74 CASEIN. 



surface to be coated. For example, a larger proportion may 

 generally be used on solid plaster and other solid substrata, 

 such as metal, cement, oil paint, paper, canvas, etc., than on 

 a surface that readily chips, such as plaster of Paris. In 

 any case, if the paint is too strong it is very liable to cause 

 the weak substratum to tear away, chip or peel off, especially 

 when organic colours are used. 



For painting on solid or prepared plaster, rough wood, 

 etc., sufficient casein medium must be taken for the paint to 

 stick to parchment paper when dry, so that it does not peel 

 off when the paper is folded, and will stand the action of a 

 strong jet of water at the end of about twenty-four hours. 



To ascertain whether the right proportion of medium has 

 been used, a small quantity of the paint is brushed on a 

 sheet of writing or printing paper. If the paper is found to 

 have shrunk much when the paint is dry, too much medium 

 has been used. 



As regards the handling of the brush, this is similar to 

 distemper work, only that the brush must be more frequently 

 cleansed with soap. A peculiar kind of curdling can be 

 noticed when the paint in the can or brush has got too dry, 

 and an attempt is made to reduce it with water. In such 

 case the brush must be washed with soap and water and a 

 fresh quantity of paint taken. Hence both paint and brush 

 should be kept moist, and the colours on the palette fre- 

 quently sprayed with water, the can kept covered up, and 

 preferably fitted with an ait tight lid. 



In connection with the storing of casein paints the fol- 

 lowing points may be mentioned. In order to prevent the 

 medium from drying, the can is either turned upside down, or 

 else the surface of the paint covered with a layer of water. 

 The paint, however, must not be thinned with this water,, 

 or a separation into layers may easily occur. 



If the paint and medium separate in the can, they may 



