Abstract:
This invention is directed to spontaneous opal glasses containing an apatite opacifying phase and exhibiting a softening point of at least 740° C., excellent chemical durability, and a temperature interval between the high temperature crystallization liquidus and the emulsion liquidus greater than 50° C. The glasses consist essentially, in weight percent on the oxide basis, of 
     
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     SiO 2    50-63   Na 2  O   5.5-10Al 2  O 3    10-14   K 2  O    0-10P 2  O 5   3.5-7    BaO          0-10PbO      2-12   CaO           0-2.5B 2  O 3     1-4    SrO          0-8F      1.5-4    BaO + CaO + SrO At Least 3 mole %______________________________________

Description:
BACKGROUND OF THE INVENTION 
     Our U.S. Pat. No. 4,298,390 discloses the production of spontaneous opal glasses wherein the opacity is the result of crystallinity in the glass and Ba 2  F(PO 4 ) constitutes the predominant crystal phase. Those glasses are asserted to manifest softening points in excess of 710° C., a white opacity, excellent chemical durability, and to consist essentially, expressed in terms of weight percent on the oxide basis, of 6-10% Na 2  O, 1-6% K 2  O, 4-11% BaO, 9-18% Al 2  O 3 , 1-5% B 2  O 3 , 50-70% SiO 2 , 3.5-7% P 2  O 5 , 1-4% F, and optionally up to 3.5% CaO and/or up to 5% total of MgO and/or SrO. 
     The patent explains that those glasses are characterized by a two-stage liquidus phenomenon; viz., a high temperature cloudiness or opacification, termed an emulsion liquidus or liquid-liquid phase separation, and the normal crystalline opal liquidus. Analysis of the phase separation found it to be rich in Na 2  O, BaO, P 2  O 5 , and F. X-ray diffraction analysis of the crystalline opal phase identified the predominant crystal phase to be of a Ba 2  (OH)PO 4  type. Nonetheless, because X-ray analysis does not distinguish between fluoride and OH, it was assumed that fluoride substituted for OH in the crystal, thereby leading to the crystals being described as Ba 2  F(PO 4 ). Minor amounts of NaBaPO 4  and other unidentifiable species were also detected. 
     Culinary ware and tableware are thermally tempered to improve the mechanical strength and thermal shock resistance thereof. Unfortunately, breakage of such ware produced according to the disclosure of U.S. Pat. No. 4,298,390 was not infrequently experienced during the thermal tempering procedure. Microscopic examination of the crystal phase indicated that a substantial number of the crystals attained relatively large dimensions with inclusions of such crystals exhibiting diameters in excess of 0.001&#34; (˜25 microns). Ware containing such large inclusions do not survive the thermal shock inherent in the air chill tempering process, viz., about 800° C. to room temperature. 
     A further and very extensive investigation of those glasses has determined that the crystallization mechanism is more complex than originally conjectured. This study demonstrated that, instead of a single crystallization liquidus, as discussed in U.S. Pat. No. 4,298,390, there appears to be a first or high temperature crystallization liquidus and a second or low temperature crystallization liquidus. Hence, at the high temperature crystallization liquidus at least one species of apatite-type crystal [classical formula Ca 10  F 2  (PO 4 ) 6  ] precipitates out of the molten glass, and at the low temperature crystallization liquidus at least one other species of apatite-type crystal is generated, as confirmed by X-ray diffraction data. Much solid solution is possible in the apatite structure which manifests itself in very minor changes in the X-ray diffraction pattern; i.e., the overall pattern is relatively indistinguishable from the general appearance of classical apatite. Hence, as used herein, apatite includes such solid solution crystals. 
     It was noted in U.S. Pat. No. 4,298,390 that the emulsion liquidus and crystallization opal liquidus data were obtained utilizing a hot stage microscope composite apparatus. Such apparatus relies upon the sensitivity of the eye of the observer and, therefore, involves a significant measure of subjectivity in the reported determinations. Accordingly, to remove the element of subjectivity, the emulsion and low temperature crystallization liquidi were determined on a number of the working examples of the patent employing laser reflectance measurements, that technique being founded in conventional laser reflectance spectroscopy. Thus, those liquidi are readily gained from laser reflectance or back scattering/temperature curves. The high temperature crystallization liquidus cannot normally be directly obtained from such curves because that deflection is hidden in the steep slope brought about by the emulsion phenomenon. Consequently, the high temperature crystallization liquidus is read from the laser reflectance or back scattering/temperature curve in the derivative mode. 
     Table A records the emulsion, high temperature crystallization, and low temperature crystallization liquidi (in °C.) measured on several of the working examples provided in Table I of the patent. The Example Nos. reflect those in Table I of the patent. 
     
