Keene's. Baked gypsum or plaster-of-Paris, steeped in a saturated solution of alum, and then recalcined and reduced to powder. For use it is mixed up with water as ordinary plaster-of-Paris. This preparation forms a stucco, rather than a cement. It takes a high polish, and when coloured is very beautiful, but does not unite pieces as strongly as:
An excellent cement for mending marble or any kind of stone, is made by mixing 20 parts of litharge and 1 of freshly burned lime in fine dry powder. This is made into a putty by linseed-oil. It sets in a few hours, having the appearance of light stone.
Resin, 8 parts; wax, 1; plaster-of-Paris, 4; mix by fusion. The pieces to be joined must be made hot.
Lac coloured to imitate the marble; may be mixed with marble dust, passed through a silken sieve.
W. F. Reid gives the following. Begin with the raw gypsum in lumps of moderate size, burning them at the usual temperature (below red heat). The solution of alum should contain 1 part of this salt in 10 of water. There is no difficulty in dissolving this quantity if the water be previously heated and the alum coarsely pulverised. By immersing the lumps of burnt gypsum in this solution while they are still warm, and leaving them in it for about 15 minutes, they will become thoroughly saturated with the liquid. They should then be allowed to drain and again burnt, but this time at a red heat. Gypsum which has been treated in this way, forms, when pulverised, a slow-setting cement which ultimately attains great hardness, and has frequently been used for making paving tiles, especially in Italy.
Into a solution of chloride of zinc, sp. gr. 1.490 to 1.652, is introduced, 3 per cent, of borax or sal ammoniac; when this is dissolved, oxide of zinc, which has been subjected to a red heat, is added, till the mass attains the desired consistence. This cement becomes as hard as marble, and may be used for moulding. (7) 12 parts Portland cement, 6 slaked lime, 6 fine sand, 1 infusorial earth, and mix into a thick paste with silicate of soda. The object to be cemented need not be warmed. The cement sets in 24 hours, and the fracture can then hardly be detected. The cemented portions are harder than the rest, and the fracture cannot by any chance be reopened.