68 CASEIN. 



it is necessary to provide a non-absorbent surface, to form 

 a suitable ground for painting on. A special preparation is 

 required for fresco painting, it being essential for the ground 

 to be thick and dry, so as to prevent the appearance of any 

 efflorescence from the brickwork underneath. For this 

 reason, the plaster should be composed of as many layers as 

 possible, each dried separately. Two to four strata of coarse 

 plaster and an upper one of fine plaster are usually sufficient. 

 Each layer should be thoroughly dry before the next one is 

 put on, the surface being well moistened with water just 

 before the next coating is applied. The bottom layer must 

 be coarse grained and thicker than the others, the total 

 thickness being 1 to 2 inches. 



The coarse plaster is prepared of mortar from 1 part by 

 volume of patent lime and 2 parts of clean, coarse, sharp 

 sand (preferably quartz sand), burnt fireclay or powdered 

 stone, together with about 10 per cent, of good Portland 

 cement. This mortar is applied to the wall surface, beaten 

 with a wooden bar to drive out air bubbles, and left to dry 

 for as long as possible. On this surface, after wetting it with 

 pure or boiled water that has been allowed to cool, the fine 

 plaster is applied, the mortar for which is compounded of 

 1 part patent lime, 1 part fine, sifted, dust-free, clean sand 

 and 1 part marble sand, the whole mixed with water that 

 has previously been boiled. This mortar is also beaten to 

 expel air bubbles in the mixing trough, and is then laid on 

 over the coarse plaster. As soon as the surface has dried dull, 

 it is wetted with a little boiled and cooled water and worked 

 over with a smoothing board, any residual roughness being 

 smoothed away by a sheet of zinc after the mortar has set to 

 such an extent that it merely gives a little under the pressure 

 of the finger. Directly this upper layer is dry it can be 

 painted on with the necessary casein paints, and retouched 

 with the same colours after drying. 



