BROWN DECOMPOSITION PRODUCTS.
=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.