OILS, ETC., USED IN THE PAINTING OF PICTURES. 461 



plied to the artist, who may thus dihite or mix it further, and we 

 shall proceed to consider the materials he may use. 



In the first place, he may merely add a little more of one of the oils 

 already referred to. In case, however, that he wishes his picture to 

 dry faster, he may use as a medium instead of raw" oil, boiled or dry- 

 ing oil. 



The property of the boiled oils depends upon the fact that if, for 

 instance, linseed oil is heated for some time with certain compounds, 

 more usually either compounds of lead, such as lead oxides or lead 

 acetates, or compounds of manganese, such as manganese borate or 

 resinate, it becomes partially oxidized, and if painted out on a surface 

 will " dry " much more quickly. 



Of the two methods of preparing drying oils described above, the 

 use of manganese is, I think, preferable, and for this reason : A cer- 

 tain amount of the substances used dissolves in the ml, and conse- 

 quently an oil prepared with lead dryers contains lead in solution 

 and is ver}" easily darkened by impure air containing sulphur com- 

 pounds, such as sulphuretted hydrogen. It is therefore probably 

 better to keep such oils out of modern pictures Avhich are exposed to 

 the impure air of cities. 



Besides diluting with oil, the artist may prefer to dilute with a 

 medium v.hich will evaporate and leave the layer of oil originally 

 present behind. The mediums most commonly used for this purpose 

 are either turpentine or petroleum. Turpentine, which is obtained 

 by distilling the natural gums of the various pines, is a very suitable 

 medium, as it not only evaporates easily, but also assists in the oxida- 

 tion of the oil. It has been objected to turpentine that it does not 

 evaporate clean, but always leaves a slight resinous residue behind. 

 This is quite true, but the amount of this residue is very small, and, 

 as far as my experience goes, it gets fairly hard in time, so that there 

 is probably no objection to its use on this ground. 



The petroleum oils have the advantage when properly rectified of 

 evaporating quite clean and leaving no residue. I have said when 

 properly rectified. It is important before using such a medium to 

 moisten a piece of blotting paper with it and expose it to the air for 

 a short time. If properly rectified, the petroleum evaporates com- 

 pletely, leaving no greasy stain behind. 



Artists sometimes forget the real property of these mediums and 

 then complain afterwards that the pigment does not adhere properly 

 to the surface of the picture. The amount of oil used for grinding 

 different pigments varies very considerably. If the ground is 

 slightly absorbent and the pigment stiffly ground and diluted by the 

 artist with petroleum, the oil in the pigment dissolves in the petro- 

 leum and passes freely into the absorbent ground, leaving the pig- 

 ment when the petroleum has evaporated without sufficient oil to 



