pasty composition of soft and plastic consistency spread or daubed on a surface where it hardens. It was traditionally made of burnt limestone (quicklime or calcium oxide) mixed with sand, water, and hair to provide a smooth surface fit to receive decorations. Plaster of Paris is lime sulphate (gypsum) deprived of its natural water-content by heat, ground to a fine powder and mixed with water to form a paste: it sets quickly, expanding at the time of setting, a peculiarity that not only makes it useful for filling cracks, but causes it to take sharp and delicate impressions from a mould.
Glass plate, aluminium sheet or other appropriate backing measuring 20 cm x 20 cm covered with a thin layer of silica gel. If the thin layer has to be prepared, silica gel mixed with about 13% plaster should be used, and it should be applied in a 0,25 mm layer with suitable apparatus in accordance with the manufacturer's instructions.
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