Humins.—Wood decomposes, like many other substances of organic origin, to form compounds of a deep brown colour, which are known as humins, on account of their occurrence in the humus of tilled soil. The numerous compounds found in humus have all a deep brown colour, which, together with their great stability, makes them very suitable for use as pigments.

By various methods substances can be made so rich in humins that they can be used as pigments. For this purpose sugar, starch, young plant fibres and beet-sugar molasses may be used, which are converted into humins with great rapidity when they are heated with water. In this way the author has obtained handsome colours from sawdust. These humins are most easily made by the following process: Thick beet-sugar molasses are cautiously heated with 5 per cent. of caustic soda in a very capacious iron pan. The already dark mass soon becomes quite black when seen in thick layers, and evolves a considerable quantity of gas; if the heating is too rapid the soft mass may boil over out of a very large vessel. When the evolution of gas diminishes the heat is increased and the mass frequently stirred. With some practice it is possible to tell when the action is finished from the smell, which is at first sweetish but later of a characteristic nature. At first tests should be repeatedly taken from the mass; these are considerably diluted with water until the liquid begins to be transparent. When two tests taken at an interval show no difference of colour the heating is stopped. The whole mass is then poured into water in order to dilute the alkali, so that it will not destroy the strainer; the soft mass, which in the wet state appears quite black, is washed with water until the washings are neutral. The humin brown made in this way, when ground with oil or gum solution, produces a very handsome brown of great covering power and warmth of shade, also distinguished by complete indifference towards chemical reagents.

The pigments we have designated humins contain a very large quantity of carbon, to which they owe their dark colour. When peat or lignite is treated with caustic soda in a similar manner, good shades of brown are obtained; they are, however, surpassed in beauty by the brown from molasses. A handsome but costly brown is obtained when crude spirits of wine are heated with fuming sulphuric acid. Equal volumes of alcohol and sulphuric acid are used; the mixture is heated in a retort connected to a condenser, which is required on account of the combustible vapours evolved from the mixture. When the mass is quite black the heat is withdrawn; the residue is diluted with water, and soda solution added so long as effervescence occurs. On filtering a very soft brown powder is left, which forms a very handsome and durable pigment.

Bistre.—When soot is produced at a very low temperature it is very lustrous, and, in addition to carbon, contains a notable quantity of the products of dry distillation. When this soot is powdered and treated with water the latter substances are dissolved; boiling water should be used. When water is no longer coloured the soot is suspended in a large quantity of water and subjected to a process of levigation. The fine powder obtained by repeated levigation has, when dry, an ugly brown colour, but when ground it acquires an extremely warm shade.