THE TECHNICS OF CASEIN PAINTING. 73 



For making up large quantities at a time, the dry colours 

 may be saturated with water and then mixed with the casein 

 preparation. In the case of heavy pigments, this precaution 

 is advisable, since the mixture easily gets too thin. The 

 colours, however, stick better when mixed with the binding 

 media dry, the particles, in that event, absorbing the vehicle 

 instead of water, and becoming more completely enveloped by 

 the former. 



Some of the organic colouring matters, e.g., bone black, 

 Cassel brown, etc., which form a good nutrient medium for 

 mould fungi, require an addition of \ to 1 per cent, of car- 

 bolic acid, 1 per mil. of formalin, or about 2 to 3 per cent, of 

 salicylic acid solution, to the water; and this admixture does 

 no harm in the case of other colours as well. 



Although not essential, it is advisable to add a little lime 

 to casein paints, especially when they are in paste form or 

 for outdoor use. 



If the amount of lime to be added is large, it is better 

 not to make up a large stock of the paint, but only just 

 enough for two to three days' consumption, since the insolu- 

 bility of the paint is greater when the combination of lime 

 and casein is fresh but is lessened when the mixture is stale. 



The proportions of medium and pigment cannot be arbi- 

 trarily laid down, owing to the divergent requirements of the 

 various pigments, and also because one and the same pigment 

 varies in this respect, according to its origin and mode of pre- 

 paration, apart from the fact that the power of the different 

 casein media is a variable quantity. 



Chalk and calcined earth pigments generally require a 

 larger proportion of casein medium than lithopone, white lead, 

 etc., whilst ochre and other raw earths usually need less. 

 Light colours generally take less than dark ones, and in- 

 organic pigments less than those of organic origin. 



The proportion of medium also varies according to the 



