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Glass transition temperature of fused quartz is 1200 °C. Does this mean it can be made by heating sand to 1200 °C, or does the melting point of $\ce{SiO2}$ has to be reached (~1700 °C) for that to happen?

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    $\begingroup$ See here for glass transition temperatures. $\endgroup$ Jul 19 '20 at 19:32
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    $\begingroup$ Most "glass" is not pure quartz (which is silicon dioxide). Usually many other ingredients are added to lower the melting temperature and/or to make the properties different. $\endgroup$
    – matt_black
    Jul 19 '20 at 20:48
  • $\begingroup$ Fused silica is not the same as fused quartz. Fused silica fluorescence cuvettes are an example. Even high purity natural quartz crystals have significant levels of impurities, e.g., aluminum. $\endgroup$
    – Ed V
    Jul 20 '20 at 1:35
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Glass is not necessarily made of pure silica $\ce{SiO2}.$ Ordinary glass is a mixture of at least $50\,\%$ $\ce{SiO2},$ plus smaller proportions of $\ce{CaO},$ $\ce{Na2O},$ $\ce{K2O},$ and $\ce{Al2O3},$ with impurities like $\ce{Fe2O3}.$ Pyrex has also some $\ce{B2O3}.$

Glass has no defined melting point. It becomes like a very viscous paste at around 800 °C. Mixtures have melting points that are always much lower than their components.

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    $\begingroup$ Is explicitly stated im talking about quartz glass, which is 100% silica. I'll change it to fused quartz to avoid such misunderstandings $\endgroup$
    – Francis L.
    Jul 19 '20 at 20:56

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