Cement for joiners. A cement for filling up cracks and holes is obtained by mixing slaked lime 50 parts, flour 100, linseed-oil varnish 15.
Cement for cracked clay crucibles and porcelain. By applying to the cracks a mixture of 10 parts of slaked lime, 10 of borax, and 5 of litharge in sufficient water to form a stiff paste, and drying after heating the crucible, the cracked places will be united so firmly that the crucible, when thrown to the ground, will generally break in any other place than the cemented one.
This cement can also be used for porcelain capable of standing a strong heat.
Lime and glue cement. Stir air-slaked lime into hot glue. This cement is especially suitable for attaching metal to glass. It forms a very hard yellowish-brown mass.
GYPSUM CEMENTS.
Sulphate of lime in combination with water is met with in nature, both in the form of transparent prisms of selenite, and in opaque and semi-opaque masses, known as alabaster and gypsum. By pulverizing the latter and heating to about 302° F. it loses its water, and is converted into anhydrous gypsum or plaster of Paris, which on mixing with water recombines with it to form a mass of hydrated sulphate of lime, the hardness of which nearly equals that of the original gypsum. When the powder is mixed with water to a cream and poured into a mould, the minute particles of anhydrous sulphate of lime combine with the water to reproduce the original gypsum, and this act of combination is attended with a slight expansion which forces the plaster into the finest lines of the mould.
By using a solution of alum instead of ordinary water, a plaster is obtained which, although it takes much longer to set than the ordinary kind, is much harder, and therefore takes a good polish.
For preparing cements only perfectly white plaster of Paris should be used, as the gray article possesses but little adhesive power.
Cement for plaster of Paris statues. To repair plaster of Paris statues so that the point of fracture cannot be detected, proceed in the following manner:
Moisten the fractured surfaces with water by means of a brush until they absorb no more and remain moist. Mix plaster of Paris with water to a thin cream and stir until the heat appearing at first has ceased, which will prevent the conversion of the plaster into a solid coherent mass. Apply quickly a thin layer of the plaster to one of the fractured surfaces, press the other against it until the plaster has set, and, when dry, carefully remove the excess by scraping.