydroxy-2-mercaptopyrimidine monohydrate, 2-amino-5-mercapto-1,3,4-thiadiazole, 3-amino-5-mercapto-1,2,4-triazole, 4-hydroxy-2-mercaptopyrimidine, 2-mercaptopyrimidine, 4,6-diamino-2-mercaptopyrimidine, 2-mercapto-4-methylpyrimidine hydrochloride, 3-mercapto-5-phenyl-1,2,4-triazole, and 2-mercapto-4-phenyloxazole. These mercapto compounds are preferably added to the emulsion layer (serving as an image forming layer) in amounts of 0.001 to 1.0 mol, more preferably 0.01 to 0.3 mol per mol of silver. In the thermographic recording element of the invention, a nucleation promoter may be added for promoting the action of the nucleating agent. The nucleation promoter used herein includes amine derivatives, onium salts, disulfide derivatives, hydroxymethyl derivatives, hydroxamic acid derivatives, acylhydrazide derivatives, acrylonitrile derivatives and hydrogen donors. Examples of the nucleation promoter include the compounds described in JP-A 77783/1995, page 48, lines 2-37, more specifically Compounds A-1 to A-73 described on pages 49-58 of the same; the compounds of the chemical formulae [21], [22] and [23] described in JP-A 84331/1995, more specifically the compounds described on pages 6-8 of the same; the compounds of the general formulae [Na] and [Nb] described in JP-A 104426/1995, more specifically Compounds Na-1 to Na-22 and Nb-1 to Nb-12 described on pages 16-20 of the same; the compounds of the general formulae (1), (2), (3), (4), (5), (6) and (7) described in Japanese Patent Application No. 37817/1995, more specifically Compounds 1-1 to 1-19, Compounds 2-1 to 2-22, Compounds 3-1 to 3-36, Compounds 4-1 to 4-5, Compounds 5-1 to 5-41, Compounds 6-1 to 6-58 and Compounds 7-1 to 7-38 described therein; and the nucleation promoters described in Japanese Patent Application No. 70908/1996. In the practice of the invention, the nucleation promoter is used as solution in water or a suitable organic solvent. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethylsulfoxide and methyl cellosolve. A well-known emulsifying dispersion method is used for dissolving the nucleation promoter with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the nucleation promoter in powder form in water in a ball mill, colloidal mill or ultrasonic mixer. The nucleation promoter may be added to an image forming layer or any other binder layer on the image forming layer side of a support, and preferably to the image forming layer or a binder layer disposed adjacent thereto. The nucleation promoter is preferably used in an amount of 1x10 -6 mol to 2x10 -1 mol, more preferably 1x10 -5 mol to 2x10 -2 mol, most preferably 2x10 -5 to 1x10 -2 mol per mol of silver. In the image forming layer, polyhydric alcohols (e.g., glycerin and diols as described in USP 2,960,404), fatty acids and esters thereof as described in USP 2,588,765 and 3,121,060, and silicone resins as described in BP 955,061 may be added as a plasticizer and lubricant. Protective layer A surface protective layer may be provided in the thermographic recording element according to the present invention for the purpose of preventing sticking of the image forming layer. The surface protective layer is based on a binder which may be any desired polymer, although the layer preferably contains 100 mg/m 2 to 5 g/m 2 of a polymer having a carboxylic acid residue. The polymers having a carboxylic acid residue include natural polymers (e.g., gelatin and alginic acid), modified natural polymers (e.g., carboxymethyl cellulose and phthalated gelatin), and synthetic polymers (e.g., polymethacrylate, polyacrylate, polyalkyl methacrylate/acrylate copolymers, and polystyrene/polymethacrylate copolymers). The content of the carboxylic acid residue is preferably 10 mmol to 1.4 mol per 100 grams of the polymer. The carboxylic acid residue may form a salt with an alkali metal ion, alkaline earth metal ion or organic cation. In the surface protective layer, any desired antisticking material may be used. Examples of the antisticking material include wax, silica particles, styrene-containing elastomeric block copolymers (e.g., styrene-butadiene-styrene and styrene-isoprene-styrene), cellulose acetate, cellulose acetate butyrate, cellulose propionate and mixtures thereof. Crosslinking agents for crosslinking, surfactants for ease of application, and other addenda are optionally added to the surface protective layer. In the image forming layer or a protective layer therefor according to the invention, there may be used light absorbing substances and filter dyes as described in USP 3,253,921, 2,274,782, 2,527,583, and 2,956,879. The dyes may be mordanted as described in USP 3,282,699. The filter dyes are used in such amounts that the layer may have an absorbance of 0.1 to 3, especially 0.2 to 1.5 at the exposure wavelength. In the image forming layer or a protective layer therefor according to the invention, there may be used matte agents, for example, starch, titanium dioxide, zinc oxide, and silica as well as polymer beads including beads of the type described in USP 2,992,101 and 2,701,245. The emulsion layer side surface may have any degree of matte insofar as no star dust failures occur although a Bekk smoothness of 200 to 10,000 seconds, especially 300 to 10,000 seconds is preferred. The thermographic photographic emulsion used in the thermographic recording element according to the one preferred embodiment of the invention is contained in one or more layers on a support. In the event of single layer construction, it should contain an organic silver salt, silver halide, developing agent, and binder, and other optional additives such as a toner, coating aid and other auxiliary agents. In the event of two-layer construction, a first emulsion layer which is generally a layer disposed adjacent to the support should contain an organic silver salt and silver halide and a second emulsion layer or both the layers contain other components. Also envisioned herein is a two-layer construction consisting of a single emulsion layer containing all the components and a protective topcoat. In the case of multi-color sensitive photothermographic material, a combination of such two layers may be employed for each color. Also a single layer may contain all necessary