                                           TABLE A__________________________________________________________________________      1  2  3  7  8  9  10 12 15 17 18__________________________________________________________________________Emulsion Liquidus      1380         1400            1390               1380                  1380                     1370                        1395                           1380                              1390                                 1400                                    1385High Temp. Liquidus      1350         1365            1370               1350                  1345                     1345                        1360                           1350                              1375                                 1370                                    1360Low Temp. Liquidus       875          860             610                700                   760                      725                         710                            820                               900                                  740                                     770__________________________________________________________________________ 
    
     As can be observed, the interval between the emulsion liquidus and the high temperature crystallization liquidus is less than 50° C., often no more than 25° C. The above-mentioned extensive investigation of those glasses has indicated that, to prevent the formation of those large crystalline inclusions which can cause mechanical breakage of ware, the temperature range between the emulsion liquidus and high temperature crystallization liquidus must be expanded. That is the principal objective of the present invention. 
     SUMMARY OF THE INVENTION 
     We have found that the incorporation of about 2-12% by weight PbO to compositions similar to those disclosed in U.S. Pat. No. 4,298,390 can substantially eliminate the above-described large inclusions of apatite crystals while retaining the chemical and physical properties of those glasses. PbO significantly lowers the high temperature crystallization liquidus such that the interval between the high temperature crystallization and emulsion liquidi is expanded by at least 25° C. and, frequently, in excess of 50° C. Thus, the crystallization liquidus is generally reduced to 1325° C. and below. The emulsion liquidus will generally remain essentially invariant with PbO contents up to about 4%. At higher values the emulsion liquidus will be depressed to some extent, but the reduction in the high temperature crystallization liquidus will generally track that decrease, such that the degree of expansion in the interval between the liquidi will be maintained. Hence, rather than a spread of no more than about 50° C. and often no more than 25° C. between the liquidi, the addition of PbO opens the interval to greater than 50° C., usually at least 75° C., and occasionally in excess of 100° C. This enlargement in the range of temperature between the crystallization and emulsion liquidi means that the exposure of the glass to a temperature between those liquidi, which is mandatory for dense opacification, can be conducted at temperatures sufficiently above the high temperature crystallization liquidus to minimize nucleation and the subsequent growth of large crystalline inclusions. 
     The glasses of the instant invention exhibit softening points in excess of 740° C., excellent chemical durability, low values of heavy metal release, and consist essentially, expressed in terms of weight percent on the oxide basis, of: 
     
         ______________________________________SiO.sub.2    50-63         Na.sub.2 O                          5.5-10Al.sub.2 O.sub.3    10-14         K.sub.2 O                           0-10P.sub.2 O.sub.5    3.5-7         BaO      0-10PbO       2-12         CaO       0-2.5B.sub.2 O.sub.3    1-4           SrO     0-8F        1.5-4______________________________________ BaO + CaO + SrO at least 3 mole % 
    
