As far as I know, glass is a non-crystalline amorphous solid. Does glass exist in crystalline forms? Or is it the definition of glass that it's non-crystalline, in which case my question is based off of a false assumption?
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3$\begingroup$ That's right, the definition of glass is that it's amorphous, so the question as it is does not make sense. Some compounds just tend to be amorphous in certain conditions; when treated differently, they may become crystalline as well. When crystalline, however, they do not qualify as glass. $\endgroup$– Ivan NeretinMar 30, 2016 at 16:19
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$\begingroup$ @IvanNeretin Thank you for explaining, I'm not well versed in chemistry. $\endgroup$– EtheryteMar 30, 2016 at 16:23
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3$\begingroup$ Some of the confusion may be that glass with a high lead oxide content is typically referred to a "crystal" because of its sparkle and the fact that the glass is usually cut to highlight the high refractive index. But it too is amorphous. en.wikipedia.org/wiki/Lead_glass $\endgroup$– MaxWMar 30, 2016 at 17:49
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$\begingroup$ Many compounds that usually form "glasses" can be cooled from the melt slow enough so they crystallise. In some cases this would take years, in others (e.g. atactic polystyrene) it is impossible. $\endgroup$– KarlMar 30, 2016 at 21:04
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3 Answers
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As Ivan explained, a glass is by definition amorphous. However, you can take a quartz crystal (silicon dioxide, which is the main ingredient of glass) if it is of high purity and optical quality, and machine it into a transparent object such as a window pane.
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1$\begingroup$ There are very very few quartz crystals in the world big enough to get a regular window pane. Even then it would almost certainly be flawed. $\endgroup$– MaxWMar 30, 2016 at 19:33
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2$\begingroup$ Sapphire (i.e. corundum) is regularly grown into huge single crystals and sawn and polished into "not-glass" windows, mostly for expensive watches. $\endgroup$– KarlMar 30, 2016 at 20:58
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4$\begingroup$ Huge crystals of quartz are also routinely produced. $\endgroup$ Mar 30, 2016 at 21:13
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"Lead crystal" usually refers to glass cut decoratively to partly resemble the facets of a cleaved crystal, so it's not really misleading, considering that most people recognise crystals by their faceted shape rather than their internal structure.
I looked into "crystalline glass" after being astonished at this term in a promotion involving what looked like ordinary glassware, with no flat surfaces. In European rules, "lead crystal" implies more than 24% lead oxide in the formula. There have been concerns about lead leaching from lead glass, so that alternatives such as barium have been replacing lead. Manufacturers are permitted to call low-lead or "lead-free" glass "crystal glass" or "crystalline glass". That seems a gross deception in the case that drew my attention, but in glass with similar designs to cut lead glass, perhaps not, although "crystalline" still seems more of a misnomer than "crystal".
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Glass is by definition amorphous. If you were to crystallize it then it would be considered a ceramic, even though compositionally you have not changed your material. Often times the crystallization may occur as a result of slow cooling rates which then allows the formation of ordered arrangements of atoms (crystals) which are usually disallowed by more rapid cooling.
Sometimes partial crystallization occurs and is even desired. This produces a "glassy-ceramic" material which can give the best of both materials. Corelleware is famously strong and durable because its structure consists of ceramic particles in a glass matrix. This also gives it it's famous million needle-like shards when broken.