components as described in USP 4,708,928. In the case of multi-dye, multi-color sensitive photothermographic material, emulsion (or photosensitive) layers are distinctly supported by providing a functional or non-functional barrier layer therebetween as described in USP 4,460,681. In the image forming layer, a variety of dyes and pigments may be used from the standpoints of improving tone and preventing irradiation. Any desired dyes and pigments may be used in the invention. Useful pigments and dyes include those described in Colour Index and both organic and inorganic, for example, pyrazoloazole dyes, anthraquinone dyes, azo dyes, azomethine dyes, oxonol dyes, carbocyanine dyes, styryl dyes, triphenylmethane dyes, indoaniline dyes, indophenol dyes, and phthalocyanine dyes. The preferred dyes used herein include anthraquinone dyes (e.g., Compounds 1 to 9 described in JP-A 341441/1993 and Compounds 3-6 to 3-18 and 3-23 to 3-38 described in JP-A 165147/1993), azomethine dyes (e.g., Compounds 17 to 46 described in JP-A 341441/1993), indoaniline dyes (e.g., Compounds 11 to 19 described in JP-A 289227/1993, Compound 47 described in JP-A 341441/1993 and Compounds 2-10 to 2-11 described in JP-A 165147/1993), and azo dyes (e.g., Compounds 10 to 16 described in JP-A 341441/1993). The dyes and pigments may be added in any desired form such as solution, emulsion or solid particle dispersion or in a form mordanted with polymeric mordants. The amounts of these compounds used are determined in accordance with the desired absorption although the compounds are generally used in amounts of 1 µg to 1 g per square meter of the recording element. In the practice of the invention, an antihalation layer may be disposed on the side of the image forming layer remote from the light source. The antihalation layer preferably has a maximum absorbance of 0.1 to 2 in the desired wavelength range, more preferably an absorbance of 0.2 to 1.5 at the exposure wavelength, and an absorbance of 0.001 to less than 0.2 in the visible region after processing, and is also preferably a layer having an optical density of 0.001 to less than 0.15. Where an antihalation dye is used in the invention, it may be selected from various compounds insofar as it has the desired absorption in the wavelength range, is sufficiently low absorptive in the visible region after processing, and provides the antihalation layer with the preferred absorbance profile. Exemplary antihalation dyes are given below though the dyes are not limited thereto. Useful dyes which are used alone are described in JP-A 56458/1984, 216140/1990, 13295/1995, 11432/1995, USP 5,380,635, JP-A 68539/1990, page 13, lower-left column, line 1 to page 14, lower-left column, line 9, and JP-A 24539/1991, page 14, lower-left column to page 16, lower-right column. It is further preferable in the practice of the invention to use a dye which will decolorize during processing. Illustrative, non-limiting, examples of decolorizable dyes are disclosed in JP-A 139136/1977, 132334/1978, 501480/1981, 16060/1982, 68831/1982, 101835/1982, 182436/1984, 36145/1995, 199409/1995, JP-B 33692/1973, 16648/1975, 41734/1990, USP 4,088,497, 4,283,487, 4,548,896, and 5,187,049. In one preferred embodiment, the thermographic recording element of the invention is a one-side recording element having at least one image forming layer on one side and a back layer on the other side of the support. In the practice of the invention, a matte agent may be added to the recording element for improving feed efficiency. The matte agents used herein are generally microparticulate water-insoluble organic or inorganic compounds. There may be used any desired one of matte agents, for example, well-known matte agents including organic matte agents as described in USP 1,939,213, 2,701,245, 2,322,037, 3,262,782, 3,539,344, and 3,767,448 and inorganic matte agents as described in USP 1,260,772, 2,192,241, 3,257,206, 3,370,951, 3,523,022, and 3,769,020. Illustrative examples of the organic compound which can be used as the matte agent are given below; exemplary water-dispersible vinyl polymers include polymethyl acrylate, polymethyl methacrylate, polyacrylonitrile, acrylonitrile-α-methylstyrene copolymers, polystyrene, styrene-divinyl-benzene copolymers, polyvinyl acetate, polyethylene carbonate, and polytetrafluoroethylene; exemplary cellulose derivatives include methyl cellulose, cellulose acetate, and cellulose acetate propionate; exemplary starch derivatives include carboxystarch, carboxynitrophenyl starch, urea-formaldehyde-starch reaction products, gelatin hardened with well-known curing agents, and hardened gelatin which has been coaceruvation hardened into microcapsulated hollow particles. Preferred examples of the inorganic compound which can be used as the matte agent include silicon dioxide, titanium dioxide, magnesium dioxide, aluminum oxide, barium sulfate, calcium carbonate, silver chloride and silver bromide desensitized by a well-known method, glass, and diatomaceous earth. The aforementioned matte agents may be used as a mixture of substances of different types if necessary. The size and shape of the matte agent are not critical. The matte agent of any particle size may be used although matte agents having a particle size of 0.1 µm to 30 µm are preferably used in the practice of the invention. The particle size distribution of the matte agent may be either narrow or wide. Nevertheless, since the haze and surface luster of coating are largely affected by the matte agent, it is preferred to adjust the particle size, shape and particle size distribution of a matte agent as desired during preparation of the matte agent or by mixing plural matte agents. In the practice of the invention, the back layer should preferably have a degree of matte as expressed by a Bekk smoothness of 10 to 1,200 seconds, more preferably 50 to 700 seconds. In the recording element of the invention, the matte agent is preferably contained in an outermost surface layer, a layer functioning as an outermost surface layer, a layer close to the outer surface or a layer functioning as a so-called protective layer. In the practice of the invention, the binder used in the back layer is preferably transparent or