460 OILS, ETC., USED IN THE PAINTING OF PICTURES. 



In selecting an oil as suitable for artists" purposes it is necessary 

 to choose what is known as a " drying oil." If, for instance, pig- 

 ments are ground in olive oil the surface would never dry, and it is 

 therefore useless for the purpose. There are certain vegetable oils 

 wdiich have this property of drying and W'hich are therefore suitable 

 for artists' purposes. I shall only refer to three here — linseed oil, 

 poppy oil, and walnut oil. There is no need to trouble you with the 

 chemical composition of these drying oils, but it is important that 

 the nature of this drying process should be clearly understood. 

 These oils do not dry in the ordinary sense of the term at all. They 

 undergo a process of oxidation when exposed to the air, which con- 

 verts them from a liquid condition into a tough elastic solid, a solid 

 which slowly undergoes further oxidation, becoming brittle, hard, 

 and resinous. It is then to this process of oxidation that their pecu- 

 liar properties are due. 



These oils are obtained from the seeds or nuts and are present in 

 those bodies as part of the reservoir of food supply for the young 

 embryo of the future plant. 



The oil from linseed is obtained by crushing, grinding, and pressure, 

 and in order to increase the yield the ground mass is heated as well as 

 pressed, thus obtaining what is known as a hot-pressed oil, which is 

 subjected to various processes of refining and bleaching. Personally, 

 while regarding such an oil as quite suitable for house painting, I 

 doubt very nmch the wdsdom of using it for artists' purposes. The 

 hot pressing results in the presence of many impurities, wdiich are 

 removed by the addition of sulphuric acid. The chemistry of the 

 wdiole subject of painters' vehicles is" so obscure that it is as well to 

 cling to tradition wdiere possible. 



The linseed oil of the earlier centuries Avas cold-pressed linseed and 

 w^as refined and bleached by the simple process of exposure to air 

 and sunlight over water. These methods yield a beautiful oil and 

 should be adhered to for artists' purposes. 



Poppy oil is obtained from the seeds of the opium poppy {Papaver 

 somniferum) by crushing and pressing or by other means of extrac- 

 tion and is easily bleached. It is often used for grinding with whites 

 or delicate blues. It dries more slowly than linseed oil, but has the 

 advantage of being almost colorless. 



Walnut oil is obtained from the common walnut {Juglans regm) 

 by allowing the nuts to decompose partially and then pressing, and 

 can be obtained almost colorless. It was largely used by the early 

 Italian painters as a drying oil. There are other drying oils, but 

 t'hey are not of special interest to artists. 



Having briefly discussed the three drying oils commonly used for 

 artists' purposes, we now^ go on to consider some of the other vehicles 

 and mediums. The jjigment having been ground stiffly in oil is sup- 



