RAW MATERIALS EMPLOYED IN THE MANUFACTURE OF PIGMENTS.
As we have mentioned before, the manufacturer of colours now generally
uses materials supplied to him by chemical works. The purer these are,
the easier it will be to work with them, and the finer will the colours
turn out. We have indicated that it is important for the manufacturer
to know accurately the properties of his materials in order to be
able to estimate their value. Many substances required in certain
cases must be made by the colour manufacturer, since, on account of
their condition, they cannot form articles of commerce—chlorine and
sulphuretted hydrogen, for example.
In addition to the substances which are not to be bought, there are
others which do occur in commerce, but are sold at so high a price that
the manufacturer is compelled to make them himself. This is the case
with the cobalt compounds, from which many beautiful colours are made.
The producers of these demand such prices that it is to the interest of
the colour maker to prepare them for his own use.
In the following chapters, we shall deal with the more important
raw materials which are employed in colour manufacturing, and shall
restrict our remarks to what is of particular importance thereto. For
more detailed accounts of these raw materials the reader is referred
to the text books of chemistry, in which he will find them minutely
described, in so far as they are chemical products.
The materials employed may be divided into assistants in the processes
and components of the manufactured pigment. The assisting substances
are those which are used in the manufacture of a colour without
entering into its composition; from the component materials the colours
are directly derived. For example, in the manufacture of Prussian blue,
yellow prussiate of potash, an iron salt, water (in which the salts are
dissolved) and nitric acid are used. In the blue obtained are contained
portions of the iron salt and of the yellow prussiate, these are,
therefore, component materials, whilst water and nitric acid are simply
assistants, since they do not enter into the composition of the pigment.
In colour making a large number of assisting materials are employed,
which comprise a considerable number of elements and compounds. Since
these are of great importance for our purpose, we shall describe their
properties, and, when necessary, briefly the method of preparation.
Among the component materials are to be reckoned a large number of
salts of the alkaline earth and earth metals and of all the heavy
metals. In addition, there are also the substances of animal or
vegetable origin used in lake making.
In the description of the raw materials, if we were to overstep the
line drawn here, we could include a great variety of compounds, those,
for example, used in the manufacture of the so-called aniline dyes.
These substances form, however, as we have stated, the object of a
particular branch of manufacture, which forms a separate division of
colour chemistry, but with which is not to be confounded what has been
hitherto designated the manufacture of colours.