"David Billington" wrote in message
...
Ed Huntress wrote:
"axolotl" wrote in message
...

Ed Huntress wrote:
But glass, and other

glassy materials, can form crystals under specific conditions.

Inquiring minds and all-

How does one make a glass crystal without introducing impurities (and
making it simply glass like)?


Kevin Gallimore


'Can't help there, Kevin. Quartz is, of course, chemically identical to
glass, but is crystalline. And practically all glasses contain
impurities.

That's a bit like someone I saw calling inconel stainless steel. While
common glasses are predominately SiO2 it does have significant additions
of other chemicals to give it the required properties and to lower the
working temperature to something acceptable. Citing wiki
http://en.wikipedia.org/wiki/Glass for some info, although I think it is
wrong with the melting point for SiO2 which is normally given as 1650C. I
do some glassblowing as a hobby and the glass is normally gathered at
about 1050C(1922F) to 1100C(2012F) depending on the glass type, the lead
glass being 24% PbO, by weight I think. The glass used for manual
glassblowing being formulated to give a long working period, whereas
machine made glass typically stiffens much more quickly to suit rapid
turnaround. A very different material to quartz.

That's certainly true. Except when it's not. g Unlike Inconel, glass is
not a specific alloy. And there certainly are glasses that are pure or
nearly pure silica: fused quartz glass, and fused silica glass, for example.

These are technical glasses that you aren't likely to encounter in your
glassblowing hobby, but they're chemically identical to crystalline quartz.

The term "glass" itself is pretty flexible, as you know. And Kevin's
question about pure silica glass does have some answers. There are products
made from pure quartz, as well as synthetic ones that wind up being pure
silica in the final product.

Concerning melting temperatures, I'm not familiar with the actual numbers,
but watch for confusion to arise over the definition of "melting
temperature" for glass. As I said earlier, it's defined arbitrarily on the
basis of a selected value of viscosity. It may not have much to do with the
working properties with which you're familiar.

--
Ed Huntress