CALCO. - It is the reproduction of objects worked in relief obtained by means of impressions taken from the original with wax, clay, plaster, or other suitable material. The material most commonly used today for making casts is plaster (a special quality for moulders).
The practice of making casts is very ancient; it seems that the ancient Egyptians knew II. Lucian, Plutarch, and Juvenal refer to it in their writings. During the Renaissance, papier-mâché or scaglia was also used to make casts, especially of relief objects. Frequent also were coloured or gilded casts depicting sacred images. The renewed interest in antiquity spread the practice of making casts of ancient sculptures; collections of casts are found in museums and galleries and in schools of archaeology and art history; the oldest of these collections is the one begun in Milan by Cardinal Federico Borromeo at the Ambrosiana for artists and scholars.
To reproduce graffiti, especially inscriptions, casts are made with blotting paper or filter paper, or, even better, with the special so-called "casting paper."