translucent and generally colorless. Exemplary binders are naturally occurring polymers, synthetic resins, polymers and copolymers, and other film-forming media, for example, gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetate butyrate, poly(vinyl pyrrolidone), casein, starch, poly(acrylic acid), poly(methyl methacrylate), polyvinyl chloride, poly-(methacrylic acid), copoly(styrene-maleic anhydride), copoly(styrene-acrylonitrile), copoly(styrene-butadiene), polyvinyl acetals (e.g., polyvinyl formal and polyvinyl butyral), polyesters, polyurethanes, phenoxy resins, poly(vinylidene chloride), polyepoxides, polycarbonates, poly(vinyl acetate), cellulose esters, and polyamides. The binder may be dispersed in water, organic solvent or emulsion to form a dispersion which is coated to form a layer. The back layer preferably exhibits a maximum absorbance of 0.3 to 2, more preferably 0.5 to 2 in the predetermined wavelength range and an absorbance of 0.001 to less than 0.5 in the visible range after processing. Further preferably, the back layer has an optical density of 0.001 to less than 0.3. Examples of the antihalation dye used in the back layer are the same as previously described for the antihalation layer. A backside resistive heating layer as described in USP 4,460,681 and 4,374,921 may be used in a photographic thermographic image recording system according to the present invention. According to the invention, a hardener may be used in various layers including an image forming layer, protective layer, and back layer. Examples of the hardener include polyisocyanates as described in USP 4,281,060 and JP-A 208193/1994, epoxy compounds as described in USP 4,791,042, and vinyl sulfones as described in JP-A 89048/1987. A surfactant may be used for the purposes of improving coating and electric charging properties. The surfactants used herein may be nonionic, anionic, cationic and fluorinated ones. Examples include fluorinated polymer surfactants as described in JP-A 170950/1987 and USP 5,380,644, fluorochemical surfactants as described in JP-A 244945/1985 and 188135/1988, polysiloxane surfactants as described in USP 3,885,965, and polyalkylene oxide and anionic surfactants as described in JP-A 301140/1994. Examples of the solvent used herein are described in "New Solvent Pocket Book," Ohm K.K., 1994, though not limited thereto. The solvent used herein should preferably have a boiling point of 40 to 180°C. Exemplary solvents include hexane, cyclohexane, toluene, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, ethyl acetate, 1,1,1-trichloroethane, tetrahydrofuran, triethylamine, thiophene, trifluoroethanol, perfluoropentane, xylene, n-butanol, phenol, methyl isobutyl ketone, cyclohexanone, butyl acetate, diethyl carbonate, chlorobenzene, dibutyl ether, anisole, ethylene glycol diethyl ether, N,N-dimethylformamide, morpholine, propanesultone, perfluorotributylamine, and water. Support According to the invention, the thermographic emulsion may be coated on a variety of supports. Typical supports include polyester film, subbed polyester film, poly(ethylene terephthalate) film, polyethylene naphthalate film, cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polycarbonate film and related or resinous materials, as well as glass, paper, metals. Often used are flexible substrates, typically paper supports, specifically baryta paper and paper supports coated with partially acetylated α-olefin polymers, especially polymers of α-olefins having 2 to 10 carbon atoms such as polyethylene, polypropylene, and ethylene-butene copolymers. The supports are either transparent or opaque, preferably transparent. The thermographic recording element of the invention may have an antistatic or electroconductive layer, for example, a layer containing soluble salts (e.g., chlorides and nitrates), an evaporated metal layer, or a layer containing ionic polymers as described in USP 2,861,056 and 3,206,312 or insoluble inorganic salts as described in US? 3,428,451. A method for producing color images using the thermographic recording element of the invention is as described in JP-A 13295/1995, page 10, left column, line 43 to page 11, left column, line 40. Stabilizers for color dye images are exemplified in BP 1,326,889, USP 3,432,300, 3,698,909, 3,574,627, 3,573,050, 3,764,337, and 4,042,394. In the practice of the invention, the thermographic photographic emulsion can be applied by various coating procedures including dip coating, air knife coating, flow coating, and extrusion coating using a hopper of the type described in USP 2,681,294. If desired, two or more layers may be concurrently coated by the methods described in USP 2,761,791 and BP 837,095. In the thermographic recording element of the invention, there may be contained additional layers, for example, a dye accepting layer for accepting a mobile dye image, an opacifying layer when reflection printing is desired, a protective topcoat layer, and a primer layer well known in the photothermographic art. The recording element of the invention is preferably such that only a single sheet of the recording element can form an image. That is, it is preferred that a functional layer necessary to form an image such as an image receiving layer does not constitute a separate member. The thermographic recording element of the invention may be developed by any desired method although it is generally developed by heating after imagewise exposure. The preferred developing temperature is 80 to 250°C, more preferably 100 to 140°C. The preferred developing time is 1 to 180 seconds, more preferably 10 to 90 seconds. Any desired technique may be used for the exposure of the thermographic recording element of the invention. The preferred light source for exposure is a laser, for example, a gas laser, YAG laser, dye laser or semiconductor laser. A semiconductor laser combined with a second harmonic generating device is also useful. Where the thermographic recording element of the invention does not contain the photosensitive silver halide, latent images can be formed by heating. Heating may be effected by various ways, for example, by direct heating using a thermal head. Indirect heating is also possible if a substance (e.g., a dyestuff or