Abstract:
Disclosed are ceramic compositions which are useful in the formulation and fabrication of microwave susceptors for disposable packages for the microwave heating of food items. The compositions include a novel microwave absorbing material and a binder. The novel microwave absorbing materials comprise ceramics with neutral lattice charges such as clays, kaolin, talc, silicates, alumina, aluminosilicates and mixtures thereof. The compositions provide good heat generation and a predeterminable upper temperature limit. The materials are common and inexpensive. Preferred compositions additionally include a temperature profile moderator which can be common salt.

Description:
This is a division of application Ser. No. 056,201, filed Jun. 1, 1987 U.S. Pat. No. 4,810,845. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Technical Field 
     This invention relates generally to the art of the microwave heating by high frequency electromagnetic radiation or microwave energy. More particularly, the present invention relates to ceramic compositions useful for fabrication in or into microwave heating susceptors, especially for disposable microwave packages for food products. 
     2. Background Art 
     The heating of food articles with microwave energy by consumers has now become commonplace. Such microwave heating provides the advantages of speed and convenience. However, heating breaded food with microwaves often gives them a soggy texture and fails to impart the desirable browning flavor and/or crispness of conventionally oven heated products due in part to retention of oil and moisture. Unfortunately, if microwave heating is continued in an attempt to obtain a crisp exterior, the interior is generally overheated or overdone. Moreover, the microwave fields in the ovens are uneven which can lead to unevenness or both hot and cold spots within food items or packaged food items being heated. 
     The prior art includes many attempts to overcome such disadvantages while attempting to retain the advantages of microwave heating. That is, the prior art includes attempts at providing browning or searing means in addition to microwave heating. Basically, three approaches exist whether employing permanent dishes or disposable packages to providing microwave heating elements which provide such browning or searing and which elements are referred to herein and sometimes in the art as microwave heating susceptors. In the art, materials which are microwave absorptive are referred to as &#34;lossy&#34; while materials which are not are referred to as &#34;non-lossy&#34; or, equivalently, merely &#34;transparent.&#34; 
     The first approach is to include an electrically resistive film usually quite thin, e.g., 0.00001 to 0.00002 cm., applied to the surface of a non-conductor or non-lossy substrate. In the case of a permanent dish, the container is frequently ceramic while for a disposable package the substrate can be a polyester film. Heat is produced because of the I 2  R or resistive loss (see for example, U.S. Pat. Nos. 3,853,612, 3,705,054, 3,922,452 and 3,783,220). Examples of disposable packaging materials include metallized films such as described in U.S. Pat. Nos. 4,594,492, 4,592,914, 4,590,349, 4,267,420 and 4,230,924. 
     A second category of microwave absorbing materials comprise electric conductors such as parallel rods, cups or strips which function to produce an intense fringing electric field pattern that causes surface heating in an adjacent food. Examples include U.S. Pat. Nos. 2,540,036, 3,271,552, 3,591,751, 3,857,009, 3,946,187 and 3,946,188. Such an approach is usually taken with reusable utensils or dishes. 
     A third approach is to form articles from a mass or bed of particles that become hot in bulk when exposed to microwave energy. The microwave absorbing substance can be composed of ferrites, carbon particles, etc. Examples of such compositions or articles prepared therefrom include, for example, U.S. Pat. Nos. 2,582,174, 2,830,162 and 4,190,757. 
     A review of the prior art, especially that art directed towards provision of heating susceptors for disposable packages for microwave heating of foods indicates at least three basic problems exist in the formulation and fabrication of heating susceptors. One difficulty with the third category of materials, generally, is that they can exhibit runaway heating, that is, upon further microwave heating their temperature continues to increase. Great care must be taken in fabrication of safe articles containing such materials. Metallized film materials of the first category can be formulated and fabricated such that they do not exhibit runaway heating. However, such films suffer from the second problem; namely that while their operating temperatures are quite hot, are at controlled temperature, and are sufficient to brown the surface of nearby food items, due to their thinness and little mass, only small quantities of heat are actually generated. Such materials are thus unsuitable for certain foods which require absorption of great amounts of heat in their preparation, e.g., cake batters. The third general problem is one of cost. Microwave susceptors frequently comprise costly materials. Also, fabrication of susceptor structures frequently is complex and expensive. 
     Accordingly, in view of the above-noted problems with present microwave susceptors, an object of the present invention is to provide a device which will heat under the influence of the microwave radiation up to an upper temperature limit at which temperatures the device comes to a steady state absorption of microwave energy and heating to a higher temperature is precluded. 
