THE TECHNICS OF CASEIN PAINTING. 71 



Another casein medium, for which, however, the same 

 claims cannot be advanced, is prepared by intimately mixing 

 an alkaline solution of casein with diatomaceous earth (kiesel- 

 gu/ir). This has the drawback of going dark-coloured in 

 time, and of not being suitable for use with pigments like 

 Berlin blue, though it can be usefully employed for coloured 

 paints and rough ornamentation work. 



On the other hand, compounds of fatty acids with alumina, 

 and similar substances, are not advisable. Prof. Linke ?n his 

 work on painters' colours says of these that though, when 

 first dried, they form a dense, horny layer which repels water, 

 the colours prepared with their aid soon alter in molecular 

 structure on exposure to the air, and fall to powder, which is 

 readily oxidised by atmospheric oxygen, chemically changed 

 and rendered useless. 



If dissociated casein is emulsified by oil, balsams or fats, 

 together or separately, the resulting medium is distinguished 

 by the property of leaving the natural tone of pigments un- 

 altered. A casein medium of this kind, or one treated with 

 resin, cannot, of course, be thinned down with water, but only 

 with benzol, petroleum, turps or other volatile substances. 



All the media hitherto described are more or less restricted 

 to use for painting indoors. For outdoor use the casein must 

 be incorporated with substances with which it forms insoluble 

 compounds. The best known of these is slaked lime, which 

 has been already mentioned in connection with cold-water 

 paints (g.v.). Cement also forms with casein a particularly 

 resistant and effective binding medium, and a patent for 

 making these media has been granted to Ant. Richard of 

 Diisseldorf. According to this specification, casein, properly 

 freed from fat, is mixed with an antiseptic, e.g., formaldehyde, 

 carbolic acid or the like, and then carefully ground along with 

 water. The resulting thin liquid mass then receives an 

 addition of cement proportionate to the amount of casein 



