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In page Cupellation:

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The base of the hearth was dug in the form of a saucepan, and covered with an inert and porous material rich in calcium or magnesium such as shells, lime, or bone ash.[6] The lining had to be calcareous because lead reacts with silica (clay compounds) to form viscous lead silicate that prevents the needed absorption of litharge, whereas calcareous materials do not react with lead.[1] Some of the litharge evaporates, and the rest is absorbed by the porous earth lining to form "litharge cakes".[4][7]