     Another object of the present invention is to provide a microwave heating device or susceptor which is disposable and adapted for use with pre-prepared foods. 
     A still further object of the present invention is to provide a heating device which can be utilized as a nondisposable utensil or tray. 
     A still further object of the present invention is to provide a heating device which by appropriate selection of manufacturing parameters can provide a predetermined upper temperature limit and moderate microwave heating of the food item through absorption and moderation of the microwave energy. 
     Another object of the present invention is to provide a heating device or utensil which is inexpensive to manufacture, safe to use and well adapted for its intended use. 
     Surprisingly, the above objectives can be realized and new compositions provided which overcome the problems associated with previous materials which have been used for the fabrication of microwave heating susceptors. The present compositions do not exhibit runaway heating yet generate relatively large amounts of heat. Indeed, the final heating temperature can be controlled quite closely. Also, the present compositions are comprised of materials which are commonly available and inexpensive. In the most surprising aspect of the present invention, the compositions comprise ceramic materials previously considered alone to be microwave transparent or used in microwave transparent ceramic compositions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a packaged food article for microwave heating constructed in accordance with the teachings of the invention; 
     FIG. 2 is a perspective view of the packaged food article with outer paperboard outerwrap opened and with an inner tray and sleeve shown disengaged; 
     FIG. 3 is a perspective view of the tray disengaged from the sleeve and holding several food pieces; 
     FIG. 4 is a perspective view of the tray with the food items removed showing a microwave heating susceptor raised above its resting position in the tray; 
     FIG. 5 is a cross sectional view of the tray taken in the direction of lines 5--5 of FIG. 3; 
     FIG. 6 is a perspective view of an alternate tray with a lid each fabricated from the present compositions with food items removed; 
     FIG. 7 is a perspective view of the alternate tray taken in the direction of lines 7--7 of FIG. 6. 
     FIGS. 8-12 depict time/temperature response curves for ceramic compositions exemplified in Examples 1-13. 
    
    
     SUMMARY OF THE INVENTION 
     The present invention provides ceramic compositions useful in the formulation and fabrication of microwave heating susceptors. The present compositions comprise a ceramic active microwave absorbing material and a binder. 
     The present microwave absorbing materials are common ceramic ingredients having a neutral lattice charge. The microwave absorbing materials can comprise about 2 to 99.1% of the ceramic compositions. The binders essentially comprise about 0.9 to 98% of the compositions. Conventional binder materials are suitable for use herein. 
     In its article aspect, the present invention resides in devices fabricated from the present compositions. Such devices are microwave heating susceptors generally in sheet form and which range in thickness from about 0.5 to 8.0 mm. In a preferred embodiment, the heating susceptor is in the form of a tray. The susceptors find particular usefulness in disposable packages for the microwave heating of food. 
     Throughout the specification and claims, percentages are by weight and temperatures in degrees Fahrenheit, unless otherwise indicated. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In its composition aspect, the present invention relates to ceramic compositions useful for fabrication into heating susceptors for disposable packages for the microwave heating of food products. The compositions comprise a defined microwave absorbing material and a binder. In its article aspect, the present invention resides in microwave heating susceptor for packaged food items, to packages for such items and to the packaged food items themselves. 
     The microwave absorbing materials useful herein surprisingly include a wide variety of ceramic materials previously regarded as microwave transparent or used in ceramic compositions transparent to microwaves. By ceramic materials are meant materials comprising oxygen attached to non-carbonaceous elements, and primarily to magnesium, sodium, calcium, iron, aluminum, silicon and mixtures thereof. 
     In the ceramic industry, a distinction is made between &#34;greenware,&#34; a ceramic composition before firing or vitrification, and the finished, fired or vitrified ceramic compositions prepared therefrom. The firing step profoundly changes a large number of the ceramic composition&#39;s properties as the individual constituents are fused into a homogeneous mass. Broadly speaking, the present invention is directed toward compositions which would be considered greenware in the ceramic arts. 
     Certain of the present microwave active materials have been used in greenware ceramic compositions, but generally at marketedly different concentrations and for different purposes than in the present invention. For example, kaolin reduces plasticity and tends to make the greenware mix short or lean. Likewise, alumina has a similar effect on the plasticity and will reduce green strength. Also, sodium metasilicate is not used at levels greater than 1% since greater amounts cause sticking and hinder mold release properties as well as decrease green strength. 
