PAINTS AND VARNISHES 371 



has a greenish yellow color. It is obtained by pressure from 

 flax seeds and after filtration is sent to the market as cold- 

 pressed oil. If the seed is pressed at a temperature near the 

 boiling point of water, more oil is obtained, but the quality 

 of the hot-pressed oil is inferior to that of the cold-pressed 

 oil. 



Linseed oil is sold on the market in two grades raw and 

 boiled. The method by which the raw oil is obtained has 

 already been explained. Boiled oil is ordinary raw oil which 

 has been heated so as to remove some of the " light " (volatile) 

 constituents. This operation produces a thicker and darker 

 oil resulting in a more resistant film. The oil is often bleached 

 to remove its color. This is done by the action of sulphuric 

 acid and steam. The traces of acid are removed by shaking 

 the oil with water. 



414. Driers. The necessity of increasing the rate of 

 drying has led to the addition of metallic salts or oxides, 

 called "driers," to hasten the oxidation of the paint. These 

 driers are added to the oil before the paint is mixed and act 

 as carriers of oxygen from air to oil. If a paint is dried too 

 quickly, the film produced by oxidation will not acquire 

 the toughness and elasticity which are essential for efficient 

 wear. 



415. Thinners. It is necessary at times to decrease 

 the thickness (viscosity) of paint so as to make it more 

 workable under the brush. It is to the interest of both the 

 workman and his employer to do this as reducing the paint 

 to as liquid a condition as possible, lessens the labor of apply- 

 ing and spiieading it, and enables a given amount to cover 

 the greatest possible surface. A limited amount of '"thin- 



