

is a name applied to varieties of two distinct minerals: gypsum (a hydrous sulfate of calcium) and calcite (a carbonate of calcium). The former is the
of the present day; the latter is generally the one known of the ancients.
The two kinds are readily distinguished from each other by their relative hardnesses. The gypsum kind is so soft as to be readily scratched by a finger-nail (hardness 1.5 to 2), while the calcite kind is too hard to be scratched in this way (hardness 3), though it does yield readily to a knife. Moreover, the calcite
, being a carbonate, effervesces on being touched with hydrochloric acid, whereas the gypsum
, when so treated, remains practically unaffected.
De Ferranti, not only commissions Ancient objects from this material, they have also worked out a way of creating partition walls in bricks creating incredible trasluscent effects.