     At least one member of the group CaO, SrO, and BaO must be present to yield an apatite opacifying phase selected from the group of Ca 5  F(PO 4 ) 3 , Sr 5  F(PO 4 ) 3 , Ba 5  F(PO 4 ) 3 , (Ca 5-x  Ba x )F(PO 4 ) 3  solid solution, (Sr 5-x  Ba x )F(PO 4 ) 3  solid solution, (Sr 5-x  Ba x )F(PO 4 ) 3  solid solution, (Ca 5-x  Sr x )F(PO 4 ) 3  solid solution, (Ba 5-x  Ca x )F(PO 4 ) 3  solid solution, and (Ba 5-x  Sr x )F(PO 4 ) 3  solid solution. All of those crystal species have the apatite structure with the general formula R 5  X(PO 4 ) 3 , wherein R is selected from the group of Ca, Sr, and Ba, and X is selected from the group of F -  and OH - . The minimum level required for each or a combination of two or more is expressed in terms of mole percent because of the obvious differences in the molecular weights of CaO, SrO, and BaO. The minimum quantities of CaO, SrO, and BaO necessary to achieve dense opacity, approximated in terms of weight percent, are about 1%, 2%, and 3%, respectively. 
     Scanning electron microscopy and X-ray emission data indicate that lead enters the separated phase to inhibit thereby the formation of the calcium and/or strontium and/or barium apatite crystals. Thus, here again is support for the suggestion that lead enters into the apatite phase to form an apatite solid solution containing lead and/or a lead apatite phase, viz. Pb 5  F(PO 4 ) 3 . 
     The most preferred levels of PbO to insure dense opacity, to minimize heavy metal release, to maintain the softening point above 760° C. to permit decorating with durable, high temperature glazes and enamels, to exhibit excellent resistance to attack by acids and bases, and to display extremely good detergent durability will comprise about 3-4%. Whereas PbO contents up to 12% are operable, the products resulting from those values do not manifest any substantial improvement in properties and the softening point may be reduced and chemical durability deleteriously affected. Opacity is also generally affected adversely. Quantities greater than 12% invariably result in reduced chemical durability and increased heavy metal release. 
     Each of the remaining base glass constituents will be constrained within the above-defined limits. For example, when the amount of CaO exceeds 2.5%, PbO becomes ineffective in controlling the size of the crystalline inclusions because of the great insolubility of calcium apatite under those conditions. Above 8% SrO the glasses display high emulsion liquidi with subsequent inclusion problems. BaO levels greater than 10% cause the coefficient of thermal expansion of the glass to become so high as to prevent satisfactory thermal tempering. At P 2  O 5  levels below 3.5%, opacity becomes poor and where P 2  O 5  is present in amounts greater than about 5, PbO cannot be counted upon to always control the growth of apatite inclusions. At least 1.5% F is necessary to form the desired fluorapatite structure, but quantities greater than 4% cause a decrease in opacity because the fluoride solubilizes the components of the apatite phase in the melt. About 1.8-2.2% F is preferred. The total Na 2  O+K 2  O will preferably not exceed about 13% since their fluxing action solubilizes the crystal components which leads to reduced opacity. K 2  O is preferably present since its absence frequently results in a glass having an undesirably high emulsion liquidus. 
     PbO when present in the defined amounts of 2-12% in the inventive opal glass system will, without fail, reduce the size of the crystalline inclusions, but may not always totally eliminate them at the higher levels of CaO, SrO, BaO, and P 2  O 5 . 
     Al 2  O 3  contents should be maintained between 10-14% to impart excellent detergent durability. Amounts in excess of 14% customarily lead to a decreased emulsion liquidus and reduced opacity. Where the B 2  O 3  level is below 1%, the emulsion liquidus rises dramatically. On the other hand, contents of B 2  O 3  greater than 4% produce non-crystalline fluorphosphate opal glasses of only medium opacity at best. 
     MgO and ZnO may form part of the glass composition in amounts not exceeding 2.5% by weight total. A combination of PbO with ZnO appears to enhance the inclusion-controlling capability of ZnO. Nevertheless, when the content of ZnO exceeds two mole percent, additions of PbO up to about 12% often result in a decrease of opacity. 
     In summary, PbO in the apatite opal glass compositions of the instant invention operates in accordance with the following factors: 
     (a) it can essentially eliminate the undesirable large apatite inclusions; i.e., it lowers the temperature for the appearance of inclusions (high temperature crystallization liquidus); and 
     (b) it can maintain or lower the emulsion liquidus while expanding the temperature interval between the high temperature crystallization and emulsion liquidi; and 
     (c) it can increase the rate at which the emulsion and crystallization phenomena occur. 
     It will be appreciated, of course, that conventional glass coloring transition metal oxides and/or rare earth metal oxides may, if desired, be incorporated into the glass compositions in the customary proportions. 
     RELATED APPLICATION 
     Ser. No. 592,929, filed concurrently by us under the same title as the instant application, discloses that the incorporation of ZnO in compositions similar to those of U.S. Pat. No. 4,298,390 and similar to those of the instant application can eliminate the large crystalline inclusions in like manner to the action of PbO. 
     PRIOR ART 
     U.S. Pat. No. 4,298,390 provides an extensive recital of U.S. patents having some relevance to spontaneous fluorophosphate opal glasses and two other U.S. patents were cited during the prosecution of the application maturing into that patent. Rather than repeating that recital, however, the text of the patent is explicitly incorporated herein by reference. Whereas none of the patents reported therein or cited during the prosecution thereof is as pertinent to the present disclosure as U.S. Pat. No. 4,298,390, the instant invention being an improvement upon the latter, U.S. Pat. No. 2,394,502 will be reviewed inasmuch as it discloses the production of opal glasses containing a fluorapatite crystalline opal phase. 
     That patent describes opal glasses having base compositions consisting essentially, in weight percent, of: 
     
         ______________________________________SiO.sub.2  54-66        PbO     0-5Al.sub.2 O.sub.3      0-6          B.sub.2 O.sub.3                            0-50Na.sub.2 O + K.sub.2 O      12-17        As.sub.2 O.sub.3                           0-1CaO         0-12        P.sub.2 O.sub.5                           4-9BaO        0-4          F       2.5-5______________________________________ 
    
     No minimum quantity of CaO+BaO+PbO is specified, although it is noted that at least one of the three must be present to participate in the formation of the fluorapatite crystals. CaO is the preferred oxide. It is stated that BaO and PbO reduce crystal formation growth. 
     The maximum Al 2  O 3  content is considerably below that required in the present inventive composition. Furthermore, BaO is stated to adversely affect crystal formation such that the maximum that can be tolerated is 4%. In contrast, the formation of crystals is not adversely affected by BaO in the present composition, and the minimum demanded to achieve dense opacity is about 3%. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Table I records exemplary compositions, expressed in terms of parts by weight on the oxide basis, illustrating the compositional parameters of the inventive glasses. Because the sum of the individual components totals or very closely approximates 100, for all practical purposes the values tabulated may be considered to represent the compositions in terms of weight percent. Furthermore, inasmuch as it is not known with which cation(s) the fluoride is combined, it is simply recited as fluoride (F), in accordance with conventional glass analysis practice. Commonly, about 20-30% by weight of the fluoride will be lost through volatilization during melting of the batch. Where desired, an oxide of arsenic or a chloride salt may be included in the batch to perform its conventional function as a fining agent. Table IA lists the components of the several glasses in terms of approximate mole percent. 
     The actual batch components may comprise any ingredients, either the oxides or other compounds, which, when melted together, will be converted into the desired oxide in the proper proportions. The fluoride will typically be added as a silicofluoride. Whereas the description below is based upon laboratory scale melting, it must be recognized that the tabulated compositions would also be useful in large scale commercial melting units. 
     The batch ingredients were compounded, tumble mixed together to assist in obtaining a homogeneous melt, and charged into platinum crucibles. The crucibles were introduced into a furnace operating at about 1500° C. and the batches melted for about four hours. The melts were poured into steel molds to yield slabs having the dimensions of 3.5&#34;×2.5&#34;×0.25&#34;, and the glass slabs immediately transferred to an annealer operating at 500°-550° C., the slabs being cooled to room temperature at a rate of about 50° C./hour. 
     Table I also reports visual appraisal of the density of the opacity (Opac.), the presence of inclusions (Incls.) in the slabs, and the identity of the crystal phase(s) present (Cryst.) as determined via X-ray diffraction data, listed in the order of extent present. 
     
