CASEIN PAINTS. -V> 



thought that the casein does not exist as such in the milk, but 

 in the form of caseinogen. When the ferment rennet is added 

 this is split up into casein and albumen ; the former is thrown 

 out as curds, the latter remains in solution. When acid is used, 

 all the caseinogen is converted into casein. The casein of 

 commerce is obtained from separated milk by precipitation 

 with acid. At present the great bulk comes from America, 

 some from Scandinavian countries. Little, if any, is made in 

 this country, because there is so great a demand for milk as a 

 food product, that there is little, if any, surplus left for casein 

 manufacture. While but little is known as to the caseins from 

 various animal milks, yet it is quite evident they vary a little 

 in degree, although closely resembling one another in many 

 properties. Casein comes into commerce as a yellowish-tinted, 

 crumbly powder. It is here that it has an advantage over glue, 

 for the latter is not obtainable as a powder, and it is this dif- 

 ference that enables dry powder distempers to be made and 

 sold, when casein forms a binder. It is much more soluble in 

 water than glue ; alkalies like soda, caustic soda, and lime 

 facilitate the solution, and so dry slaked lime is one of the 

 constituents of distemper. On exposure to the air this lime 

 is converted into carbonate, and the casein separates out and 

 exerts a binding effect on the pigment. Casein alone does 

 not produce a reliable washable distemper, although it helps ; 

 there is needed the presence of a little linseed oil, which, 

 when used in the paint, oxidises and binds the pigment 

 firmly. When dry, casein will keep for a long time, but when 

 moist, like all other nitrogenous bodies, it undergoes putrefac- 

 tive decomposition, and to prevent this there may be added 

 preservatives such as carbolic acid, thymol, salicylic acid, zinc 

 sulphate, or zinc chloride. 



CASEIN-SILICATE PAINTS. 



Casein-silicate paint is a fermented compound of casein 

 with potassium or sodium silicate (" water-glass "), the 