     The present ceramic microwave absorbing materials and their other general properties are well known and described generally, for example, in &#34;An Introduction to the Rock Forming Materials,&#34; by Deer, Howie, and Zussman, Longman Group Ltd., Essex, England, 1966 or in &#34;The Potter&#39;s Dictionary of Materials and Techniques&#34; by Frank and Janet Hamer, Watson-Guptill Publications (1986) and which is incorporated herein by reference. Materials as therein described are generally and conventionally classified as ortho and ring silicates, chain silicates, sheet silicates, framework silicates and non-silicates. However, the materials useful herein can fall into any of these classifications although not all materials in those classifications are useful herein. 
     As indicated above, the microwave absorbing materials useful herein surprisingly include a wide variety of ceramic materials previously regarded as microwave transparent. It is speculated herein that these materials have heretofore been unappreciated as being useful as consumer microwave absorbing materials since most investigations of their electromagnetic interactions, i.e., absorption/transparency has been done at very different frequencies or have been investigated as fired ceramics. The present materials are further essentially characterized by a neutral lattice charge. &#34;Neutral lattice charge&#34; is used herein in its conventional usage and means that the net relative electron surface charge densities of the material is essentially zero or that the cation exchange capability is essentially zero for the constituent chemical make-up of the ceramic material. The present ceramic materials are further characterized by relatively low electrical resistivity, i.e., about 0.1 to 35 ohm.cm and are thus classifiable as semiconductors in the broad sense of the term. 
     Exemplary specific materials include: 
     Sodium Metasilicate, Na 2  SiO 3  ; 
     Talc, Mg 6  [Si 8  O 20  ](OH) 4  ; 
     Kaolin, Al 4 .[Si 4  O 10  ](OH) 8 .4H 2  O; 
     Alumina and activated alumina, Al 2  O 3  ; 
     Clays (fine grained, natural, early argillareous materials); 
     Aluminosilicates; 
     Non-siliceous ceramics. 
     Of course, mixtures of these materials can also be used. Preferred materials include sodium aluminum silicate, clay, sodium metasilicate and kaolin and mixtures thereof due to the relatively flat or uniformity of their final heating temperature. 
     The present compositions include an effective amount of the above described microwave absorbing materials. The precise level will depend on a variety of factors including end use application, desired final temperature, and thickness of the susceptor to be fabricated from the present compositions. Good results are generally obtained when the microwave absorbing material comprises from about 0.1% to about 98% by weight of the present ceramic compositions. Preferred compounds include from about 20% to 98% by weight of the microwave absorbing material. For best results, the ceramic compositions comprise about 40% to 98% by weight of the microwave absorbing materials. The particle size of the microwave absorption material or refactory is not critical. However, finely ground materials are preferred inasmuch as the ceramic susceptors produced therefrom are smooth and uniform in texture. 
     Another essential component of the present ceramic compositions is a conventional ceramic binder. By the term &#34;ceramic binder&#34; is meant that the binder is capable of binding the present ceramic heating materials into a solid mass. The term is not meant to imply or require that the binder material itself is necessarily ceramic in composition although it well may be. Such ceramic binders are well known in the ceramic art and the skilled artisan will have no problem selecting suitable binder materials for use herein. The function of the binder is to form the particulate microwave absorbing material into a solid form or mass. Exemplary materials include both ceramic and plastic binders, respectively, such as cement, plaster of Paris, i.e., calcium sulphate, silica fiber, feldspar, Pulverized Kelvar® (a polyamide fiber), colloidal silicas, fumed silicas, fiberglass, wood pulp, cotton fibers, and mixtures thereof. The binder can comprise from about 2% to 99.9% by weight of the present ceramic compounds, preferably from about 20% to 80%. Exemplary, conventional plastic based binders, both thermoplastic and thermosetting, are described in U.S. Pat. No. 4,003,840 (issued Jan. 18, 1977 to Ishino et al.) which is incorporated herein by reference. 
     In one preferred embodiment, the present compositions include binders which are organic thermoplastic resins especially those approved as food packaging material such as polyvinyl chloride, polyethylene, polyamides, polyesters, polycarbonates, polyimides, epoxies, etc. In these embodiments, the thermoplastic resin binders can range from as little as 20% up to 60% of the composition and preferably about 30% to 50%. Such compositions are especially well suited for fabrication into shaped microwave susceptors, especially food trays, e.g., for TV dinners or entrees. 
     In one highly preferred embodiment, the present ceramic compositions additionally desirably comprise common salt or sodium chloride as a temperature profile modulator. The temperature profile modulator can assist in reaching more quickly the final operating temperature of the ceramic composition. Also, the salt increases modestly the final operating temperature of the ceramic composition. The preferred ceramic compositions comprise from about 0.1% to about 6% by weight salt. While ceramic compositions can be formulated having higher amounts of salt, no advantage is derived therefrom. 
