OILS, ETC., USED IN THE PAINTING OF PICTURES. 463 



difficulty of preventing it from getting too dark on fusing. This 

 can be overcome, but in so far as I have been able to test its properties 

 it has no advantage over the kauri resins or copals. These, then, are 

 the principal mediums in everyday use for painting oil pictures. I 

 have excluded all special fancy mediums, as I do not know their 

 composition. They maj^ be quite harmless, but I object to them as a 

 doctor objects to a patent medicine. An artist, if he is wise, will 

 only use such mediums as are of known composition and have stood 

 the test of time and experience. 



We shall next, then, 2:)roceed to consider various problems which 

 arise in connection with these mediums, and which I may frankly 

 say are far from being solved. 



In the first place, then, how far do pigments act chemically on each 

 other when mixed in an oil vehicle ? To take a typical example : Will 

 a mixture of white lead, which is so sensitive to sulphur compounds, 

 turn black Avhen mixed with vermillion (sulphide of mercury) or with 

 cadmium yellow (sulphide of cadmium) ? It is sometimes stated in 

 the text-books that these pigments must not be mixed together, but all 

 practical experience is against this view, and when we examine a 

 pigment ground in oil under the microscope and notice how the 

 particles are each protected by a layer of oil it is difficult to see how, 

 unless the pigment is soluble in the oil, any action can take place. 

 There is one well-known case when such action does take place, the 

 turning brown of a green made with emerald green and cadmium 

 yelloAV, but this, I take it, is due to emerald green dissolving slightly 

 in the linseed oil. I once made an experiment which I think is of 

 interest in this connection. I rubbed out some cadmium yellow 

 ground in oil on a glass plate, allowed it to dry, and then coated it 

 with a layer of linseed oil, allowed this to dry and then coated this 

 with emerald green in oil. At the end of six months the combination 

 had turned brown and a section under the microscope revealed the 

 fact that the top layer of cadmium yellow had turned black. This 

 must have been due to the solution of the copper in the emerald green 

 in the linseed oil and the slow diffusion of the copper salt molecules 

 through the solid oil, with formation of l^lack copper sulphide. It 

 is evident then that pigments solul)le in linseed oil will slowly diffuse 

 through the solid oil and attack other pigments, but if they are in- 

 soluble no change seems to take place. 



The next question is, How far does the oil protect pigments from 

 external influences — air. moisture, and injurious gases? I read many 

 years ago a paper dealing with this subject at the Society of Arts," 

 Ignited sulphate of copper is a white opaque powder Avhich is in- 



o Journal, Vol. XXXIX, 1891, p. 392. 