         TABLE I   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24   SiO.sub.2 58.75 56.9 55.5 56.3 58.1 56.35 54.7 51.7 59.4 58.6 57.2 59.35 58.0 58.2 58.0 57.8 55.6 56.5 58.9 57.85 58.6 57.3 58.8 57.6 Al.sub.2 O.sub.3 15.4 14.95 14.6 14.8 12.8 12.4 12.1 11.4 13.2 12.9 12.6 13.1 12.9 12.8 12.8 12.7 12.25 13.0 13.0 12.7 12.9 12.6 12.9 12.7 CaO 4.05 3.1 2.3 0.8 1.55 1.5 1.5 1.4 1.7 0.8 -- 1.6 1.6 1.55 0.8 -- -- 1.7 1.6 1.55 1.6 1.55 1.6 1.55 BaO -- -- -- -- 6.4 6.2 6.0 5.7 6.7 6.45 6.3 4.35 2.2 -- 4.3 2.15 -- 6.2 6.5 6.4 6.45 6.3 6.5 6.35 Na.sub.2 O 8.5 8.3 8 .1 8.2 8.2 7.95 7.7 7.3 8.7 8.3 8.1 8.4 8.3 8.2 8.2 8.2 7.9 7.5 8.3 8.2 7.4 6.4 7.8 7.3 K.sub.2 O 1.95 1.9 1.85 1.9 3.3 3.2 3.1 2.9 -- 3.3 3.25 3.35 3.3 3.3 3.3 3.3 3.15 -- 2.0 0.65 3.3 3.25 2.65 1.95 P.sub.2 O.sub.5 4.5 4.3 4.25 4.3 4.15 4.0 3.9 3.7 4.3 4.2 4.1 4.25 4.2 4.15 4.15 4.15 4.0 4.1 4.2 4.15 4.2 4.1 4.2 4.1 B.sub.2 O.sub.3 3.45 3.3 3.3 3.3 2.4 2.35 2.3 2.15 2.7 2.45 2.4 2.5 2.45 2.4 2.4 2.4 2.3 2.5 2.45 2.4 2.45 2.4 2.45 2.4 PbO 3.35 6.5 9.5 9.6 3.1 6.0 8.8 13.8 3.2 3.1 6.1 3.2 6.3 9.3 6.2 9.3 14.9 6.1 3.15 6.2 3.15 6.1 3.15 6.15 F 2.28 2.2 2.15 2.18 2.12 2.05 1.99 1.9 2.2 2.1 2.1 2.2 2.15 2.1 2.1 2.1 2.0 2.3 2.15 2.1 2.15 2.1 2.15 2.1 MgO 0.9 0.87 0.85 0.86 -- -- Incls. No No No No No No No No  No No No No No No No No No Few No No No No No No Opac. Medi- Medi- Very Poor Dense Dense Dense Dense Dense Medi- Poor Poor Medi- Medi- Medi- Poor Poor Very Very Very Very Very Dense Very  um um Denseum   um um um   Dense Dense Dense Dense Dense  Dense Dense Cryst. -- -- Apatite -- Apatite -- -- Apatite Apatite -- -- -- Apatite -- Apatite -- -- Apatite Apatite Apatite Apatite Apatite -- Apatite    25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44   SiO.sub.2 59.9 58.55 57.25 59.3 58.0 58.0 58.2 59.15 57.5 58.7 56.85 58.2 56.3 54.9 57.5 55.6 53.65 57.0 53.0 58.25 Al.sub.2 O.sub.3 13.2 12.9 12.6 13.0 12.75 12.75 12.8 13.0 12.65 12.9 12.5 12.9 12.5 12.15 12.75 13.6 13.75 13.3 13.7 12.8 CaO 1.45 1.3 1.1 1.6 1.55 1.4 1.4 1.6 1.55 1.6 1.5 1.2 0.4 -- 1.55 1.5 1.45 1.55 1.45 1.55 BaO 6.6 6.45 6.3 6.5 6.4 5.1 5.85 5.4 3.2 6.5 6.3 5.4 3.15 -- -- 6.1 5.9 6.3 5.9 -- Na.sub.2 O 6.85 6.0 5.2 6.35 5.5 7.25 7.5 8.35 8.1 8.3 8.0 8.3 8.0 7.8 8.16 7.85 7.6 8.1 7.5 8.2 K.sub.2 O 2.0 2.0 1.95 1.7 1.5 2.9 3.0 2.65 1.3 3.3 3.2 3.3 3.2 3.15 3.25 3.15 3.0 3.25 3.0 3.3 P.sub.2 O.sub.5 4.3 6.25 4.1 4.25 4.15 4.15 4.15 4.2 4.1 4.2 4.1 5.2 5.0 4.9 5.1 4.0 3.85 5.1 4.7 4.15 B.sub.2 O.sub.3 2.5 2.45 2.4 2.45 2.4 2.45 2.45 2.45 2.4 1.45 -- 2.45 2.35 2.3 2.4 2.3 2.25 2.4 2.2 2.4 PbO 3.2 4.2 9.2 4.75 7.75 6.2 4.7 3.15 9.2 3.15 7.6 3.1 9.1 14.8 9.3 5.95 8.6 3.05 8.55 9.35 F 2.2 2.15 2.1 2.15 2.1 2.1 2.1 2.15 2.1 2.15 2.1 2.15 2.1 2.0 2.1 2.0 1.95 2.1 1.9 -- SrO                   -- -- Incls. No Few Many Few Few No No No No No No No No No No No No No No No Opac. Dense Dense Dense Dense Dense Very Very Poor- Poor- Poor- Poor Medium Poor Very Medium- Dense Medium Dense Dense Poor       Dense Dense Medium Medium Medium    Poor Poor Cryst. -- Apatite Apatite + Apatite -- Apatite Apatite -- Apatite + -- -- -- -- -- -- Apatite -- Apatite Apatite Apatite +    Pb.sub.3 (PO.sub.4).sub.2      Pb.sub.3 (PO.sub.4).sub.2           Pb.sub.3 (PO.sub.4).sub.2    45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63   SiO.sub.2 56.7 56.6 54.9 58.5 58.1 57.2 57.0 56.6 57.6 54.0 53.1 56.15 5 5.7 57.9 57.4 56.4 54.4 56.5 59.3 Al.sub.2 O.sub.3 12.5 12.4 13.5 12.9 12.8 12.6 12.5 12.45 12.7 12.15 12.1 12.5 12.5 12.95 12.9 12.9 12.15 12.8 13.0 CaO -- 1.5 1.5 1.6 1.55 1.5 -- -- -- 1.5 1.5 1.55 1.55 1.6 1.6 1.5 1.5 1.55 1.5 BaO 4.2 5.15 6.1 4.3 -- -- 2.1 -- -- 5.9 5.9 6.1 6.1 6.3 6.3 5.9 5.9 6.25 6.15 Na.sub.2 O 8.0 8.0 7.8 8.25 8.2 8.1 8.05 8.0 8.1 7.6 7.6 7.85 7.8 8.1 8.1 8.1 7.6 8.0 7.0 K.sub.2 O 3.2 3.2 3.1 3.3 0.65 0.65 1.9 1.9 3.25 3.0 3.0 3.05 3.1 3.2 3.2 3.2 3.0 1.85 3.15 P.sub.2 O.sub.5 4.05 4.0 4.9 4.2 4.15 4.1 4.1 4.05 4.1 3.8 3.75 3.9 3.9 4.0 4.0 4.0 3.75 4.0 4.25 B.sub.2 O.sub.3 2.4 -- 2.3 2.45 2.4 2.4 2.4 2.35 2.4 2.2 2.2 2.3 2.3 2.4 2.4 2.4 2.25 2.35 2.5 PbO 9.1 9.1 5.9 3.1 9.3 9.2 9.15 9.1 6.15 8.8 8.75 6.0 6.0 3.1 3.1 3.1 8.8 6.15 3.2 F -- -- 2.0 2.15 2.1 2.1 2.1 2.05 2.1 2.0 2.0 2.05 2.05 2.1 2.1 2.1 2.0 2.1 2.2 SrO -- -- -- 1.5 2.9 4.3 2.85 5.65 5.75 -- ZnO     -- -- -- -- -- 0.55 2.1 0.6 1.1 0.6 1.15 2.25 0.55 0.6 -- As.sub.2 O.sub.3     -- -- -- -- -- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 -- Incls. No No No No No No No No No No No No No No No No No No No Opac. Clear Poor Very Dense Dense Dense Dense Dense Very Medium Poor- Dense Dense Dense Dense Medium Medium Very Dense    Dense      Dense  Medium       Dense Cryst. -- -- Apatite Apatite Apatite + Apatite + Apatite Apatite + Apatite + -- -- Apatite Apatite + Apatite Apatite -- -- Apatite Apatite      Pb.sub.3  (PO.sub.4).sub.2 Pb.sub.3 (PO.sub.4).sub.2  Pb.sub.3 (PO.sub.4).sub.2 Pb.sub.3 (PO.sub.4).sub.2    Pb.sub.3  (PO.sub.4).sub.2 
    