     The present ceramic compositions can be fabricated into useful microwave heating susceptor articles by a simple admixture of the materials into a homogeneous blend, and addition of sufficient amounts of water if needed to hydrate the binder. When plaster of Paris is used as the binder, typically, water will be added in a weight ratio to binder ranging from about 0.4 to 0.7:1. While the wet mixture is still soft, the ceramic compositions can be fabricated into desirable shapes, sizes and thicknesses and thereafter allowed to harden or dry to a moisture content ranging from about 2.5% to 10%. 
     Of course, one advantage of the present invention is that upon heating in a conventional microwave oven, e.g., 2450 MHz, the ceramic compositions will relatively quickly (e.g., within 30 to 300 seconds) heat to a final temperature ranging from about 300° to 600° F. which temperature range is very desirable in providing crisping, and browning to foods adjacent thereto and consistent with safe operation of the microwave oven. 
     Another advantage of the present ceramic compositions is that they can be dried at temperatures above 180° F. Still another advantage of the present invention is that susceptors fabricated from the present ceramic compositions provide a microwave field modulating effect, i.e., evening out peaks and nodes, i.e., standing wave points and, it is believed independent of wattage. This benefit is especially useful when sensitive foods such as cookie doughs are being microwave heated. 
     Still another advantage of the present ceramic compositions is that they are believed to be useful not only with microwave ovens operating at 2450 MHz but at all microwave frequencies, i.e., above as low as 300 MHz. 
     Another advantage of the present invention is that the ceramic compositions can absorb oil and/or moisture from food items to be microwave heated, e.g., par-fried fish portions, without substantial adverse affect on heating performance. 
     It is important that the susceptors fabricated herein be unvitrified, i.e., not subjected to a conventional firing operation generally above 800° F. to 1000° F. (426° C. to 538° C.). Conventional firing can result in a fused ceramic composition substantially transparent to microwave and thus devoid of the desirable microwave reactive properties of the present invention. 
     The present ceramic compositions are useful in any number of microwave absorption applications. The present ceramic compositions are particularly useful for fabrication into microwave susceptors which in turn are useful as components in packages for foods to be heated with microwaves. 
     For example, FIG. 1 illustrates generally a packaged food item 10 fabricated in accordance with the teachings of the present invention and suitable for microwave heating. FIG. 2 shows that the article 10 can optionally comprise a six-sided outerwrap 12 which can be plastic, paper or other conventional packaging material such as the paperboard package depicted. The article can further comprise an inner assembly 14 disposed within the outerwrap 12 which can comprise a sleeve 16 fabricated from a dielectric material (e.g., cardboard, paper, polyester) and disposed therein a tray 18. In conventional use, the consumer will open the article 12, remove and discard the overwrap 12, and insert the entire assembly into the microwave oven. The sleeve 16 is helpful although not esssential not only to prevent splattering in the microwave oven, but also to assist in securing the food items against excessive movement during distribution. 
     In FIG. 2, it can be seen that the sleeve 16 can comprise an opposed pair of open ends, 20 and 22, an upper major surface or top wall 24, a lower major surface or bottom wall 26 and an opposed pair of minor side or wall surfaces 28 and 30. As can be seen in FIG. 3, the tray 18 holds or contains one or more food items 32. FIG. 4 shows the tray 18 with the food items 32 removed. Disposed within the tray 18 is one or more microwave heating susceptors such as microwave susceptor heating panel 34. In this preferred embodiment, the susceptors are generally flat or planar and range in thickness from 0.020 to 0.250 inch. 
     Still referring to FIGS. 3 and 4, with the cooking of certain foods, it may be desirable to heat the food items 32 from only or primarily one side by use of the heating susceptor panel 34 while at the same time minimizing the heating of the food item 32 by exposing it to microwave radiation through the walls of the package assembly 14. To allow microwave radiation to reach the susceptor 34, the bottom wall 26 is microwave transparent at least to the extent that sufficient microwave energy can enter the package to heat the susceptor 34. Side walls 28 and 30 can each optionally be shielded with shielding 29 as can top wall 24 thereby restricting the entry of microwave radiation through these walls to the food product as is known in the art. The shielding 29 can be of any suitable type material of which aluminum foil is a currently preferred material. With the use of shielding, the microwave radiation penetrates the microwave transparent bottom 26 only. Accordingly, cooking of the food product 32 in this embodiment is accomplished substantially totally by the heat transferred to the food product 32 from the susceptor 34 although some microwave entry through the open ends 20 and 22 occurs. It is pointed out that the terms microwave transparent and microwave shield are relative terms as used herein and in the appended claims. 
