U.S. Pat. No. 2,920,971, the fundamental disclosure in the field of glass-ceramics, describes the manufacture of such bodies as requiring three general steps. First, a glass-forming batch, normally containing a nucleating agent, is melted. Second, the resulting melt is simultaneously cooled to a substantially crystal-free glass and an article of a desired configuration shaped therefrom. Third, this glass article is exposed to a specifically-defined heat treatment which causes the article to crystallize in situ. As is also explained in that patent, this crystallization heat treatment is commonly undertaken in two steps. Hence, the glass article is initially heated to a temperature somewhat above the transformation range of the glass to promote the development of nuclei therein. Thereafter, the nucleated glass is heated to a higher temperature, often above the softening point of the glass, to cause the growth of crystals on the nuclei.
Because a glass-ceramic article is derived through the essentially simultaneous growth of crystals on innumerable nuclei dispersed throughout the parent glass, the microstructure thereof comprises fine-grained crystals of relatively uniform size, homogeneously dispersed and randomly oriented in a residual glassy matrix. Glass-ceramic articles are normally highly crystalline, i.e., greater than 50% by volume crystalline. In view of this, the physical properties of such articles are more closely similar to those of the crystal phase than to those demonstrated by the residual glassy matrix. In addition, the residual glass will generally have a very different composition from that of the original or parent glass inasmuch as the components constituting the crystal phase will have been removed therefrom.
Glass-ceramics have been employed extensively in the manufacture of culinary ware. Thus, cooking vessels have been marketed under the trademark CORNING WARE and flat sheeting that has been utilized as cooking surfaces for stoves has been marketed under the trademark THE COUNTER THAT COOKS, both of these being the products of Corning Glass Works, Corning, New York.
In the field of culinary ware, and in particular the manufacture of cooking vessels, market surveys have indicated a desire for transparent utensils. Thus, transparent glass cooking vessels have been marketed under the trademark PYREX for many years. However, many glass-ceramic compositions are intrinsicly stronger and exhibit lower coefficients of thermal expansion than the borosilicate glass constituting the basis for the PYREX brand articles. Numerous transparent glass-ceramic compositions have been developed in the past but such have not found success in the marketplace for culinary ware.
To be useful for the application, not only must the material be mechanically strong, transparent, and have a low coefficient of thermal expansion, but it also must be chemically durable and resistant to food staining. Furthermore, the material as a glass must exhibit the necessary capabilities for large scale melting and forming. Thus, the commercial product must not only demonstrate desirable chemical and physical properties, but also must be amenable to practical, high speed production techniques. It is on this latter factor that many of the former candidates for cooking vessels have failed. For example, extremely high batch melting temperatures have been required, the glass derived from the melt has been very unstable, the viscosity of the glass has been such as to render it difficult to work and form, fire-polishing of the glass has not been successful, etc. In essence, previous transparent glass-ceramic compositions did not present the combination of production characteristics which is a practical necessity for conventional large scale melting and forming techniques.
The glass-ceramic materials, such as the CORNING WARE culinary ware noted above, which have been utilized in cooking vessels have customarily been opaque to visible light and very poorly transmitting in the infra-red portion of the radiation spectrum. An exception to those materials is disclosed in Ser. No. 603,544, filed Aug. 11, 1975 by the present applicant. The glass-ceramic articles described therein are opaque to visible light but have exhibited transmittances of up to 60% of radiations having a wavelength of 3.5 microns in samples having 4.25 mm. cross sections. The compositions thereof are encompassed within a very circumscribed range of the Li.sub.2 O--ZnO--Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 system wherein beta-spodumene solid solution is the predominant crystal phase.
The hexagonal trapezohedral modification of SiO.sub.2, viz., beta-quartz, is recognized to exhibit a slightly negative coefficient of thermal expansion. The crystal also demonstrates very low birefringence, i.e., optical anisotropy. This combination of characteristics has led to the development of transparent glass-ceramic bodies wherein the primary crystal phase is a solid solution having a beta-quartz structure (also frequently denominated as beta-eucryptite solid solution). The basis of the solid solution is considered to be the replacement of some of the silicon ions in beta-quartz with aluminum ions, with the concomitant charge deficiency being compensated with the entry of lithium, magnesium, and/or zinc ions into the quartz structure.
U.S. Pat. No. 3,157,522 discloses the production of glass-ceramic articles demonstrating transparency to visible light having compositions within the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 -- TiO.sub.2 quaternary. However, those compositions are difficult to melt and form properly. This has led to the addition of modifying ingredients thereto which would improve those characteristics while not adversely affecting the physical properties of the final product to any great extent. The introduction of such conventional fluxes as Na.sub.2 O, K.sub.2 O, and B.sub.2 O.sub.3 can raise the coefficient of thermal expansion excessively and/or impair the chemical durability and/or reduce the high temperature capability of the crystalline article. In an effort to avoid those phenomena, alkaline earth oxides have been added (note Examples 10-14 in Table II of the patent). The use of these components will improve the melting and forming capabilities of the parent glasses. but has the deleterious side effect of severely decreasing the transmittance of the final product to infra-red radiations.