pigment) capable of absorbing radiation of a specific wavelength and converting it into heat is incorporated in the recording element. The light source used in this embodiment is preferably a laser as mentioned above. A combination of these techniques is possible. Where a latent image is formed by heating, the process may involve two stages, a first stage of heating to form a latent image and a second stage of heating to form an image. A single stage of heating can complete image formation. Developing apparatus Referring to FIG. 1, there is schematically illustrated one exemplary heat developing apparatus for use in the processing of the photothermographic recording element according to the invention. FIG. 1 is a side elevation of the heat developing apparatus which includes a cylindrical heat drum 2 having a halogen lamp 1 received therein as a heating means, and an endless belt 4 trained around a plurality of feed rollers 3 so that a portion of the belt 4 is in close contact with the drum 2. A length of photothermographic recording element 5 is fed and guided by pairs of guide rollers to between the heat drum 2 and the belt 4. The element 5 is fed forward while it is clamped between the heat drum 2 and the belt 4. While the element 5 is fed forward, it is heated to the developing temperature whereby it is heat developed. In the heat developing appratus of the drum type, the luminous intensity distribution of the lamp is optimized so that the temperature in the transverse direction may be controlled to a variation within ±1°C, for example. The element 5 exits at an exit 6 from between the heat drum 2 and the belt 4 where the element is released from bending by the circumferential surface of the heat drum 2. A correcting guide plate 7 is disposed in the vicinity of the exit 6 for correcting the element 5 into a planar shape. A zone surrounding the guide plate 7 is temperature adjusted so that the temperature of the element 5 may not lower below 90°C. Disposed downstream of the exit 6 are a pair of feed rollers 8. A pair of planar guide plates 9 are disposed downstream of and adjacent to the feed rollers 8 for guiding the element 5 while keeping it planar. Another pair of feed rollers 10 are disposed downstream of and adjacent to the guide plates 9. The planar guide plates 9 have such a length that the element 5 is fully cooled, typically below 30°C, while it passes over the plates 9. The means associated with the guide plates 9 for cooling the element 5 are cooling fans 11. Although the belt conveyor type heat developing apparatus has been described, the practice of the invention is not limited thereto. Use may be made of heat developing apparatus of varying constructions such as disclosed in JP-A 13294/1995. In the case of a multi-stage heating mode which is preferably used in the practice of the invention, two or more heat sources having different heating temperatures are disposed in the illustrated apparatus so that the element may be continuously heated to different temperatures. EXAMPLE Examples of the invention are given below by way of illustration and not by way of limitation. Tg is glass transition temperature and MFT is minimum film-forming temperature. The trade names used in Examples have the following meaning. Denka Butyral: polyvinyl butyral by Denki Kagaku Kogyo K.K. CAB 171-15S: cellulose acetate butyrate by Eastman Chemical Products, Inc. Sildex: spherical silica by Dokai Chemical K.K. Sumidur N3500: polyisocyanate by Sumitomo-Bayer Urethane K.K. Megaface F-176P: fluorochemical surfactant by Dainippon Ink Chemicals K.K. LACSTAR 3307B: styrene-butadiene rubber (SBR) latex by Dainippon Ink & Chemicals K.K. The polymer has an equilibrium moisture content of 0.6 wt% at 25°C and RH 60%, Tg = 17°C, and MET = 25°C. The dispersed particles have a mean particle diameter of about 0.1 to 0.15 µm. MP-203: polyvinyl alcohol by Kurare K.K. PVA-217: polyvinyl alcohol by Kurare K.K. The compounds used in Examples have the following structural formulae. (corresponding to CN-03 in USP 5,545,515) (corresponding to HET-01 in USP 5,635,339) Example 1 Preparation of silver halide grains A In 900 ml of water were dissolved 7.5 g of inert gelatin and 10 mg of potassium bromide. The solution was adjusted to pH 3.0 at a temperature of 35°C. To the solution, 370 ml of an aqueous solution containing 74 g of silver nitrate and an aqueous solution containing potassium bromide and potassium iodide in a molar ratio of 94:6 and K 3 [IrCl 6 ] were added over 10 minutes by the controlled double jet method while maintaining the solution at pAg 7.7. Note that [IrCl 6 ] -3 was added in an amount of 3x10 -7 mol/mol of silver. Thereafter, 0.3 g of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene was added to the solution, which was adjusted to pH 5 with NaOH. There were obtained cubic silver iodobromide grains A having a mean grain size of 0.06 µm, a coefficient of variation of projected area of 8%, and a {100} face ratio of 87%. The emulsion was desalted by adding a gelatin flocculant thereto to cause flocculation and sedimentation and then adjusted to pH 5.9 and pAg 7.5 by adding 0.1 g of phenoxyethanol. Preparation of organic acid silver emulsion A A mixture of 10.6 g of behenic acid and 300 ml of distilled water was mixed for 15 minutes at 90°C. With vigorous stirring, 31.1 ml of 1N sodium hydroxide was added over 15 minutes to the solution, which was allowed to stand at the temperature for one hour. The solution was then cooled to 30°C, 7 ml of 1N phosphoric acid was added thereto, and with more vigorous stirring, 0.13 g of N-bromosuccinimide (C-2) was added. Thereafter, with stirring, the above-prepared silver halide grains A were added to the solution in such an amount as to give 2.5 mmol of silver halide. Further, 25 ml of 1N silver nitrate aqueous solution was continuously added over 2 minutes, with stirring continued for a further 90 minutes. With stirring, 37 g of a 1.2 wt% butyl acetate solution of polyvinyl acetate was slowly added to the aqueous mixture to form flocs in the dispersion. Water was removed, and water washing and water removal were repeated twice. With stirring, 20 g of a solution of 2.5% by weight polyvinyl butyral (Denka Butyral #3000-K) in a 1/2 