     In FIG. 5, it can be seen that the heating panel 34 can optionally comprise a thin finish layer 36, e.g., 0.00005 to 0.001 inch (0.001 to 0.025 mm) to impart desirable surface properties, e.g., color, water repellency, smooth appearance, stick free, etc. In the simplest form, such a layer can comprise ordinary paraffin or a sodium silicate polymerized with zinc oxide. The finish layer does not substantially adversely affect the performance of the microwave susceptor. Such surface property modification finds particular usefulness when the microwave susceptors are used in medical settings. For example, it is known to fabricate surgical implants, e.g.. discs, cylinders, from ferrites which absorb microwave radiation to thermally treat tumors. In such applications wherein the present compositions are employed, water repellency may be particularly desirable. 
     Other types of packages can be utilized with the ceramic microwave heater compositions of the present invention. It is an important advantage that the present compositions can be fabricated into susceptors of different configurations whether regular, e.g., corrugated, or irregular. 
     Another embodiment is depicted in FIG. 6. Thermoplastic resins are preferred for use as the binder materials. In this embodiment, the article 10 in addition to outerwrap 12 as shown in FIG. 2 can comprise a microwave heating susceptor 40 fabricated into trays or shallow pans whether square, rectangular, circular, oval, etc. which serve both to contain and heat the food items. Such tray shaped susceptors 40 find particular suitability for use in connection with a batter type food item 44, especially cake batters or with casseroles, baked beans, scalloped potatoes, etc. In one particular embodiment the tray 40 can additionally include a cover 42 also fabricated from the present ceramic compositions. Trays 40 with covers 42 are especially useful for batter food items like brownies in which it is desired to form an upper or top skin to the food item 44. 
     In still another embodiment shown in FIG. 5A, the panel susceptor 34 can additionally comprise a backing layer(s), especially a metal foil, e.g., aluminum 46. The foil serves to reflect back to the susceptor 34 microwave energy passing through the susceptor 34. The incorporation of a microwave shielding or reflecting layer 29 in close proximity on the opposite surface of the ceramic susceptor 34 also serves to act as a susceptor temperature booster to elevate the operating temperature substantially above the temperature obtained without a microwave shielding or reflective layer 29. Final temperature reached can be as high as 100° F. or more over similar structures without the metal foil. Also, the use of the temperature booster can reduce the need for a thicker ceramic susceptor to obtain the same temperature thereby reducing both production costs as well as final weights of the microwave package. Since the ceramic compositions adhere to the metal foil with some difficulty and cause an in heating interference due to conductor-wave phenomena interaction, it is preferable to treat the surface of the metal foil with an intermediate or primer layer (not shown) for better adherency, i.e., ordinary primer paints, or to have an intermediate silicone layer, paper layer or other polymer layer, or to select those binders for the ceramic compositions with increased capacity to adhere to metal foils. 
     The skilled artisan will also appreciate that the present compositions absorb microwave radiation at a wide range of frequencies and not merely at those licensed frequencies for consumer microwave ovens. 
     The ceramic susceptor compounds of the present invention can also be utilized in non-disposable utensils adapted for a limited number of repetitive heating cycles by embedding the heating compositions or otherwise associating with a non-disposable utensil body. The susceptor is associated with the remainder of the utensil in a manner such that it will be in heat transfer relation to a product to be heated in or on the utensil. The utensil can be in the form of an open top dish, griddle or the like. However, the present compositions will exhaust their ability to heat upon microwave exposure relatively quickly, i.e., after only a few cycles of operation. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure whatsoever. It will be appreciated that other modifications of the present invention, within the skill of those in the food arts, can be undertaken without departing from the spirit and scope of this invention. 
     EXAMPLE 1 
     Sodium metasilicate pentahydrate (100 grams) was mixed with 25 grams deionized distilled water, cast into a mat 5/32 inch (0.160 inch) thick and air dried overnight at 85° F. (29.4° C.). During drying the tile exhibited no shrinkage or breaking. Cast tile weight 3.5&#34;×3.5&#34;×5/32&#34;  was 31.21 grams, density 0.992 g cm -3 . The tile subjected to a 750 watt, 2460 MHz microwave field for a period of five minutes while the temperature of the tile surface was monitored using a Luxtron 750® Fluoroptic temperature monitor equipped with ceramic clad fiber optic temperature probes and interfaced with an IBM PC/AT computer for data collection and handling. The recorded temperature profile of the tile is shown in FIG. 8 as line 1. 
