50 CASEIN. 



although sufficiently white, is of a crystalline structure, which 

 causes the poor covering power. This could be remedied by 

 the mixture with a certain amount of artificially precipitated 

 carbonate of calcium. Artificial chalk is quite amorphous, 

 and therefore has a better binding and covering power than 

 the natural product ; the price of tempera colours permits its 

 use in certain quantities. Sometimes the superior kinds of 

 china clay are mixed with chalk and used for casein-lime 

 colours. An excess of china clay must not be used if a good 

 article is to be produced ; the use of china clay, it is believed, 

 causes the colour to crack more readily under the influence of 

 atmosphere. The pure white casein-lime colour can only be 

 obtained by using chalk alone. A yellowish tint is sometimes 

 caused by the presence of minute quantities of iron in the chalk 

 or hydroxide of lime. This can be remedied by means of 

 ultramarine. If other shades are required, the chalk is mixed 

 with certain quantities of earth colours fast to lime. Heavy 

 pigments such as white lead, and, of course, all pigments which 

 are not fast to lime, must be avoided. Good earth colours, 

 such as ochre (iron oxide), are most suitable, and can be used 

 in a high percentage, 30 per cent, or more. 



The pigments must, of course, be mixed in a perfectly dry 

 state. If the casein-lime colour is required for interior work, 

 pigments only can be employed. Sometimes boracic acid and 

 other antiseptics are combined with the colour for sanitary 

 purposes, 1 per cent, being the usual proportion. It has been 

 found best to mix the casein with the lime and the pigment 

 colours first, and then to add the hydroxide of lime. This method 

 retards the action of the two vehicles. The colours when ready 

 are stored and despatched in waterproof receptacles to prevent 

 any binding of the lime and casein. Casein-lime colours do 

 not hold well on wood or metal ; the coat very soon shows a 

 tendency to become detached from the ground, as iron and 

 wood are more readily influenced by temperature ; on the other 



