CHAPTER V. 



CASEIN PAINTS. 



THOUGH it has long been known that casein combines with 

 certain substances to form agglutinant compounds, which 

 become more or less insoluble on exposure to the air, and 

 though it has been stated that casein has been detected in 

 old paintings, it is nevertheless only within the past forty to 

 fifty years (indeed only within a couple of decades) that any 

 extensive use has been made of it for this purpose. That 

 an addition of milk to lime-washes greatly increases their 

 durability has also long been known ; and, in fact, such a 

 mixture constituted the first casein paint, the casein of the 

 milk forming with the caustic lime a compound which is at 

 the basis of all such paints. The casein is rendered soluble, a 

 result that can be produced, not merely with quicklime, but 

 also with various substances having an alkaline reaction. 

 The chief casein paints we shall here consider are those met 

 with in commerce in the form of paste or liquid, containing 

 the casein in a dissociated condition, and in some cases 

 already mixed with other substances used in paint, such as 

 linseed oil, boiled oil, resins or petroleum, in order to pro- 

 duce special effects. Others again and of late these have 

 become the most important of all are supplied in the form 

 of powder by the makers, and contain casein and alkali in 

 the dry state. It is. only when these paints are mixed with 

 water, to make them fluid for use, that the alkali is dissolved 

 and in turn acts as a solvent of the casein. 



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