     EXAMPLE 1A 
     Sodium metasilicate pentahydrate was mixed with sufficient distilled water to form a cohesive mass (10% moisture). The mixture was compressed into a disc 2.969 inches (7.540 cm) diameter and 0.160 inch thick. The disc weight was 22.44 grams, density 1.236 g cm -3 . After air drying overnight at 85° F. (29.4° C.) the temperature profile was determined as described above in Example 1. The temperature profile of the tile is not shown but is very similar to line 1 in FIG. 8. The dielectric constant at 20° C. and 1000 MHz is 11.3 with a dissipation factor D or loss tangent &#34;tan δ&#34; of 0.227. 
     EXAMPLE 2 
     100 grams of sodium metasilicate pentahydrate was mixed with 30 grams of calcium sulfate hemihydrate and after blending to a uniform mix 30 grams of distilled water was added. The resulting mix was stiff and displayed a positive heat of reaction (exothermic). The mass was cast into tiles 3.5&#34;×3.5&#34;×0.175 inches and air dried at 85° F. (29.4° C.) for 24 hours. The cast tile weight was 42.58 grams, density 1.212 g cm -3 . The tile did not display cracking or mold shrinkage. The tile was treated as described in Example 1 with the recorded temperature profile shown as line 2 in FIG. 8. Weight loss upon heating was 29.76%. 
     EXAMPLE 2A 
     To the dry mix prepared in Example 2 was added 11.0 grams distilled water so as to form a cohesive mass upon compression. The mixture was then compressed into a disc 3.00 inches (7.620 cm) diameter and 0.130 inch thick. The disc weight was 19.98 grams, density 1.327 g cm -3 . After air drying at 85° F. (29.4° C.) for 24 hours, the temperature profiles of the tile in a 2460 MHz microwave field was determined as described in Example 1. The temperature profile of the tile is similar to that shown as line 2 in FIG. 8. The dielectric constant at 20° C. at 1000 MHz is 12.1 with a loss tangent &#34;tan δ&#34; of 0.125. Weight loss upon heating was 19.9%. 
     EXAMPLE 2B--CAST 
     100 grams of sodium metasilicate pentahydrate was mixed with 30 grams of calcium sulfate hemihydrate, 40 grams of Hawthorn Bonding Fireclay and 40 grams of A. P. Green Fireclay. After blending to a uniform mix 210 grams of distilled water was added. The resulting mix was plastic and easily workable. The mass was cast into tiles 3.5&#34;×3.5&#34;×0.125 inches and air dried at 85° F. (29.4°hours. The cast tile weight was 31.61 grams, density 1.259 g cm -3 . The tile did not display cracking or mold shrinkage. The heating structure was treated as described in Example 1 with the recorded temperature profile shown as line 2B in FIG. 9. 
     EXAMPLE 2C--PRESSED 
     To 69 grams of dry mix as prepared in Example 2B was added 15.0 grams of distilled water. The resulting damp mix was compressed into a disc 3.00 inches (7.620 cm) diameter and 0.135 inches thick. The disc weight was 24.0 grams, density 1.530 gm cm -3 . After drying in warm air at 85° F. (29.4° C.) for 24 hours, the temperature profile of the heater tile was determined as previously outlined. The temperature profile of the tile is shown as line 2C in FIG. 9. 
     EXAMPLE 3A--CAST 
     100 grams of calcined activated high alumina X-5111 (Englehard Corporation, Edison, N.J. 08818) was dry blended with 40 grams of magnesium silicate (Ceramitalc HDT, R. T. Vanderbilt Company, Inc., Norwalk, Conn. 06855). 65 grams of distilled water was added and a slurry prepared. The slurry was cast into 31/2 inch square tile frames 0.125 inches thick and allowed to dry at 120° F. (48.9° C.) for 12 hours. The resulting tile was cracked but exhibited minimal mold shrinkage. The tile was measured for heating performance in a microwave field as previously detailed. The temperature profile of the heating structure is shown in FIG. 8 as line 3C. Weight loss upon heating was 3.2%. 
     EXAMPLE 3B--PRESSED 
     A second dry mix was prepared as detailed above with 20 grams of distilled water. The resulting mix was compressed into a 3.00 inch (7.620 cm) disc, 0.125 inches thick with a density of 1.920 g cm -3 . Evaluation for heating performance was made after drying at 120° F. (48.9° C.) for 24 hours. The heating profile is shown in FIG. 8 as line 3P. Weight loss upon heating was 3.3%. The dielectric constant at 20° C. and 1000 MHz is 11.7 with a loss tangent &#34;tan δ&#34; of 0.172. 
