CASEIN PAINTS. 49 



hydroxide of calcium. Casein used for tempera colours .should 

 always be of the finest quality pure white and the finest powder, 

 and quite free from any suspicion of decomposition. Inferior 

 brands sometimes contain chemical compounds produced by 

 over-drying the casein. These are not fit for tempera colours, 

 notwithstanding their low price. They do not possess sufficient 

 binding power, and very often the decomposition of the matter 

 referred to causes strips of yellowish colour on the painted 

 surface. These substances also sometimes contain soluble salts 

 which are more or less dissolved by rain, and this is often the 

 cause of bad colours in outside work. 



The properties of hydroxide of calcium used for tempera 

 colours are also important. It should always be in the form 

 of very fine powder, and, if possible, be freshly made ; in no 

 case can it be kept in stock for any considerable period of time, 

 as it takes up carbonic acid from the air. The hydroxide of 

 calcium, therefore, should be prepared on the spot. Freshly 

 burned white lime entirely free from iron, if possible, should 

 be treated with exactly the quantity of water which it will 

 take up ; an excess of water must be avoided, and great care 

 must be taken to prevent the formation of lumps ; the fine 

 powder thus obtained should be machine-sifted and then stored 

 in air-tight drums or casks. The lime should not be slaked 

 with water in the open air, but the operation should be carried 

 out in drums ; a very fine powder hydroxide of calcium is thus 

 obtained, and it can be passed on to the sifting machines in 

 enclosed trucks. 



It is highly important that the chalk for casein-lime colours 

 should be perfectly dry ; if it contains any trace of water this 

 causes lumps in the product, owing to the imperfect formation 

 of casein-lime lakes, and these lumps are not broken up when 

 the colour is mixed with water. On account of the price, a 

 good natural chalk must be chosen, and the covering power 

 must be thoroughly tested. Sometimes the natural product, 



