Devitrification - Shitto

This is the phenomenon where crystals precipitate from glass. Thermodynamically, glass is not in a stable state, so when it is kept in a temperature range above the glass transition point and below the liquidus temperature, crystals will precipitate and the glass will often become opaque, which is known as devitrification. The viscosity at which devitrification is most likely to occur is said to be 10 ⁴ P (poise, 1 P = 10 ^-1 kg m ^-1 s ^-1 ). This viscosity range often occurs during molding and processing, so in practical glass, an appropriate amount of Al ₂ O ₃ is added to the composition to expand the safe viscosity range against devitrification. Conversely, milky glass and glass ceramics make use of devitrification.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

ガラスから結晶が析出する現象．熱力学的にガラスは安定状態にないから，ガラス転移点以上，液相温度以下の温度域に保つと結晶を析出し，不透明になる場合が多いので失透という．もっとも失透しやすい粘度は 10⁴ P(ポアズ，1 P ＝ 10^－1 kg m^－1 s^－1)といわれている．この粘度域は成形，加工時にしばしば起こるので，実用ガラスでは適量のAl₂O₃を組成に加えて，失透に対する安全な粘度域の拡大をはかっている．逆に失透を利用したものに乳白ガラスやガラスセラミックスがある．

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