solvent mixture of butyl acetate and isopropyl alcohol was added. To the thus obtained gel-like mixture of organic acid silver and silver halide, 7.8 g of polyvinyl butyral (Denka Butyral #4000-2) and 57 g of 2-butanone were added. The mixture was dispersed by a homogenizer, obtaining a silver behenate salt emulsion A of needle grains having a mean minor diameter of 0.04 µm, a mean major diameter of 1 µm and a coefficient of variation of 30%. Preparation of emulsion layer coating solution A The following chemicals were added to the above-prepared organic acid silver salt emulsion A in amounts per mol of silver. With stirring at 25°C, 10 mg of sodium phenylthiosulfonate, 25 mg of Sensitizing Dye A, 20 mg of Sensitizing Dye B, 18 mg of Sensitizing Dye C, 2 g of 2-mercapto-5-methylbenzimidazole (C-1), 21.5 g of 4-chlorobenzophenone-2-carboxylic acid (C-3), 580 g of 2-butanone and 220 g of dimethylformamide were added to the emulsion, which was allowed to stand for 3 hours. With stirring, 4 g of 4,6-ditrichloromethyl-2-phenyltriazine (C-4), 2 g of Disulfide Compound A, 170 g of 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (C-5), 15 g of phthalazine (C-6), 5 g of tetrachlorophthalic acid (C-7), 1.1 g of fluorochemical surfactant Megaface F-176P, 590 g of 2-butanone, and 10 g of methyl isobutyl ketone were added to the emulsion. Further with stirring, the nucleating agent shown Table 9 was added in the amount shown in Table 9. Preparation of emulsion surface protective layer coating solution A A coating solution A for an emulsion layer surface protective layer was prepared by dissolving 75 g of CAB 171-15S, 5.7 g of 4-methylphthalic acid (C-8), 1.5 g of tetrachlorophthalic anhydride (C-9), 8 g of tribromomethylsulfonylbenzene (C-12), 6 g of 2-tribromomethylsulfonylbenzothiazole (C-10), 3 g of phthalazone (C-11), 0.3 g of fluorochemical surfactant Megaface F-176P, 2 g of spherical silica Sildex H31 (mean size 3 µm), and 6 g of polyisocyanate Sumidur N3500 in 3070 g of 2-butanone and 30 g of ethyl acetate. Preparation of coated sample A back layer coating solution was prepared by adding 6 g of polyvinyl butyral Denka Butyral #4000-2, 0.2 g of spherical silica Sildex H121 (mean size 12 µm), 0.2 g of spherical silica Sildex H51 (mean size 5 µm), and 0.1 g of Megaface F-176P to 64 g of 2-propanol and mixing them into a solution. Further, a mixed solution of 210 mg of Dye A and 210 mg of Dye B in 10 g of methanol and 20 g of acetone and a solution of 0.8 g of 3-isocyanatomethyl-3,5,5-trimethylhexyl isocyanate in 6 g of ethyl acetate were added to the solution. A polyethylene terephthalate film having a moistureproof undercoat of vinylidene chloride on either surface was coated on one surface with the back surface coating solution so as to give an optical density of 0.7 at 780 nm. On the thus prepared support, the emulsion layer coating solution was coated so as to give a coverage of 2 g/m 2 of silver and the emulsion surface protective layer coating solution was then coated on the emulsion layer so as to give a dry thickness of 5 µm. In this way, samples of thermographic recording element were prepared. Exposure and Development The samples prepared above were exposed to xenon flash light for an emission time of 10 -4 sec through an interference filter having a peak at 780 nm and a step wedge and heated for development at 115°C for 25 seconds. The resulting images were determined for density by a densitometer, from which a characteristic curve was obtained. Contrast The gradient of a straight line connecting points of density 0.3 and 3.0 on the characteristic curve is reported as gradation (γ). Gamma values of 10 and more are satisfactory. Dependency on developing temperature It was determined how the sensitivity (S) of a sample changed with developing temperature. The standard developing conditions were set at 115°C and 25 seconds. A change ΔS of sensitivity with a change of the developing temperature ±2°C was determined. ΔS = S(117°C/25 s) - S(113°C/25 s) The sensitivity (S) was expressed by a logarithmic value of an exposure providing a density of 1.5. Values of ΔS closer to 0 indicate stability to developing conditions. Values of ΔS of 0 to -0.1 are practically acceptable, with values of 0 to -0.05 being preferred. Fog After a sample was developed by heating at 117°C for 30 seconds, a fog was measured as the difference of a density of an unexposed area from the base line. The difference from that under the standard developing conditions (115°C, 25 seconds) was obtained. ΔFog = Fog(117°C/30 s) - Fog(115°C/25 s) For practical use, ΔFog values of 0.05 or less are necessary, with values of 0.03 or less being preferable. The results are shown in Table 9. Sample No. Nucleating agent Photographic properties Remarks No. Amount (mol/m 2 ) γ ΔS ΔFog 1-1 -- -- 5.8 -0.03 0.01 comparison 1-2 RF-1 0.3×10 -4 8.2 -0.08 0.09 comparison 1-3 RF-1 1.0×10 -4 12.7 -0.28 0.54 comparison 1-4 RF-2 0.3×10 -4 8.5 -0.09 0.08 comparison 1-5 RF-2 1.0×10 -4 12.9 -0.32 0.68 comparison 1-6 1 0.3×10 -4 13.0 -0.02 0.00 invention 1-7 4 0.3×10 -4 12.6 -0.03 0.01 invention 1-8 22 0.3×10 -4 13.1 -0.02 0.01 invention 1-9 34 0.3×10 -4 12.1 -0.02 0.01 invention 1-10 40 0.3×10 -4 11.2 -0.02 0.00 invention 1-11 45 0.3×10 -4 12.8 -0.03 0.00 invention 1-12 58 0.3×10 -4 13.4 -0.03 0.01 invention 1-13 54 0.3×10 -4 10.3 -0.08 0.04 invention It is evident that using the nucleating agents within the scope of the invention, thermographic recording elements satisfying the requirements of ultrahigh contrast, minimal dependency on developing temperature and minimal fog are obtained. The samples within the scope of the invention showed fully high values of sensitivity and Dmax, and low fog whenever developed under the above developing conditions. Example 2 Preparation of silver halide emulsion B In 700 ml of water were dissolved 22 g of phthalated gelatin and 30 mg of potassium bromide. The solution was adjusted to pH 5.0 at a temperature of 40°C. To the solution, 159 ml of an aqueous solution containing 18.6 g of silver nitrate and an aqueous solution containing potassium bromide were added over 10 minutes by the controlled double jet method while maintaining the solution at pAg 7.7. Then, an aqueous solution containing 