ON THE NATURE OF DRYING OILS. 261 



The ingredients added to oils to make them dry fafter, When oils are i 

 viz. calces of lead, faline fubftances, earths or gums, are Jf " c d ^ deprave 

 fuch as unite with and increafe the quantity of thofe parts the colours j and 

 which float to the top, and form a (kin, more or lefs dark, now * 

 over the colours originally mixed with them. If we confider 

 the nature of thefe ingredients, we fhall be at once enabled to 

 account for a fact univerfally known, viz. that in proportion 

 to the ftrength of the drying oil ufed in painting a picture 

 its colour becomes depraved. It will be injured and finally 

 deftroyed, by being kept in a damp fituation, excluded from 

 a free circulation of air, or placed under a glafs. 



The defideratum is to prepare oil or other vehicle for Oils for painting 



painting, fo that the colours, when mixed with it, (hall not ° u . ght not t0 

 f° " w . . r "ikinj nor con - 



be debated under any of the above-mentioned circumuances, tain changeable 



It muft be fo prepared or ufed, that it fhall ferve as a cement ingredients, , 

 to unite and bind the colours, without fkinning over them* 

 It muft likewife not contain thofe principles which always 

 cxiftin the calces of lead, faline, or earthy fubftances, which 

 from the firft deprave the colours, and attract particles from, 

 the air, under peculiar circum fiances, which increafe that de- 

 pravity, till at lafl the appearance of the colours is totally 

 deftroyed. 



It is only among the refins or bitumens that we can expect Re/ins or bitu. 

 to find a fubftance poffefTing the properties requifite to g^ v ^Xs character • 

 to colours all the brilliancy and durability of which they are 

 fufceptible. My firft attempts and experiments were made 

 with folutions of maftic and fandarac in the painters oils ; but Mafttc and fan- 

 though thefe compofitions pofTefTed more brilliancy than the ^JjJJ^dS? 

 common drying oils, . they were liable to a confiderable objec-. rable. 

 tion ; for they did not dry readily, and when dry, were eafily 

 acted upon by all the common folvents for refinous fubftances, 

 and on that account muft be very deficient in durability, which 

 is one of the moft neceffary qualities I wifhed to difcover. 



The difficulty with which amber is in any way diffolved, Amber poffefles 

 fuggefted the propriety of trying that fubftance. Accordingly J^ ior * uaU " 

 I diffolved it, in each of the painters oils, by Dr. Lewis's 

 procefs, without injuring its colour ; and this folution was 

 made in the common way. It was much darker coloured in 

 itfelf, but produced fcarcely any difference in effect when 

 mixed with colour. By experiments with each of thefe folu- 

 tions I afcertained the following facts, viz, 



Every 



