Lime prepared by calcining limestone in a kiln at about 500 C to form a white amorphous powder that may contain small amounts of silica, iron, magnesium, and/or aluminum oxides. Oyster shells have been used to make a very pure quicklime. Once water is added, quicklime becomes slaked lime.
Source: AAT
http://vocab.getty.edu/page/aat/300011740
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