CASEIN PAINTS. 39 



The coatings furnished by the different kinds of casein 

 paint, dry without gloss, like all water-colour paints, and 

 therefore exhibit a property which is difficult to impart to 

 oil paints without impairing their durability. Under the 

 influence of the air, the paint, too, acquires a certain firm- 

 ness, and does not rub off like lime-washes. It is fairly 

 durable when exposed to the air, but when in situations 

 where it is continuously exposed to wet or heavy rains, 

 i.e., on the weather side of a building, it suffers more 

 quickly than oil paints. On the other hand, all casein paints 

 have the advantage of drying quickly and being easy to 

 work, without requiring any special preparation of the 

 surface to be painted, drying without gloss, on brick work y 

 plaster, wood, canvas, etc., and without stopping the pores 

 of brickwork. These paints are soaked in cold water in 

 order to swell up the soluble binding constituents, salts and 

 similar compounds, so that they may dissolve completely 

 when diluted further with water, and also perform their 

 chemical functions. In addition to the binding constituents 

 the paint powders of this kind contain mineral matters 

 (chalk, china-clay, clay, caustic lime, etc.), forming the bulk 

 of the mass and giving covering power, whilst body colours 

 are also present to produce the desired shade of colour. 

 These pigments, which are present in merely minute pro- 

 portion in the case of light colours, must be perfectly in- 

 different to lime and alkalies. Consequently, a green, for 

 instance, compounded of Prussian blue and chrome yellow, 

 cannot be used ; and, besides, the pigments must be as pure 

 as possible. 



The media of these cold-water paints, and on which 

 their quality and utility chiefly depend, are mostly albumin- 

 oid substances of vegetable or animal origin, and generally 

 consist of casein, dissolved by means of alkalies and after- 

 wards rendered insoluble by the action of air on the coat of 