     
                                           TABLE IA__________________________________________________________________________    1  2  3  4  5  6  7  8  9  10 11 12 13 14 15  16 17 18  19__________________________________________________________________________SiO.sub.2    65.2  65.2     65.2        65.2           69.5              69.5                 69.5                    69.5                       69.3                          69.5                             69.5                                69.5                                   69.5                                      69.5                                         69.5                                             69.5                                                69.5                                                   69.6 69.5Al.sub.2 O.sub.3    10.1  10.1     10.1        10.1           9.0              9.0                 9.0                    9.0                       9.1                          9.0                             9.0                                9.0                                   9.0                                      9.0                                         9.0 9.0                                                9.0                                                   9.4  9.0CaO 4.8  3.8     2.8        1.0           2.0              2.0                 2.0                    2.0                       2.1                          1.0                             -- 2.0                                   1.0                                      -- 2.0 -- -- 2.2  2.0BaO -- -- -- -- 3.0              3.0                 3.0                    3.0                       3.1                          3.0                             3.0                                2.0                                   1.0                                      -- 2.0 1.0                                                -- 3.0  3.0Na.sub.2 O    9.2  9.2     9.2        9.2           9.5              9.5                 9.5                    9.5                       9.8                          9.5                             2.0                                9.5                                   9.5                                      9.5                                         9.5 9.5                                                9.5                                                   9.0  9.5K.sub.2 O    1.4  1.4     1.4        1.4           2.5              2.5                 2.5                    2.5                       -- 2.5                             9.5                                2.5                                   2.5                                      2.5                                         2.5 2.5                                                2.5                                                   --   1.5P.sub.2 O.sub.5    2.1  2.1     2.1        2.1           2.1              2.1                 2.1                    2.1                       2.1                          2.1                             2.5                                2.1                                   2.1                                      2.1                                         2.1 2.1                                                2.1                                                   2.1  2.1B.sub.2 O.sub.3    3.3  3.3     3.3        3.3           2.5              2.5                 2.5                    2.5                       2.7                          2.5                             2.1                                2.5                                   2.5                                      2.5                                         2.5 2.5                                                2.5                                                   2.7  2.5PbO 1.0  2.0     3.0        3.0           1.0              2.0                 3.0                    5.0                       1.0                          1.0                             2.5                                1.0                                   2.0                                      3.0                                         2.0 3.0                                                5.0                                                   2.0  1.0F   8.0  8.0     8.0        8.0           8.0              8.0                 8.0                    8.0                       8.0                          8.0                             8.0                                8.0                                   8.0                                      8.0                                         8.0 8.0                                                8.0                                                   8.0  8.0MgO 1.5  1.5     1.5        1.5           -- --__________________________________________________________________________    20 21 22 23 24 25 26 27  28 29 30 31 32 33  34 35 36 37 38__________________________________________________________________________SiO.sub.2    69.5  69.5     69.5        69.5           69.5              69.5                 69.5                    69.5                        69.5                           69.5                              69.5                                 69.5                                    69.5                                       69.5                                           69.5                                              69.5                                                 69.0                                                    69.0                                                    69.0Al.sub.2 O.sub.3    9.0  9.0     9.0        9.0           9.0              9.0                 9.0                    9.0 9.0                           9.0                              9.0                                 9.0                                    9.0                                       9.0 9.0                                              9.0                                                 9.0                                                    9.0 9.0CaO 2.0  2.0     2.0        2.0           2.0              1.8                 1.6                    1.4 2.0                           2.0                               1.76                                  1.82                                    2.0                                       2.0 2.0                                              2.0                                                 1.5                                                    0.5 --BaO 3.0  3.0     3.0        3.0           3.0              3.0                 3.0                    3.0 3.0                           3.0                               1.64                                  2.73                                    2.5                                       1.5 3.0                                              3.0                                                 2.5                                                    1.5 --Na.sub.2 O    9.5  8.5     7.5        9.0           8.5              7.7                 6.9                    6.1 7.2                           6.4                              8.4                                  8.67                                    9.5                                       9.5 9.5                                              9.5                                                 9.5                                                    9.5 9.5K.sub.2 O    0.5  2.5     2.5        2.0           1.5              1.5                 1.5                    1.5 0.5                           0.5                              2.2                                  2.28                                    2.0                                       1.5 1.0                                              2.5                                                 2.5                                                    2.5 2.5P.sub.2 O.sub.5    2.1  2.1     2.1        2.1           2.1              2.1                 2.1                    2.1 2.1                           2.1                              2.1                                 2.1                                    2.1                                       2.1 2.1                                              2.1                                                 2.6                                                    2.6 2.6B.sub.2 O.sub.3    2.5  2.5     2.5        2.5           2.5              2.5                 2.5                    2.5 2.5                           2.5                              2.5                                 2.5                                    2.5                                       2.5 1.5                                              -- 2.5                                                    2.5 2.5PbO 2.0  1.0     2.0        1.0           2.0              1.0                 2.0                    3.0 1.5                           2.5                              2.0                                 1.5                                    1.0                                       3.0 1.0                                              2.5                                                 