56 CASEIN. 



casein being dissolved in caustic soda or potash and stirred 

 up with the alkali silicate and a sufficient quantity of diato- 

 maceous earth until the resulting grey-brown mass is per- 

 fectly homogeneous. The paint can be shaded by the 

 addition of lime- proof colours up to 10 per cent. It forms 

 a useful indoor and outdoor coating for stone, brick and 

 fresh dry plaster (lime or cement), and it can also be very 

 advantageously used in very damp rooms. Cheap fireproof 

 paints for wood, canvas, etc., can be easily made by mix- 

 ing these silicate paints with asbestos or other fireproof 

 material. 



The addition of casein to water-glass causes gradual 

 silication, so that, unlike ordinary silicate paints, the coat- 

 ing is not completely hard in a few hours, but only at the 

 end of two to three days. This circumstance is owing to 

 the formation of a compound between the casein and the 

 alkali of the water-glass. If the paint is desired to possess 

 greater elasticity it is advisable to add a small quantity of 

 saponified water-glass along with the ordinary silicate solu- 

 tion ; and a higher gloss may be obtained by means of a 

 solution of shellac and water-glass. 



Though silicate paints offer many advantages over oil 

 paints they have failed to make much headway up to the 

 present, owing to the difficulties encountered in their pre- 

 paration and use. Among these difficulties are the low range 

 of colours attainable and inability to stand the weather. In 

 fact, only the following pigments have been found suitable 

 for use in silicate paints : 



For Blue. Ultramarine and smalt. 



Yellow. Barium chromate, uranium oxide and ochre. 

 Black. Lampblack, boneblack and graphite. 

 Green. Chrome oxide, ultramarine green and cobalt green. 

 Red. Iron reds. 

 Orange. Chrome red. 



Brown. Colcothar, burnt sienna, brown manganese oxide ; these do not 

 curdle the silicate. 