8x10 -6 mol/liter of K 3 [IrCl 5 ] and 1 mol/liter of potassium bromide was added over 30 minutes by the controlled double jet method while maintaining the solution at pAg 7.7. There were obtained cubic grains having a mean grain size of 0.07 µm, a coefficient of variation of the projected area diameter of 8%, and a (100) face proportion of 86%. The thus obtained silver halide grains B were heated at 60°C, to which 8.5x10 -5 mol of sodium thiosulfate, 1.1x10 -5 mol of 2,3,4,5,6-pentafluorophenyldiphenylsulfine selenide, 2x10 -6 mol of Tellurium Compound 1, 3.3x10 -6 mol of chloroauric acid, and 2.3x10 -4 mol of thiocyanic acid were added per mol of silver. The emulsion was ripened for 120 minutes and then quenched to 50°C. With stirring, 8x10 -4 mol of Sensitizing Dye C was added, and 3.5x10 -2 mol of potassium iodide was added to the emulsion, which was stirred for 30 minutes and then quenched to 30°C, completing the preparation of a silver halide emulsion B. Preparation of organic acid silver microcrystalline dispersion A mixture of 40 g of behenic acid, 7.3 g of stearic acid, and 500 ml of distilled water was stirred at 90°C for 15 minutes. With vigorous stirring, 187 ml of 1N NaOH aqueous solution was added over 15 minutes, 61 ml of 1N nitric acid was added, and the solution was cooled to 50°C. Then, 124 ml of an aqueous solution of 1N silver nitrate was added and stirring was continued for 30 minutes. Thereafter, the solids were separated by suction filtration and washed with water until the water filtrate reached a conductivity of 30 µS/cm. The thus obtained solids were handled as a wet cake without drying. To 34.8 g as dry solids of the wet cake were added 12 g of polyvinyl alcohol and 150 ml of water. They were thoroughly mixed into a slurry. A vessel was charged with the slurry together with 840 g of zirconia beads having a mean diameter of 0.5 mm. A dispersing machine (1/4G Sand Grinder Mill by Imex K.K.) was operated for 5 hours for dispersion, completing the preparation of a microcrystalline dispersion of organic acid silver grains having a volume weighed mean grain diameter of 1.5 µm as measured by Master Sizer X (Malvern Instruments Ltd.). Preparation of solid particle dispersions of chemical addenda Solid particle dispersions of tetrachlorophthalic acid (C-7), 4-methylphthalic acid (C-8), 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (C-5), phthalazine (C-6), and tribromomethylsulfonylbenzene (C-12) were prepared. To tetrachlorophthalic acid were added 0.81 g of hydroxypropyl cellulose and 94.2 ml of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 10 hours. A vessel was charged with the slurry together with 100 ml of zirconia beads having a mean diameter of 0.5 mm. A dispersing machine as above was operated for 5 hours for dispersion, obtaining a solid particle dispersion of tetrachlorophthalic acid in which particles with a diameter of up to 1.0 µm accounted for 70% by weight. Solid particle dispersions of the remaining chemical addenda were similarly prepared by properly changing the amount of dispersant and the dispersion time to achieve a desired mean particle size. Preparation of emulsion layer coating solution B An emulsion layer coating solution B was prepared by adding the following compositions to the organic acid silver microparticulate dispersion prepared above. Organic acid silver particle dispersion 1 mol Silver halide emulsion B 0.05 mol Binder: LACSTAR 3307B SBR latex 430 g Addenda for development: Tetrachlorophthalic acid 5 g 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane 98 g Phthalazine 9.2 g Tribromomethylphenylsulfone 12 g 4-methylphthalic acid 7 g Nucleating agent shown in Table 10 (see Table 10) Preparation of emulsion surface protective layer coating solution B A surface protective layer coating solution B was prepared by adding 0.26 g of Surfactant A, 0.09 g of Surfactant B, 0.9 g of silica microparticulates having a mean particle size of 2.5 µm, 0.3 g of 1,2-bis(vinylsulfonylacetamide)ethane and 64 g of water to 10 g of inert gelatin. Preparation of back surface coating solution B A back surface coating solution B was prepared by adding 5 g of Dye C, 250 g of water, and 1.8 g of spherical silica Sildex H121 (mean size 12 µm) to 30 g of polyvinyl alcohol. Coated sample The emulsion layer coating solution B was applied to a polyethylene terephthalate support so as to give a silver coverage of 1.6 g/m 2 . The emulsion surface protective layer coating solution B was coated thereto so as to give a gelatin coverage of 1.8 g/m 2 . After drying, the back surface coating solution B was applied to the back surface of the support opposite to the emulsion layer so as to give an optical density of 0.7 at 780 nm. Coated samples were prepared in this way. Photographic property tests The samples were exposed, developed and tested as in Example 1. The results are shown in Table 10. Nucleating agent Photographic properties Sample No. No. Amount (mol/m 2 ) γ Δ S Δ Fog Remarks 2-1 -- -- 5.2 -0.03 0.03 comparison 2-2 RF-1 0.6×10 -4 7.8 -0.11 0.18 comparison 2-3 RF-1 2.0×10 -4 13.2 -0.38 1.35 comparison 2-4 RF-2 0.6×10 -4 7.9 -0.10 0.21 comparison 2-5 RF-2 2.0×10 -4 12.8 -0.28 1.58 comparison 2-6 2 0.6×10 -4 14.2 -0.04 0.02 invention 2-7 7 0.6×10 -4 14.0 -0.04 0.02 invention 2-8 33 0.6×10 -4 13.8 -0.04 0.03 invention 2-9 35 0.6×10 -4 14.5 -0.05 0.03 invention 2-10 43 0.6×10 -4 13.9 -0.04 0.02 invention 2-11 51 0.6×10 -4 14.4 -0.04 0.03 invention 2-12 60 0.6×10 -4 14.1 -0.04 0.03 invention 2-13 63 0.6×10 -4 14.3 -0.09 0.05 invention It is evident that using the nucleating agents within the scope of the invention, thermographic recording elements satisfying the requirements of ultrahigh contrast, minimal dependency on developing temperature and minimal fog are obtained. The samples within the scope of the invention also showed fully high values of sensitivity and Dmax, and low fog whenever developed under the above developing conditions. Example 3 Using the same inventive nucleating agents and comparative compounds as used in Example 2 whose type and amount are shown in Table 10, thermographic recording element samples were prepared in accordance with the following formulation. Preparation of silver halide emulsion