I know about a process of Annealing where a crystalline solid is heated to its melting point and then cooled rapidly so as to form amorphous solid which has random crystallite in uts structure.

But how is the reverse done. How amorphous is converted to Crystalline. Does it have to do anything with the fact that glass panes look milky after long time. Almost all Indian textbooks have this fact written in one of the chapters.

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  • $\begingroup$ I'm not sure if it happens over time but we observe something similar here when we make glass ampoules from quartz glass tubes. Once we use the big oxyhydrogen torch to melt the thick-walled tubes you can see both, SiO2 gas formation as well as surfaces becoming rough and opaque. We always assumed it has to do with the glass melting and crystallizing again under the conditions. But many glasses also contain additives to avoid any far-range network formation. $\endgroup$ Jun 14, 2022 at 11:40
  • $\begingroup$ The crystal nucleates and grows. The kinetics are slower, of course, but it happens just like it would in a melt. And, generally speaking, the quote from the textbook is, well, wrong - fused silica will remain fused silica at near-human temperatures for longer than we care to watch. (Now, when your intense pulsed laser self focuses inside a fused silica optic and rapidly nucleates crystal, well, so much for that optic... $\endgroup$
    – Jon Custer
    Jun 14, 2022 at 12:46
  • $\begingroup$ @SamyakMarathe Comments are not supposed to provide answers. $\endgroup$
    – Poutnik
    Jun 14, 2022 at 13:22

1 Answer 1


It is only a question of time. Crystalline substances are more stable than amorphous substances. So when heated to near its melting point, any amorphous substance has from time to time the possibility of rearranging its atoms and molecules in a crystalline order. The barrier separating random order and crystal state can, contrary to the general practice, be exceeded for a small minority of energetic atoms and molecules, according to Maxwell theory. And once a crystalline arrangement is started at some point of the solid it is more stable. So it will not go back as easily to random structure. The crystalline structure will be growing and growing. Of course, rather slowly. This is how quartz crystal have been formed from silica in nature.

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    $\begingroup$ Well, the time may be a significant fraction of the age of the universe. Nobody worries much about their diamonds turning to graphite. $\endgroup$
    – Jon Custer
    Jun 14, 2022 at 14:09
  • $\begingroup$ @Jon Custer. The activation energy between diamond and graphite is huge, because both phases are highly ordered, and one is cubic, and the second is hexagonal. The transition diamond-graphite will never be obtained within even the age of the universe. But the transition from amorphous silica to ordered silica (quartz) has a much lower activation energy. This transition may occur within a couple of centuries. $\endgroup$
    – Maurice
    Jun 16, 2022 at 8:02
  • $\begingroup$ indeed, diamond to graphite is harder. But network glasses such as fused silica aren't particularly easy, and the more pure they are the slower they are. Highly pure fused silica tubes are used in furnaces operating at 1100C for years without nucleation. Even obsidian, which is not pure by any means, will last millions of years. Your modern window glass going crystalline? Not in your lifetime or that of generations to come. $\endgroup$
    – Jon Custer
    Jun 16, 2022 at 13:12

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