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48 Alumine It is an established doctrine that the coloured Earths constitute the most durable Pigments - yet these natural Earths, or okers, do not furnish us with a sufficient variety, nor of hues sufficiently brilliant; & therefore we have recourse to chemistry to supply the deficiency- Hence we procure Vermillion, Lake, Blue &c. It is found by experience that the most pleasant texture of pigments, ground in oil, is afforded by Argillaceous Earths which are tinctured with the oxides of Iron - yellow, red and brown. Some of these are deficient in the requisite quantity of the earthy base, and others which are manufactured are entirely without it. To remedy these defects, I Employ Alumine, the Earth of Alum, itself. It is prepared [[strikethrough]] by the chemists [[/strikethrough]] by precipitation from a solution of alum, by means of an Alkali (ammonia) - carefully washing & drying the Earth. This alumine, when ground with oil, resembles flour Paste - dries slowly, but very cohesively & tough. Vermillion ground with it may be kept in tubes, or bladders, or under water, as well as any other colour. Indian Red with it is rendered more delicate and brilliant, grinds finer & does not run on the Palette, effectively resembling a natural oker. Naples Yellow also is made to stand up on the Palette & keeps better. In short it may be added to every colour that is deficient of the base; some of which are thereby rendered more beautifully transparent. Ivory Black is rendered more delicate as a glazing colour - Brown Red is much improved by it - but Umber, Sienna & White do not require the addition. Prussian Blue sometimes has not enough alumine in it, as well as the Lakes. Prussiate of Copper requires as much as to prevent it running or flattening. Violet de Mars cannot be used without it, for the same reason. If Alumine cannot be procured perfectly pure & devoid of colour, the Carbonate of Magnesia may be employed, and perhaps, from many experiments which I have tried with it, may be found preferable - as the colours are rendered more fluid, & dry, with greater brilliance.
