Sepia

The dark-brown colouring-matter from the ink-bag of Sepia officinalis, Loligo tunicata, and other species of cuttlefish common in the Mediterranean and Adriatic, has not been thoroughly investigated from a chemical point of view. The pure pigment, which constitutes four-fifths of the weight of the dried ink-bags as they occur in commerce, partakes of the character of a weak organic acid, and is soluble in alkalies and precipitated by acids. In preparing the substance for artists' use, it is commonly first removed from the sacs containing it, dissolved in soda or ammonia solution, and then, after straining the solution, thrown down by neutralizing the alkali with hydrochloric acid; the precipitate is then washed by de-cantation, and dried. Sometimes the filtered ammoniacal solution is used directly as a liquid ink. The chief impurities of the natural sepia are salts of lime and magnesia, which may be partly removed by washing the dried and crushed sepia first with dilute hydrochloric acid and then with water, previous to dissolving it.

Sepia is of a redder or warmer brown hue than bistre, but it is not so reddish as Vandyke brown. Of these three pigments, there is no question that sepia is the least alterable. It is not, however, permanent when exposed to sunshine, although in darkness or in diffused daylight it suffers no appreciable change either in depth or quality. It might, indeed, have been supposed that sepia would prove unalterable, from the consideration of the observed fact of the pigment from the ink-bags of fossil cuttle-fish showing now, when used as a water colour, the same hue and the same chemical characters as recent sepia from the Mediterranean cuttle-fish of to-day. But it must be recollected that this fossil pigment has been excluded completely from the adverse influence of light, and in great measure from that of air, during the long ages which have elapsed since it was embedded in rock. And it is light which, in the case of the majority of organic pigments, is the determining cause of the changes brought about by the combined presence of moisture and atmospheric oxygen.

Sepia is not employed as an oil colour.

There are several grey pigments which, being compounded of other paints already described, need hardly be mentioned here. Amongst these are neutral tint and Payne's grey, which in water-colour are still prepared by commingling indigo, crimson lake, and ivory black, but which have been replaced in oil by artificial ultramarine, ochre and ivory black - an entirely trustworthy mixture. Slate grey, prepared from a rather soft and very dark-coloured slate, is a satisfactory pigment both in artistic quality and in stability.