High Temperature Glass

The word "Pyroceram" is a generalized brand name that Corning used with respect to its glass-ceramic products. There is a belief that it specifically refers only to the Corning Ware formula, but this is incorrect. In company literature Centura is stated to be "Fashioned from remarkable Pyroceram brand glass ceramic ..." and there is no doubt that Centura has a different composition than Corning Ware.

     Visions cookware was also called Pyroceram at times and the backstamps of Centura and Suprema restaurant ware also contain the word "Pyroceram". These products are not Corning Ware. So Pyroceram is just a brand name that encompasses the entire family of glass-ceramics rather than one specific formulation.

     "Pyroceram" is a family of different glass-ceramics. Corning Ware and Centura are two types of Pyroceram. Image from 1963 leaflet.

     The scientific field of glass-ceramics was first pioneered in the 1950s and quickly led to the introduction of Corning Ware in 1958. Corning Ware, Visions, Centura and Suprema are all glass-ceramics. This means they are materials that are glass in structure initially, but through heat treatment they become a ceramic.

     An article like a casserole or saucepan is formed from melted glass, then it is re-heated and cooled under controlled conditions. The heat treatment makes crystals grow within the glass, turning it into a ceramic and dramatically improving the strength of the item.

     The transformation from glass to ceramic is visible in the material's molecular structure. Glass has an amorphous structure, meaning that its atoms are certainly bound, but in irregular patterns. In contrast, the atoms of a ceramic are bound in very regular patterns, or crystals, i.e.: a crystalline structure.

     Heat treatment is not the only requirement for encouraging crystals to grow. The glass must contain an ingredient that provides nucleation during heat treatment, i.e.: an atom that the crystal can grow around, or its nucleus. Titanium Dioxide serves this purpose in most glass-ceramic formulations.

     Before heat treatment, Corning Ware is a transparent glass with an amber hue. Its appearance is very similar to amber Visions, but paler in colour. After heat treatment, it is almost perfectly white and very densely opaque.

     Opal Pyrex is also strengthened by crystallization, but it is not transformed sufficiently to be classified as a glass-ceramic. After the opal glass article is formed, it is re-heated in the annealing, or tempering, process by passing through a long oven known as a lehr and then cooled again.

     The amount of crystal growth in opal glass measures 10% or even less, and overall it is still glass because its structure is predominantly amorphous. Materials in the glass-ceramics family can attain crystallization of 50% or more.

     The substantial degree of crystallization in glass-ceramics translates to a huge gain in mechanical strength compared to ordinary annealed glass. The modulus of rupture of annealed glass ranges from 5000 to 10,000 psi, while this measurement for glass-ceramics is usually between 10,000 to 20,000 psi. Centura possesses even greater strength once it is glazed. The glaze acts as a compression layer, boosting its MoR to over 40,000 psi.

     Glass-ceramics also differ greatly from traditional ceramics. Clay products are formed from a mix that is cold and wet, then air dried, usually at room temperature, and fired to gain the crystalline structure that provides strength. But the strength of traditional ceramics does not compare with the extraordinary durability of glass-ceramics.

     The usual crystallization method for glass-ceramics creates an opaque product, and researchers were challenged with inventing a transparent version. Their first success came in 1966, but this clear colourless product did not reach the market due to a fear of hurting Pyrex and Corning Ware sales. There were also concerns that consumers would not be able to distinguish it from ordinary clear Pyrex. One proposed application for this glass-ceramic was a coffee percolator.

     The progress made in this earlier project led directly to Visions stovetop ware, an amber-hued transparent glass-ceramic. The official name of Visions glass is Calexium and it was developed at Corning in France. The product line became available in France in the late 1970s, and it entered the North American market in 1983.

Sources:

Introduction to Manufacturing Processes, John A. Schey

The Generations of Corning, Davis Dyer and Daniel Gross.

"Method of Making Ceramics and Product Thereof", Stanley D. Stookey, United States Patent 2,920,971

"Profiles in Ceramics: S. Donald Stookey", The American Ceramic Society Bulletin, March 2000

"Profiles in Ceramics: George H. Beall" The American Ceramic Society Bulletin, June 2000