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Corning Ware and Transparency
Name: David
Status: Teacher
Grade: 9-12
Location: IL
Country: USA
Date: Summer 2009
Question:
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.
Replies:
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).
Regards,
Bob Wilson.
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