C In 700 ml of water were dissolved 11 g of phthalated gelatin, 30 mg of potassium bromide, and 10 mg of sodium benzenethiosulfonate. The solution was adjusted to pH 5.0 at a temperature of 55°C. To the solution, 159 ml of an aqueous solution containing 18.6 g of silver nitrate and an aqueous solution containing 1 mol/liter of potassium bromide were added over 6.5 minutes by the controlled double jet method while maintaining the solution at pAg 7.7. Then, 476 ml of an aqueous solution containing 55.5 g of silver nitrate and an aqueous halide solution containing 1 mol/liter of potassium bromide were added over 28.5 minutes by the controlled double jet method while maintaining the solution at pAg 7.7. The solution was then desalted by lowering its pH for flocculation and sedimentation. With 0.17 g of Compound A and 23.7 g of deionized gelatin (having a calcium content of less than 20 ppm) added, the solution was adjusted to pH 5.9 and pAg 8.0. There were obtained cubic grains having a mean grain size of 0.11 µm, a coefficient of variation of the projected area of 8%, and a (100) face proportion of 93%. The thus obtained silver halide grains were heated at 60°C, to which 76 µmol of sodium benzenethiosulfonate was added per mol of silver. After 3 minutes, 154 µmol of sodium thiosulfate (per mol of silver) was added to the solution, which was ripened for 100 minutes. Thereafter, while the solution was kept at 40°C, 6.4x10 -4 mol of Sensitizing Dye D and 6.4x10 -3 mol of Compound B were added per mol of silver halide with stirring. After 20 minutes, the emulsion was quenched to 30°C, completing the preparation of silver halide emulsion C. Preparation of organic acid silver dispersion While a mixture of 4.4 g of arachidic acid, 39.4 g of behenic acid, and 770 ml of distilled water was stirred at 85°C, 103 ml of a 1N NaOH aqueous solution was added over 60 minutes. The mixture was stirred for a further 240 minutes for reaction and then cooled to 75°C. Then 112.5 ml of an aqueous solution containing 19.2 g of silver nitrate was added over 45 seconds to the solution, which was allowed to stand for 20 minutes and then cooled to 30°C. Thereafter, the solids were separated by suction filtration and washed with water until the filtrate reached a conductivity of 30 µS/cm. The thus collected solids were handled as wet cake without drying. To 100 g calculated as dry solids of the wet cake were added 5 g of polyvinyl alcohol (trade name: PVA-217) and water. This was further diluted with water to a total weight of 500 g and pre-dispersed by a homomixer. The pre-dispersed liquid was processed three times by a dispersing machine Micro-Fluidizer M-110S-EH (with G10Z interaction chamber, manufactured by Microfluidex International Corp.) which was operated under a pressure of 1,750 kg/cm 2 . There was obtained an organic acid silver dispersion. This dispersion contained needle grains of organic acid silver having a mean minor diameter of 0.04 µm, a mean major diameter of 0.8 µm, and a coefficient of variation of 30%. The grain size was measured by Master Sizer X (Malvern Instruments Ltd.). Cooling was carried out by mounting serpentine heat-exchangers before and after the interaction chamber and adjusting the temperature of the coolant, thereby setting the desired dispersion temperature. Solid particle dispersion of 1,1-bis (2 -hydroxy -3,5-dimethylphenyl)-3,5,5-trimethylhexane To 20 g of 1,1-bis(2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane were added 3.0 g of MP polymer MP-203 (by Kurare K.K.) and 77 ml of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 3 hours. A vessel was charged with the slurry together with 360 g of zirconia beads having a mean diameter of 0.5 mm. A dispersing machine 1/4G Sand Grinder Mill (Imex K.K.) was operated for 3 hours for dispersion, obtaining a solid particle dispersion of the reducing agent in which particles with a diameter of 0.3 to 1.0 µm accounted for 80% by weight. Solid particle dispersion of tribromomethylphenylsulfone To 30 g of tribromomethylphenylsulfone were added 0.5 g of hydroxypropyl methyl cellulose, 0.5 g of Compound C, and 88.5 g of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 3 hours. The subsequent procedure was the same as in the preparation of the solid particle dispersion of reducing agent. There was obtained a solid particle dispersion of the antifoggant in which particles with a diameter of 0.3 to 1.0 µm accounted for 80% by weight. Solid particle dispersion of nucleating agent To 10 g of the nucleating agent were added 2.5 g of polyvinyl alcohol (trade name, PVA-217) and 87.5 g of water. They were thoroughly agitated to form a slurry, which was allowed to stand for 3 hours. The subsequent procedure was the same as in the preparation of the solid particle dispersion of reducing agent. There was obtained a solid particle dispersion of the nucleating agent in which particles with a diameter of 0.3 to 1.0 µm accounted for 80% by weight. Preparation of emulsion layer coating solution To the above-prepared organic silver salt microcrystalline dispersion (corresponding to 1 mol of silver), the above-prepared silver halide emulsion C, a binder and the dispersions of developing addenda were added, and water added, obtaining an emulsion layer coating solution. Binder : LACSTAR 3307B SBR latex (as solids) 470 g Developing addenda: 1,1-bis (2-hydroxy-3,5-dimethylphenyl)-3,5,5-trimethylhexane (as solids) 110 g Tribromomethylphenylsulfone (as solids) 25 g Sodium benzenethiosulfonate 0.25 g Polyvinyl alcohol (MP-203) 46 g Compound F 0.12 mol Nucleating agent (used in Example solid particle dispersion 2) in (Table 10) Dye D 0.62 g Silver halide emulsion C 0.05 mol Ag Emulsion surface protective layer coating solution An emulsion surface protective layer coating solution was prepared by adding 3.75 g of H 2 O to 109 g of a polymer latex having a solid content of 27.5% (methyl methacrylate/styrene/2-ethylhexyl acrylate/2-hydroxyethyl methacrylate/acrylic acid = 59/9/26/5/1 copolymer, Tg = 55°C, MFT = 66°C), adding 4.5 g of benzyl alcohol as a film-forming aid, 0.45 g of Compound D. 0.125 g of Compound E, 0.0125 mol of Compound G, and 0.225 g of polyvinyl alcohol PVA-217, and further adding water to a total weight of 