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Glass Ingredients and Structure
Name: Gabby S.
Status: student
Grade: 9-12
Location: US
Country: USA
Date: N/A
Question:
Hello, I have been
researching the properties of Glass, I am aware
that it is basically formed from SiO2, but in order
to make it cheaper to produce other ingredients are
added. Commonly Sodium Carbonate and limestone.
Once these components are added, how does this
affect the molecular structure, bonds or properties?
Replies:
Hi Gabby,
The "glass" made from pure silica (SiO2) you refer to is in
fact called "Fused Silica" or "Fused Quartz" It is rather
inconvenient to process, since it melts at around 2000°C
(almost 4000°F). As you have stated, other compounds are
added, primarily Sodium Carbonate and Lime (not "Limestone"
as you stated) in order to reduce its melting point to the
area of 1000°C. (The difference between lime and limestone is
that Lime is CaO, and Limestone is CaCO3). This type of
glass, which is the most common type available, is called
"Soda-Lime Glass".
Both Fused Silica and Soda Lime glasses are both amorphous
"glasses" (there, the word "glass" refers to the molecular
structure, which it totally amorphous, just like a liquid).
These materials are in fact called supercooled liquids,
because like normal liquids, they actually flow at room
temperature.... albeit very, very slowly and imperceptably.
Comparing their properties, Soda-Lime glass will block UV
radiation, whereas Fused Silica does not. Fused Silica also
can withstand much higher temperatures than "normal" Soda-
Lime glass, which is why it is used as an envelope for
halogen light bulbs that operate at temperatures high enough
to soften Soda-Lime glass. Otherwise, these two types of
glass are very similar.
Regards,
Bob Wilson.
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Update: June 2012
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