1.0                                                    3.0 5.0F   8.0  8.0     8.0        8.0           8.0              8.0                 8.0                    8.0 8.0                           8.0                              8.0                                 8.0                                    8.0                                       8.0 8.0                                              8.0                                                 8.0                                                    8.0 8.0__________________________________________________________________________    39 40 41  42 43  44                  45 46  47 48 49 50 51 52 53 54 55   56__________________________________________________________________________SiO.sub.2    69.0  69.5     69.5         69.0            69.0               69.5                  69.5                     69.5                         69.5                            69.5                               69.5                                  69.5                                     69.5                                        69.5                                           69.5                                              68.66                                                 67.66                                                      69.16Al.sub.2 O.sub.3    9.0  10.0     10.5         9.5            10.5               9.0                  9.0                     9.0 9.0                            9.0                               9.0                                  9.0                                     9.0                                        9.0                                           9.0                                              9.1                                                 9.1  9.1CaO 2.0  2.0     2.0 2.0            2.0               2.0                  -- 2.0 2.0                            2.0                               2.0                                  -- -- -- -- 2.02                                                 2.02 2.02BaO -- 3.0     3.0 3.0            3.0               -- 2.0                     2.5 2.5                            3.0                               -- -- 1.0                                        -- -- 2.95                                                 2.95 2.95Na.sub.2 O    9.5  9.5     9.5 9.5            9.5               9.5                  9.5                     9.5 9.5                            9.5                               9.5                                  9.5                                     9.5                                        9.5                                           9.5                                              9.37                                                 9.37 9.37K.sub.2 O    2.5  2.5     2.5 2.5            2.5               2.5                  2.5                     2.5 2.5                            2.5                               0.5                                  0.5                                     1.5                                        1.5                                           2.5                                              2.41                                                 2.41 2.41P.sub.2 O.sub.5    2.6  2.1     2.1 2.6            2.6               2.1                  2.1                     2.1 2.1                            2.1                               2.1                                  2.1                                     2.1                                        2.1                                           2.1                                              2.02                                                 2.02 2.02B.sub.2 O.sub.3    2.5  2.5     2.5 2.5            2.5               2.5                  2.5                     --  2.5                            2.5                               2.5                                  2.5                                     2.5                                        2.5                                           2.5                                              2.44                                                 2.44 2.44PbO 3.0  2.0     3.0 1.0            3.0               3.0                  3.0                     3.0 3.0                            3.0                               3.0                                  3.0                                     3.0                                        4.0                                           3.0                                              3.0                                                 3.0  3.0F   8.0  8.0     8.0 8.0            8.0               -- -- --  4.0                            8.0                               8.0                                  8.0                                     8.0                                        8.0                                           8.0                                              8.0                                                 8.0  8.0SrO              -- -- -- --  -- 3.0                               3.0                                  3.0                                     3.0                                        3.0                                           4.0                                              --ZnO                                 -- -- -- -- -- 1.0                                                 2.0  0.52As.sub.2 O.sub.3                    -- -- -- -- --  0.036                                                  0.036                                                       0.036__________________________________________________________________________                               57  58  59  60  61  62  63__________________________________________________________________________                           SiO.sub.2                               68.66                                   69.16                                       68.66                                           67.66                                               69.16                                                   68.16                                                       69.5                           Al.sub.2 O.sub.3                               9.1 9.1 9.1 9.1 9.1 9.1 9.0                           CaO 2.02                                   2.02                                       2.02                                           2.02                                               2.02                                                   2.02                                                        1.88                           BaO 2.95                                   2.95                                       2.95                                           2.95                                               2.95                                                   2.95                                                        2.82                           Na.sub.2 O                               9.37                                   9.37                                       9.37                                           9.37                                               9.37                                                   9.37                                                        7.45                           K.sub.2 O                               2.41                                   2.41                                       2.41                                           2.41                                               2.41                                                   1.41                                                        2.35                           P.sub.2 O.sub.5                               2.02                                   2.02                                       2.02                                           2.02                                               2.02                                                   2.02                                                       2.1                           B.sub.2 O.sub.3                               2.44                                   2.44                                       2.44                                           2.44                                               2.44                                                   2.44                                                       2.5                           PbO 2.0 1.0 1.0 1.0 3.0 2.0 1.0                           F   8.0 8.0 8.0 8.0 8.0 8.0 8.0                           ZnO 1.0 0.52                                       1.0 2.0 0.52                                                   0.52                                                       --                           As.sub.2 O.sub.3                                0.036                                    0.036                                        0.036                                            0.036                                                0.036                                                    0.036                                                       --__________________________________________________________________________ 
    
