When water is cooled below its normal freezing point, it normally freezes to form hexagonal ice, or Ice I, though it can exist in other solid forms. Solid water can exist in a number of non-crystalline forms; amorphous solid water (ASW or "low density glass," 0.94 grams per cubic centimeter), is formed from the slow deposition of water vapor, at less than 0.2 nm per second, on a very cold metal crystal surface below 120 K. If it is very pure and cooled carefully, it may be supercooled to about -42 °C. If water is cooled very rapidly then it forms an amorphoric glass.
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