150 g. PET support with back and subbing layers (1) Preparation of support Using terephthalic acid and ethylene glycol, a polyethylene terephthalate (PET) having an intrinsic viscosity of 0.66 as measured in a phenol/tetrachloroethane 6/4 (weight ratio) mixture at 25°C was prepared in a conventional manner. After the PET was pelletized and dried at 130°C for 4 hours, it was melted at 300°C, extruded through a T-shaped die, and quenched to form an unstretched film having a thickness sufficient to give a thickness of 120 µm after heat curing. The film was longitudinally stretched by a factor of 3.3 by means of rollers having different circumferential speeds and then transversely stretched by a factor of 4.5 by means of a tenter. The temperatures in these stretching steps were 110°C and 130°C, respectively. Thereafter, the film was heat cured by heating at 240°C for 20 seconds and then transversely relaxed 4% at the same temperature. Thereafter, with the chuck of the tenter being slit and the opposite edges being knurled, the film was taken up under a tension of 4.8 kg/cm 2 . In this way, a film of 2.4 m wide, 3,500 m long and 120 µm thick was obtained in a roll form. (2) Subbing layer (a) Polymer Latex 1 (styrene/butadiene/hydroxyethyl methacrylate/divinyl benzene = 67/30/2.5/0.5 wt% copolymer) 160mg/m 2 2,4-dichloro-6-hydroxy-s-triazine 4 mg/m 2 Matte agent (polystyrene, mean particle size 2.4 µm) 3 mg/m 2 (3) Subbing layer (b) Alkali treated gelatin (Ca -+ content 30 ppm, jelly strength 230 g) 50 mg/m 2 Dye C coverage to give an optical density of 0.7 at 780 nm (4) Conductive layer Jurimer ET-410 (Nippon Junyaku K.K.) 38 mg/m 2 SnO 2 /Sb (9/1 weight ratio, mean particle size 0.25 µm) 120 mg/m 2 Matte agent (polymethyl methacrylate, mean particle size 5 µm) 7 mg/m 2 Melamine 13 mg/m 2 (5) Protective layer Chemipearl S-120 (Mitsui Petro-Chemical K.K) 500 mg/m 2 (Mitsui Petro-Chemical K.K) 500 mg/m 2 Snowtex C (Nisssan Chemical K.K.) 40 mg/m 2 Tenachol EX-614B (Nagase Chemicals K.K.) 30 mg/m 2 On each surface of a support, the subbing layer (a) and the subbing layer (b) were successively coated and dried at 180°C for 4 minutes. On one surface of the support where subbing layer (a) and subbing layer (b) had been coated, the conductive layer and the protective layer were successively coated and dried at 180°C for 4 minutes. There was obtained the PET support with the back/subbing layers. The PET support with the back/subbing layers was automatically fed at a feed speed of 20 m/min. and a tension of 3 kg/m 2 through a heat treating zone of 200 m in overall length which was set at a temperature of 200°C. The PET support was then passed through a zone of 40°C for 15 seconds and taken up into a roll under a take-up tension of 10 kg/cm 2 . Photothermographic samples On the PET support with the back/subbing layers, the emulsion layer coating solution was applied to the subbing layer to a silver coverage of 1.6 g/m 2 . The emulsion surface protective layer coating solution was applied thereon to a coverage of 2.0 g/m 2 of the polymer latex. Photographic test The samples were examined for photographic properties as in Example 1. The changes in the exposure and heat development steps are described below. In the exposure step, the coated samples were exposed to xenon flash light for an emission time of 10 -6 sec. through an interference filter having a peak at 780 nm and a step wedge. In the heat development step, the heat developing apparatus shown in FIG. 1 was modified by incorporating two heat sources in accordance with the construction of the heat developing apparatus shown in FIG. 3 of JP-A 13294/1995 such that the sample might be heated in two continuous stages. The exposed sample was developed through this heat developing apparatus so that it was heated at 105°C for 10 seconds (conditions under which no images were developed) and then at 117°C for 20 seconds. The results were equivalent to those of Example 2. It is thus evident that using the nucleating agents within the scope of the invention, thermographic recording elements satisfying the requirements of ultrahigh contrast, minimal dependency on developing temperature and minimal fog are obtained. The samples within the scope of the invention also showed fully high values of sensitivity and Dmax, and low fog whenever developed under the above developing conditions. There has been described a thermographic recording element featuring high sensitivity, high Dmax, satisfactory contrast, low fog, and minimal dependency of photographic properties on developing temperature.

Claims: A thermographic recording element having at least one image forming layer and comprising an organic silver salt, a reducing agent, and at least one nucleating agent of the following formulas (A) and (B): wherein Z 1 and Z 2 each are a group of non-metallic atoms capable of forming a 5- to 7-membered ring structure with the carbon atoms, Y 1 and Y 2 each are -C(=O)- or -SO 2 -, X 1 and X 2 each are a hydroxy or salt thereof, alkoxy, aryloxy, heterocyclic oxy, mercapto or salt thereof, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, arylamino, heterocyclic amino, acylamino, sulfonamide or heterocyclic group, and Y 3 is hydrogen or a substituent, provided that the following compound is exempt from formula (B): The recording element of claim 1 wherein the compound of formula (A) has at least 6 carbon atoms in total, and the compound of formula (B) has at least 12 carbon atoms in total. The recording element of claim 1 wherein in formula (A), the total number of carbon atoms in Z 1 is at least 3, and in formula (B), the total number of carbon atoms in Z 2 and Y 3 is at least 8. The recording element of claim 1, further comprising a photosensitive silver halide. The recording element of claim 1 wherein in formulas (A) and (B), the ring structures formed by Z 1 and Z 2 are 5-membered rings, and both Y 1 and Y 2 are -C(=O)-.

Inventors: EZOE TOSHIHIDE, HOSHIMIYA TAKASHI, KAWATO KOHJI, SASAKI HIROTOMO, SUZUKI HIROYUKI, YAMADA KOHZABUROH, EZOE, TOSHIHIDE, HOSHIMIYA, TAKASHI, KAWATO, KOHJI, SASAKI, HIROTOMO, SUZUKI, HIROYUKI, YAMADA, KOHZABUROH
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1998
Main classification: G03C1
IPC codes: G03C1