     Table II records the emulsion liquidus, the high temperature crystallization liquidus, and the low temperature crystallization liquidus, each expressed in terms of °C., determined on a number of the exemplary compositions listed in Table I utilizing laser reflectance measurements, along with ΔT, defined as the interval between the emulsion liquidus and the high temperature crystalline liquidus, and a visual appraisal of the density of the opacity exhibited by each. 
     Table III reports those three liquidus values, ΔT, and visual opacity appraisals for the same glasses as recited in Table II except that PbO is absent from the compositions thereof. The batches for the glasses (designated with an A) were compounded, mixed, melted, poured into slabs, and the slabs annealed in like manner to the above-described laboratory procedure. 
     
                       TABLE II______________________________________Ex-am-  Emulsion High Temp. Low Temp.ple  Liquidus Liquidus   Liquidus                            ΔT                                 Opacity______________________________________ 2   1380     1315       800     65   Medium 4   1300     1210       N.O.*   90   Poor 5   1370     1310       730     60   Dense 6   1440     1380       800     60   Dense12   1350     1275       700     75   Poor13   1360     1300       660     60   Medium14   1250     1200       N.O.*   50   Medium30   1380     1315       770     65   Very Dense31   1460     1400       880     60   Very Dense32   1400     1320       720     80   Poor-Medium40   1360     1305       770     55   Dense48   1440     1390       780     50   Dense51   1380     1300       890     80   Dense52   1370     1300       940     70   Dense61   1310     1230       820     80   Medium63   1375     1315       800     60   Dense______________________________________ *Not Observed 
    
