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Quartz, Glass, and Ultraviolet Radiation
Name: Chuck
Status: educator
Grade: post 12
Location: MS
Question:
I teach Analytical Chemistry and Instrumental Analysis at
a small university in Mississippi. My colleagues and I were
discussing the differences between quartz and amorphous glass
recently and the topic of UV transmission came up. We are all aware
that amorphous glass does not transmit UV well, but we could not
arrive at a satisfactory reason why. Can you explain why?
Replies:
Chuck,
While both standard glass and quartz glass use SiO2 as its main ingredient,
the difference between standard glass or "soda-lime glass" from quartz glass
(may be fused quartz or synthetic fused silica) is in the manufacturing
process.
Soda-lime glass uses SiO2 and adds sodium carbonate to lower the melting point
(think freezing point depression) and allow working the material at lower
temperatures. Since the addition of soda makes the material somewhat water
soluble, calcium oxide is added to have better chemical durability. Depending
on the use, other substances may also be added (lead for luster, boron to have
Pyrex, iron for heat filtering, etc.)
Quartz glass, on the other hand, is usually made from pure silica (either by
fusing SiO2 or from other sources of Si and O).
Thus, the purity of quartz minimizes interaction with UV, whereas standard
glass have additives that can absorb or disperse UV.
Greg (Roberto Gregorius)
Glass is a mixture of various metal oxides, boron oxides in particular. The
composition of this, or any of numerous other glasses, depends upon the
composition of the particular glass. Quartz on the other hand is usually reserved
to pure SiO2, which may be amorphous or crystalline. The composition and structure
of the quartz determines its UV transmission.
Vince Calder
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Update: June 2012
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