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Updated: May 16, 2025
The following qualities are desirable in a glass for ordinary working: moderately low working temperature, freedom from air bubbles, striations and irregularities, proper composition, so that the glass will not devitrify or crystallize while being handled at its working temperature, ability to withstand rapid heating without cracking.
Even good glass will begin to devitrify in this way if heated too long at the highest temperature of the flame, so care should always be taken to reduce the time of heating of any spot of glass to a minimum; i.e., get the desired result at the first attempt, if possible, or at least with the minimum of reheating and "doctoring," and avoid keeping the glass at the highest temperature of the flame any longer than necessary.
Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing action of underground waters; but their flow lines and perlitic and spherulitic structures remain to tell of their original state. Ancient tuffs are known by the fragmental character of their volcanic material, even though they have been altered to firm rock.
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