120 WRITINGS OF JAMES SMITHSON. 



assayer in Cornwall, to whom he had made it known ; and I 

 have since heard, from another person, of an assayer in that 

 county, who, finding the assays he was employed to make, 

 cost him more in fuel than he was paid for them, had con- 

 trived means of making them at the blowpipe on one grain 

 of matter. I presume him to have been the same Dr. Black 

 had spoken of. 



LONDON, May 12, 1825. 



A METHOD OF FIXING CRAYON COLOURS. 



From Thomson's Annals of Philosophy, Vol. XXVI ; New Series, Vol. X, 



1825, page 236. 



LONDON, August 23, 1825. 



GENTLEMEN : Wishing to transport a crayon portrait to a 

 distance for the sake of the likeness, but without the frame 

 and glass, which were bulky and heavy, I applied to a man 

 from whom I expected information for a method of fixing 

 the colours. He had heard of milk being spread with a 

 brush over them, but I really did not conceive this process 

 of sufficient promise to be disposed to make trial of it. 



I had myself read of fixing crayon colours by sprinkling 

 solution of isinglass from a brush upon them, but to this 

 too, I apprehended the objections of tediousncss, of dirty 

 operation, and perhaps of incomplete result. 



On thinking on the subject, the first idea which presented 

 itself to me was that of gum-water applied to the back of 

 the picture ; but as it was drawn on sized blue paper, pasted 

 on canvass, there seemed little prospect of this fluid pene- 

 trating. But an oil would do so, and a drying one would 

 accomplish my object. I applied -drying oil diluted with 

 spirit of turpentine ; after a day or two when this was grown 

 dry, I spread a coat of the mixture over the front of the 

 picture, and my crayon drawing became an oil painting. 



