70 CASEIN. 



however, be adopted in order to secure the intimate connec- 

 tion between the painting and the ground, so necessary to 

 render the painting durable. 



COMPOUNDING THE BINDING MEDIUM. 



Next to the substratum, the most important thing in casein 

 painting is the medium, since on this depends not only the 

 durability of the paint but also several of its chief character- 

 istics. Hence, it can be easily understood that the composi- 

 tion of the medium may vary in many ways, according to 

 the purpose for which it is intended. Thus, one used for 

 ordinary painting on walls is not also suitable for decorative 

 painting, whilst, on the other hand, a medium for indoor 

 work cannot be employed for outdoor painting. 



Casein and its properties have been already described, and 

 all that is now necessary to mention is that the dry casein 

 must be converted into a soluble form before it can be used 

 as a bind. 



The usual solvents for this purpose are caustic potash 

 or soda, borax, sodium bicarbonate, ammonia, potassium or 

 sodium silicate (water-glass), slaked lime, cement, etc. Since 

 it would hardly pay the painter to prepare casein from milk, 

 it is preferable to use the commercial soluble casein. A 

 simple solution of this is made by adding to 1 part of casein 

 0*3 to 0*4 part of strong ammonia, the solution being pro- 

 tected from putrefaction by means of a little carbolic acid. 

 To convert these solutions into a usable medium they are 

 treated with a little aqueous solution of resin, containing a 

 small admixture of soap or wax. These latter substances 

 are intended to surround the particles of pigment with a pro- 

 tective envelope against atmospheric influences. A medium 

 prepared in this way is suitable for fine artistic painting in- 

 doors, enabling difficult pigments to be used and imparting 

 to these a fine sheen. 