     EXAMPLE 4--CAST 
     75 grams of calcined activated high alumina X-5111 (Englehard Corporation) was dry blended with 75 grams of air floated kaolin #6 tile (Georgia Kaolin Company, Inc., Union, N.J. 07083) and 13 grams of Q-Fiber Amorphous High Purity Silica Fiber (Johns-Manville, Denver, Colo. 80217). 84 grams of distilled water was added and a paste prepared. The paste was cast into 3.5 inch square×0.125 inch tiles and dried at 200° F. (93.3° C.) for 1 hour. The resulting tile was intact and displayed a 13.8% shrinkage. Tile weight was 25.99 grams, density 1.201 g cm -3 . The microwave performance of the heater tile is shown in FIG. 10 as line 4C. Weight loss upon heating was 2.3%. 
     EXAMPLE 4--PRESSED 
     A second dry mix was prepared as detailed above with 24 grams distilled water. The resulting mix was compressed into a 3.00 inch (7.620 cm) disc, 0.125 inches thick, density 1.833 g cm -3 . Evaluation for heating performance was made after drying at 200° F. (93.3° C) for 5 hours. The heating performance is shown in FIG. 10 as line 4P. Weight loss upon heating was 1.5%. The measured dielectric constant at 20° C., and 1000 MHz is 11.1 with a loss tangent &#34;tan δ&#34; of 0.147. 
     EXAMPLE 5 
     5.0 grams of sodium metasilicate pentahydrate, 30.0 grams calcium sulfate hemihydrate, 10.0 grams of calcined activated high alumina X-5111 (Englehard Corporation), 35.0 grams Kentucky Clay #6 (Kentucky-Tennessee Clay Company, Mayfield, Ky.), 50.0 grams Hexafil--a semi-reinforcing clay (Hammill and Gillespie, Inc. Livingston, N.J.) and 7.5 grams of Goldart--Cedar Heights air floated secondary clay (Minnesota Clay, Bloomington, Minn.) were dry blended together to a uniform consistency. 62 grams of distilled water was added to the dry powder mix and a paste formed upon mixing. The paste was cast into 3.5 inch square by 0.125 inch thick tiles and dried for 8 hours at 150° F. (65.6° C.). The resulting tiles were intact and displayed a 23.4% shrinkage upon drying. The tile weight was 27.58 grams, density 1.435 g cm -3 . The microwave performance of the heater tile is shown in FIG. 10 as line 5C. 
     A second dry mix was prepared as detailed above with 25.8 grams of distilled water added to the mix. The resulting mix was compressed into a 3.00 inch (7.620 cm) disc, 0.125 inches thick, density 1.554 g cm -3 . Evaluation for heating performance in a microwave field was made after drying at 150° F. (65.6° C.) for 8 hours. The measured heating profile is shown in FIG. 10 as line 5P. 
     EXAMPLE 5A--CAST 
     A formulation similar to the one prepared in Example 5 was prepared with the following modifications. 15 grams of calcined activated alumina X-5111 (Englehard Corporation), 30 grams of Kentucky Clay #6 (Kentucky-Tennessee Clay Company, Mayfield, Ky.) and 7.5 grams of Yellow Banks #401 air floated clay (Minnesota Clay, Bloomington, Minn.) were dry blended with the other ingredients. 65 grams of distilled water was added to the dry powder mix and a paste formed upon mixing. The paste was cast into 3.5 inch square by 0.125 inch thick tiles and air dried for 8 hours at 150° F. (65.6° C.). The resulting tiles were intact and exhibited a 21.9% shrinkage upon drying. The tile weight was 27.21 grams, density 1.388 g cm -3 . The microwave performances of the heater tile is shown in FIG. 11 as line 5-1. 
     EXAMPLE 5B--PRESSED 
     A second dry mix was prepared as detailed above with 25.8 grams of distilled water added to the mix. The resulting mix was compressed into a 3.00 inch (7.620 cm) disc, 0.125 inches thick, density 1.498 g cm -3 . Evaluation for heating performances in a microwave was made after drying for 8 hours at 150° F. (65.6° C.). The measured heating profile is shown in FIG. 11 as line 5-2. 
