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



tensely hygroscopic, and in attracting moisture forms a transparent 

 green hydrate. If, then, it is ground up Avith yarious media, which 

 are alloAyed to dry in dry air and then exposed to moist air, the white 

 opaque enamel will become green and transparent if moisture pene- 

 trates. One result was to show that while pure resins dissolyed in 

 turpentine formed moist-tight surfaces, oils and oil yarnishes were all 

 quickly permeable. 



The same is true of other gases. Sulphuretted hydrogen quickly 

 blackens white lead in oil through eyen an oil yarnish, but if the white 

 is ground in Canada balsam and then yarnished with Canada balsam 

 the white is protected. Verdigris forms a permanent green in such 

 a medium. It does not, however, stop, though it checks the fading of 

 crimson lake, showing that there is probably some decomposition of 

 this color in sunlight apart from air and moisture. 



It is evident, then, that while a freer range can be allowed to the 

 oil color than to the water color palette it is safer to select pigments 

 which are permanent in themselves, and not to trust to their pro- 

 tection by the oil from change. 



The next question to be considered is the durability of the oil and 

 yarnish surfaces themselves. This has not received very much investi- 

 gation as far as I am aware. I have been recently making experiments 

 with a view to getting some comparative test for the combined tough- 

 ness and hardness of a varnish. The method is to place the varnish 

 painted out on a glass plate, and dry, under a blunt steel point, the 

 pressure of which on the varnish can be increased by known amounts 

 with a spiral spring. 



The yarnish is then drawn under the point, and the pressure is in- 

 creased until the varnish shows a clear definite scratch. Under this 

 test the brittle spirit varnishes break down at a pressure of 100 grams 

 on a steel point of 1 millimeter radius, oil varnishes made from soft 

 gums at from 300 to 500 grams, and oil varnishes made from hard 

 gums at from 900 to 1.200 grams. Moreover, the character of the 

 scratch is very different. Varnishes with an excess of resin in them, 

 and therefore made from soft, easily dissolved gums, give a splintery 

 scratch, while the tough oil varnishes, made from hard gums, give 

 a tear. On exposure to weather during winter the yarnishes are all 

 soon reduced to a brittle surface which scratches at 100 grams. In 

 summer, however, they are improved by exposure. This clearly in- 

 dicates that frost has probably much to do with this, and it is worthy 

 of further investigation. It is easy by this machine to pick out a 

 good yarnish for the artist to use as a medium, and if this is done, 

 no doubt the life of the picture, if kept under proper conditions, will 

 be very much increased. 



