Thanks for the info wwh - concurs with what I've heard. Taking it further though, somebody once told me that the fragments of safety glass (basketball's "ice shower") had achieved solid status (from being a super cooled liquid).
Also heard that glass retains its fluidity throughout its "working" life. One of the characteristics of a fluid is the ability to flow - which is exactly what glass does. Apparently the glass in windows ever so slowly succumbs to gravity, to the point that the window panes in old churches (frinstance) are thicker at their base than at their top.
Just another couple of bits of trivia about the colouring of glass....
A friend of mine collects old (say 1st half of the 20th century) glassware. Things like cake plates and so on. He leaves them on top of his garage roof in the blazing Perth sun for a couple of years until they turn that beautiful amethyst colour I presume we've all seen. They look stunning. Obviously (help Bean!) it's the sun's gamma(?) radiation that sets off the trace elements in the glass. My friend cites it as being due to the presence of uranium in the glass - but obviously not to the level that produces the deep yellows mentioned earlier in this thread. He also told me that the inclusion of uranium in glass was banned in the 50's (unsure of the date).
Gamma radiation is used commercially to produce coloured quartz - presumably in a similar (but shorter) process than my friend's garage roof! Clear quartz is turned a whole range of yellows, lilac and so on through this process. Some of you may have seen the bi-coloured gem "ametrine" - half lilac, half yellow. Very pretty but, for me, being artificial, not that desirable.
In a similar fashion, the vast quantities of amethyst produced by Brazil and India are often heat treated (I believe they are simply baked in an oven) to produce butterscotch orange "citrine" or black "morion". Both are naturally occuring varieties of Quartz but, IMHO, buyers should be aware that they are acquiring an "enhanced" or "altered" product. (It'll never happen so I won't lose any sleep over it!!)
Finally, whilst I haven't seen (first hand) the reds and yellows discussed above, the mineral world has a couple of magnificent offerings - realgar and orpiment. These are closely related sulfides of arsenic with stunning depth of colour. They were used historically by icon artists, the crushed mineral being mixed with egg yolk to make the paint.
Well that turned out to be a bit of a mixed bag hey!
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