No. 5. yellow-spotted, as if varnished with gall.
The foregoing tints were all fixed with virgin-wax, which I thought the best; but having at the same time and with the same colours painted upon cloth waxed with common yellow bees-wax, I found that the latter in the open air preserved the colours rather better.
Experiment the second.
I washed the foregoing tints with a strong lixivium of pot-ash, vinegar, spirit of wine, a solution of sea salt, and aqua fortis.
By this operation the oil-colours were entirely destroyed, the encaustic suffered nothing, only smalt grew darker; but after scraping it and bringing it again to the fire, it recovered its tone.
I have still a little scrap of a picture, a landscape, by me, which has undergone all the abovementioned trials and more, for I took it from the frame and folded it in four, put it upon the frame again, and brought to the fire and the folds disappeared,—the colours are as fresh as if painted but yesterday. On examining it close one may perceive it suffered violence, but at a yard’s distance no marks appear.
Experiment on oil-colours.
Having perceived that oil-colours, painted upon a waxed ground always appeared brighter upon an oil-cloth; I, to come at the knowledge of the cause of this effect, contrived various experiments, but without success; at last I made microscopical observations, and found that oil-colours painted upon an oil-cloth undergo a great fermentation, five or six hours after being laid on, and continue so till they are dry. Then they begin to overcast, and by degrees cover the surface with a yellowish, grey substance, not to be washed or rubbed off but with a knife.
Among the very same colours painted upon an encaustic ground I could perceive no such fermentation, or overcasting.—From this we may conjecture that the priming, or ground we work upon is more the cause of the colours changing than the colours themselves, very likely owing to the desecated saline particles of the oil, which are dissolved by and mix with the new oil and colours; or to the superabundant quantity of salts contained in the ground or priming, which is generally composed of the coarsest oil and colours, and frequently half chalk.
Though this latter experiment has nothing to do with encaustic, it will find its application and owner.