     
                       TABLE III______________________________________Ex-am-  Emulsion High Temp. Low Temp.ple  Liquidus Liquidus   Liquidus                            ΔT                                 Opacity______________________________________ 2A  1380     1350       840     30   Medium 4A  1320     1290       N.O.*   30   Medium 5A  1380     1350       760     30   Dense 6A  1450     1430       820     20   Dense12A  1360     1330       720     30   Medium13A  1380     1350       650     30   Medium14A  1320     1300       N.O.*   20   Medium30A  1390     1370       800     20   Dense31A  1390     1375       880     15   Dense32A  1410     1390       775     20   Dense40A  1380     1350       790     30   Dense48A  1440     1415       800     25   Very Dense51A  1380     1360       880     20   Dense52A  1400     1360       900     40   Medium-                                 Dense61A  1320     1275       840     45   Medium63A  1385     1360       800     25   Dense______________________________________ *Not Observed 
    
     A comparison of Tables II and III illustrates that the inclusion of PbO raises the value of ΔT by at least 25° C. and sometimes over 50° C. It has been found that values of ΔT less than 40° C. generally result in the growth of apatite inclusions during the forming process (each of the glasses in Table III being subject thereto), accompanied with reduced mechanical and thermal strength. 
     Experience has also demonstrated that the presence of PbO in certain compositions may decrease the high temperature crystallization temperature to such a degree that opacity is reduced. That phenomenon is evidenced in a comparison of Examples 4 with 4A, 12 with 12A, and 32 with 32A, respectively. Although the mechanism underlying that phenomenon has not been fully characterized, the following explanation has been proposed. When ΔT becomes very large, the lowered glass viscosity at the high temperature crystallization liquidus results in lower diffusion rates for ions in the glass. That circumstance translates into reduced crystal growth and, consequently, decreased opacity. Increased density of opacity, however, can normally be readily restored via minor modifications in glass composition within the specified ranges of the constituents. The greatest opacity (without crystalline inclusions) appears to occur when ΔT is about 50°-60° C. 
     It can be observed that several of the exemplary compositions reported in Table I exhibited some Pb 3  (PO 4 ) 2  crystallization. The occurrence of that crystallization is believed to be due to low levels of CaO, SrO, and/or BaO, note, for example, Examples 44, 49, 50, 52, and 53. The reduced quantities of those components decreases the solubility of Pb 3  (PO 4 ) 2  in the glass. 
     In order to determine the resistance of the inventive glasses to detergent attack, the following tests were devised: 
     First, a 0.3% by weight aqueous solution of SUPER SOILAX detergent, marketed by Economics Laboratories, St. Paul, Minn., was heated to 95° C. and a sample of several of the exemplary compositions of Table I having the dimensions of 2&#34;×1&#34;×0.25&#34; immersed therein for 96 hours. Table II reports the results of those tests in terms of lead release (Pb Release) derived from analyzing the detergent solution after the test. The standard promulgated by the Federal Drug Administration for food contact surfaces is a maximum lead release of 7 PPM (parts/million). 
     Second, a 0.3% by weight aqueous solutions of SUPER SOILAX detergent was prepared and heated to 95° C. in like manner to the above test procedure. A sample of several of the exemplary compositions of Table I having the dimensions of 2&#34;×1&#34;×0.25&#34; was immersed into the solution for 96 hours, withdrawn from the solution, rinsed in tap water, and dried. At least a portion of the sample surface was coated with SPOTCHECK dye penetrant, marketed by Magnaflux Corporation, Chicago, Ill., and the dye allowed to rest thereon for 20 seconds. The dye was thereafter dried with a clean cloth and the surface then cleaned with a household cleansing powder for about 30 seconds. An (A) rating in the Detergent Test signifies that no stain was observed. A (B) rating indicates a light stain which can be removed with a cloth wetted with detergent. Table IV reports the results of tests conducted on several of the inventive glasses. 
     Also reported in Table IV are values of softening points (Soft. Pt.) in °C. where measured on several of the glasses. 
     
                                           TABLE IV__________________________________________________________________________   1  2  5  6  7  9  18 19 20 21__________________________________________________________________________Pb Release   1.0      1.12         1.0            1.2               0.92                  3.5                     1.6                        1.22                           1.3                              1.05Detergent Test   A  A  A  A  B  B  A  A  A  ASoft. Pt.   -- 835         -- -- -- 810                     802                        785                           771                              810__________________________________________________________________________   23 24 25 28 31 40 42 48 53 63__________________________________________________________________________Pb Release   1.3      1.0         0.85            1.0               1.1                  1.25                     0.90                        0.84                           1.6                              0.92Detergent Test   A  A  A  A  A  A  A  A  A  ASoft. Pt.   811      826         818            -- 810                  -- 824                        782                           848                              785__________________________________________________________________________ 
    
     The most preferred composition is Example 63 from the standpoint of automatic pressing processing.