What are Glass-Ceramics?

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, Centura and Suprema are examples of glass-ceramics.  To put it briefly, they are materials that are initially glass in structure, but after heat treatment they become a ceramic.  For example, an item such as a casserole is first formed from melted glass, then it is re-heated and cooled under controlled conditions.  It is the heat treatment which grows crystals within the glass turning it into a ceramic which vastly improves the strength of the item.  

The transformation from glass to ceramic is visible in the material’s atomic 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; it has a crystalline structure.  The heat treatment of the glass article is not the only thing required to encourage the crystals to grow, however.  The glass must contain an ingredient which provides nucleation; i.e. an atom which kick-starts the growth of a crystal during the heat treatment.  Titanium Dioxide serves this purpose, and it has the added feature of being brilliantly white as well. 

Opal Pyrex is strengthened in a similar manner.  The item is made of glass and then re-heated in the annealing (or tempering) process by passing through a long 'oven' known as a lehr.  The glass is strengthened by virtue of a certain amount of crystallization occurring during annealing.  The amount of crystal growth in opal glass can be measured at 10% or even less, and overall it is still glass since its structure is predominantly amorphous.  For materials in the glass-ceramics family the amount of crystallization achieved can be 50% or higher.  

This degree of crystallization translates to a huge gain in terms of mechanical strength for glass-ceramics when compared to annealed glass.  The modulus of rupture of annealed glass is in the range of 5000 to 10 000 psi, while this measurement for glass-ceramics is usually between 10 000 to 20 000 psi.  Centura exhibits phenomenal 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 more traditional and natural types of ceramics.  Clay items for example, are formed from a mix that is basically cold and wet.  This is in contrast to forming items from melted glass.  Clay articles are air dried, usually at room temperature, and then fired to gain the crystalline structure that provides its strength.  This is not to say that the strength of traditional ceramics is anywhere near that of glass-ceramics.

The word 'Pyroceram' is a generalized brand name that Corning used with respect to its glass-ceramic products.  It has been commonly believed that it specifically refers to the Corning Ware formula, but in the literature Centura is described as "Fashioned from remarkable Pyroceram brand glass ceramic..."  It is certain that Centura has a different composition than Corning Ware, and even Visions, a clear glass-ceramic, was sometimes called Pyroceram too.  So obviously, Pyroceram is just a brand name invented to encompass the family of glass-ceramics, and it is not a specific glass formula.  Centura products made for restaurant use are branded 'Corning Pyroceram Tableware'; these items are not Corning Ware.

The usual crystallization method results in an opaque product, and scientists rose to the challenge of inventing a transparent crystalline material.  The first success in this area came in 1966, but this clear and completely colourless product did not reach production due to a fear of hurting sales of Pyrex and Corning Ware, and 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 achieved on this earlier project directly led to Visions cookware, made of a transparent amber-tinted glass-ceramic.  The official name of Visions glass is Calexium and it was developed at Corning in France.  It became available in France in the late 1970s, and it hit the North American market in 1983.

Approximately six years prior to the North American debut of Visions, brown clear amber Pyrex was launched as the new 'Fireside' product line.  It is of the utmost importance not to confuse the two types of glass; this is easy enough to do since they are almost the same colour.  It is vital to remember that Visions is a glass-ceramic formulated for use on the stovetop, but if amber Pyrex is used in this way the result will certainly be disastrous with a real risk of personal injury. 

It is very simple to determine which is which, since Visions is always clearly marked with the proper brand name.  If the piece is marked simply Pyrex, or even Corning, then it is not Visions.  Real Visions cookware is stamped with 'Vision' or 'Visions'.  However, the lids which go with Visions are frequently made of amber Pyrex, and this is alright because with proper use, the lid will never be in direct contact with the stove.  Besides the obvious brand name difference, it is possible to distinguish between amber Pyrex and Visions by colour.  With a close comparison, Visions glass has a golden orange tint, while amber Pyrex is simply brown.  Both Visions and clear Pyrex were also available in Cranberry. 

Sources:

Introduction to Manufacturing Processes, John A. Schey

The Generations of Corning, Davis Dyer and Daniel Gross.

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

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

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

Related Article: 

Comparing Amber Pyrex & Amber Visions