As was explained in U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, the production of such articles contemplates three fundamental steps. First, a glass-forming batch, to which a nucleating agent is commonly added, is melted. Second, this melt is simultaneously cooled to an essentially crystal-free glass and a body of a desired geometry shaped therefrom. Third, the glass body is subjected to a controlled heat treatment which causes the glass body to crystallize in situ. Normally, this third step is conducted in two parts. Thus, the glass is first heated to a temperature somewhat above the transformation range of the glass to cause the development of nuclei therein. Subsequently, the nucleated glass is heated to a higher temperature, frequently above the softening point of the glass, to effect the growth of crystals on the nuclei.
Inasmuch as a glass-ceramic article results from the substantially simultaneous growth of crystals on countless nuclei dispersed throughout the parent glass, the microstructure thereof consists of fine-grained crystals of relatively uniform size, homogeneously dispersed and randomly oriented in a residual glassy matrix. Glass-ceramic articles are generally highly crystalline, i.e. greater than 50% crystalline, such that the physical properties thereof are more closely akin to those of the crystal phase than to those exhibited by the residual glassy matrix. Furthermore, the residual glassy matrix will commonly have a far different composition from that of the original or parent glass body because the constituents composing the crystal phase will have been removed therefrom.
Glass-ceramic compositions have found their greatest utility to date in the fields of dinnerware and culinary ware; an example of the former being CENTURA.RTM. dinnerware and that of the latter being CORNING WARE.RTM. cooking vessels, both being products of Corning Glass Works, Corning, N.Y. More recently, flat sheeting of glass-ceramic material has been utilized as cooking surfaces for stoves, e.g., THE COUNTER THAT COOKS.RTM., also a product of Corning Glass Works.
In the field of cooking ware, and in particular the use of flat sheeting for stove top applications, it has been recognized that compositions exhibiting good transmission to infra-red radiation could be useful in improving the rate at which food could be cooked on top of the stove. Thus, the heat from the burner source underneath would pass more quickly through the heating surface. However, such an application requires a material having a complex matrix of physical properties. Hence, the material must be mechanically strong to withstand substantial impacts; it must be chemically durable and stain resistant to withstand attack by food contact; and it must demonstrate a uniformly low coefficient of thermal expansion to withstand sharp thermal shocks. These properties are, of course, in addition to the melting and forming capabilities demanded for practical large-scale production techniques. Finally, the composition must be such that the parent or precursor glass body crystallizes uniformly in situ to a fine-grained body exhibiting homogeneous properties.
To date, the glass-ceramic bodies utilized for flat cooking surfaces have commonly been opaque to visible radiations and very poorly transmitting in the infra-red portion of the radiation spectrum. Examples of such materials have been prepared in accordance with U.S. Pat. Nos. 2,920,971, 3,148,994, and 3,582,371. To achieve the desired low coefficient of thermal expansion, compositions have been designed to yield beta-spodumene solid solution as the principal crystal phase. Although the classic formula for spodumene is Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2, the crystals developed in the glass-ceramic articles do not comply exactly with that formulation. However, the X-ray diffraction pattern yielded by the crystals very closely approximates that of the classic spodumene. Therefore, the crystals have been deemed to be a solid solution Li.sub.2 O.Al.sub.2 O.sub.3.nSiO.sub.2, wherein "n" can range from about 3.5 to 10. This, then, is the sense in which the expression "beta-spodumene solid solution" has been employed.
The simple ternary Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 nucleated with TiO.sub.2 is very difficult to melt and form properly, so various additions have been made thereto in order to improve those properties without deleteriously affecting the physical characteristics thereof. For example, additions of such conventional fluxes as Na.sub.2 O, K.sub.2 O, and B.sub.2 O.sub.3 can unduly raise the coefficient of thermal expansion and/or impair the chemical durability and/or reduce the refractoriness of the final product. This has resulted in the use of the alkaline earth metal oxides and, particularly, MgO and/or CaO. Such additions did, indeed, improve the melting and forming capabilities of the compositions while not substantially altering the physical properties of the final product. However, the crystallized articles were essentially opaque to infra-red radiations.