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Poor glass, badly prepared window-glass, and glass which has been subjected to strain tend to devitrify on exposure to air, some of the ingredients separating in a crystalline form.

Good glass does not readily devitrify when held in the blow-pipe flame.

Bad soda-glass or that which has been kept for many years, tends to devitrify when worked.

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.

If either of these precautions are neglected most glass will devitrify badly.