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Name: David
Status: Teacher
Grade: 9-12
Location: IL
Country: USA
Date: Summer 2009

I have discussed the making of Corning Ware with my class many times over the years. I have gone through the seeding process using very small titanium oxide crystals to nucleate the silica. Using devitrification, the materials becomes nearly totally free of porosity. The material goes from a glass to a microcrystalline ceramic with small amounts of glass between the tiny crystals. The crystalline arrangement of atoms takes up less room than the amorphous material and causes shrinkage in the process. The other change, due to the numerous small crystals and their associated boundaries, changes the material from transparent to opaque. I hold up a white, opaque, Corning Ware baking dish. Question from student: Then how is the transparent, colorless lid formed so that it is both heat resistant AND transparent? In my research, I have found Visions Ware that is transparent, but red or amber. I am stumped. Please explain the process that allows for colorless, transparent light transmission.

Hi David,

The answer is much simpler that you are imagining. The lid of a Corning Ware dish is simply made of plain ordinary borosilicate glass (also known by one trade name "Pyrex"), not any type of nucleated glass. Borosilicate glass is perfectly resistant to heat, so long as the heat is evenly applied (such as when the lid is placed in an oven, or is not directly exposed to heat from a heating element of gas flame).

Bob Wilson.

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