     EXAMPLE 6 
     5.0 grams of sodium metasilicate, 30 grams calcium sulfate hemihydrate, 15 grams of calcined activated high alumina X-5111 (Englehard Corporation), 80 grams of Tennessee Clay #6 (Kentucky-Tennessee Clay Company, Mayfield, Ky.) and 7.5 grams of Hawthorn Bonding Fireclay (Minnesota Clay, Bloomington, Minn.) were dry blended together to a uniform consistency. 70 grams of distilled water was added to the dry powder and a paste formed upon mixing. The paste was cast into 3.5 inch square by 0.125 inch thick tiles and dried for 8 hours at 150° F. (65.6° C.), the resulting tiles were intact and displayed a 7.0% shrinkage upon drying. The tile weight was 28.34 grams, density 1.215 g cm -3 . The microwave performance of the heater tiles is shown in FIG. 11 as line 6C. 
     A second dry mix was prepared as detailed above with 26.0 grams of distilled water. The resulting damp mix was compressed into a 3.0 inch (7.620 cm) disc, 0.110 inches thick, density 1.694 g cm -3 . Evaluation for heating performance in a microwave field was made after drying at 150° F. (65.6° C.) for 8 hours. The measured heating profile is shown in FIG. 12 as line 6P. As discernible from the shown profiles, a pressed embodiment in this example is preferable to the cast embodiment due to the plateauing profile shape observed. 
     EXAMPLE 7 
     50 grams of sodium metasilicate pentahydrate, 30 grams of calcium sulfate hemihydrate, 10 grams of Hawthorn Bonding Fireclay and 50 grams of sodium aluminum silicate were dry blended together to a uniform consistency. 70 grams of the dry mix was added with stirring to 35 grams of distilled water. The resulting paste was cast into a 3.5 inch square by 0.125 inch thick tile and dried for 8 hours at 150° F. (65.6° C.). The tile exhibited no shrinking or cracking upon drying. The microwave performance of the heater tile is shown in FIG. 12 as line 7C. 
     To the remaining 70 grams of dry mix as prepared above, 13 grams of distilled water was added. The damp mix was compressed into a 3.0 inch (7.620 cm) disc, 0.110 inches thick, density 1.726 g cm -3 . Evaluation for microwave heating performance was made after drying at 150° F. (65.6° C.) for 8 hours. The measured heating profile is shown in FIG. 2 as line 7P. 
     EXAMPLE 8 
     50 grams of Tennessee #6 Clay, 50 grams of Hawthorn Bonding Fireclay, 20 grams of calcined activated high alumina X-5111 and 25 grams of sodium aluminum silicate were dry blended to a uniform consistency. To 70 grams of the dry mix was added 35 grams of distilled water, after mixing the resultant paste was formed into 3.5 inch square by 0.125 inch thick tiles and dried for 2 hours at 150° F. (65.6° C.). The tile displayed no shrinking or cracking after drying. Tile weight was 27.78 grams, density 1.107 g cm -3 . The microwave heating performance of the tile is shown in FIG. 9 as line 8C. 
     To the remaining 75 grams of dry mix prepared above was added 15 grams of distilled water with mixing. The damp mix was then compressed into discs 3.00 inches (7.620 cm) diameter and 0.110 inches thick, density 1.723 g cm -3 . The discs were dried as described above and evaluated for microwave heating performance in the usual manner. The heating curve is shown in FIG. 12 as line 8P. 
     EXAMPLES 9-13 
     Ceramic compositions were prepared having the compositions indicated in the following table: 
     
         ______________________________________         Amount (grams)Component       9        10    11    12  13______________________________________Sodium metasilicate             5Tennessee Clay #6        10    30    10Hexafil                  10X-5111 calcined bauxite  10    15        15Hawthorn Bonding Fireclay           20       10    15    30  25A.P. Green Fireclay           20       10          10Goldart-Cedar Heights Clay           20       10    15    20  20Yellow Banks 401           20       10           5  10Old Hickory Ball Clay    10     5     5Redart Cedar Heights Clay           20              5Nytal ® Talc         10              25Georgia Kaolin #6 Tile Clay           20             10    50  25Cornwall Stone           10Gerstley Borate 20       10     5Sodium aluminum silicate 20    10        15Feldspar                 20           5Kelvar ® Fiber (pulverized)                          10______________________________________ 
    
     Seventy grams of the above mixtures were each separately mixed with 35 g deionized distilled water, individually cast or pressed into a mat 0.160 inch thick and air dried overnight at 85° F. (29.4° C.). The heat profiles are shown in FIGS. 13-15 with &#34;c&#34; indicating cast and &#34;p&#34; indicating pressed.