THE PHYSICO-CHEMICAL BEHAVIOUR OF PIGMENTS.
In a work which, as its title indicates, is devoted to a description of
the manufacture of pigments, the properties of those substances which
are necessary for the preparation of colours cannot be exhaustively
considered; we must, therefore, presuppose a knowledge of the elements
of chemistry. We have to consider in this book the chemistry of
colours; the reader will, therefore, not expect an exposition of
general chemical laws; we shall only state certain facts which are of
value to the manufacturer. With the description of the manufacture of
each pigment and of the materials required for that manufacture, we
shall still discuss the chemical processes which must be conducted in
the preparation of the colours, so far as it is necessary in order to
understand them. In this chapter we shall say a few words about the
physical and chemical behaviour of pigments in general.
The great majority of pigments are prepared by the process of
precipitation, generally by mixing the solutions of two substances,
upon which an interchange of the constituents occurs and the less
soluble compound separates in pulverulent form from the solution as a
precipitate. Most of these colours are obtained by the admixture of
the solutions of two salts; the preparation of the so-called chrome
yellow may be taken as an example. In the preparation of this pigment,
a solution of a lead salt, sugar of lead (lead acetate), is mixed
with a solution of bichromate of potash, whereupon a precipitate of
lead chromate (chrome yellow) is formed, whilst potassium acetate
remains dissolved. The lead chromate is formed because the acetic acid
has a greater affinity for potash than for lead oxide, wherefore an
interchange of acid and base takes place, but the lead chromate being
insoluble in water consequently separates in the form of a precipitate.
Many mineral pigments are produced in the form of precipitates by
passing sulphuretted hydrogen or carbonic acid gas into certain metal
solutions. In these cases a similar exchange takes place between the
reacting substances to that given in the case of chrome yellow; the
metals have a greater affinity for the sulphur or for the carbonic
acid than for the substances with which they are already united, they
unite with the former, and the new compound separates as an insoluble
substance. We have examples of such compounds in cadmium sulphide,
which is obtained by passing sulphuretted hydrogen into the solution
of cadmium in an acid, and in white lead, which is formed by the
saturation of a solution of lead acetate by carbonic acid.
Many organic colouring matters, soluble in water, have the property
of forming compounds with metallic oxides, soluble with great
difficulty, when their solutions are mixed with a salt of lead, tin or
aluminium, and the oxide is separated from the solution by an alkali.
The precipitates obtained in this way are insoluble compounds of the
colouring matter and the oxide of the metal; they are called lake
pigments, or, briefly, lakes. A large number of pigments, often of
great beauty, is obtained in this manner. The lakes are widely used in
all branches of painting and dyeing.
Of great importance for the quality of the pigment is the physical
condition of the precipitate; this is either crystalline or amorphous,
that is, non-crystalline. When a crystalline precipitate is examined
under the microscope, it is seen to consist of very small, coloured,
transparent crystals. The amorphous precipitates are, however, in such
a fine state of division that even with the highest magnification
they transmit little or no light, and consequently appear opaque.
These different characters of precipitates have the greatest influence
on that property of pigments which we call _covering power_. In
consequence of its transparency, a crystalline precipitate will allow
the colour of the surface upon which it is spread to appear through,
hence it must be laid on much more thickly than is necessary with an
opaque pigment, of which a thin coating is sufficient to make the
colour of the surface beneath invisible.
How extremely important is the crystalline or non-crystalline nature
of a precipitate in practice is seen by a consideration of white lead.
This pigment, lead carbonate, can be made by mixing solutions of a
lead salt and a soluble carbonate (soda); but in this case a lead
carbonate of crystalline nature is formed, which, being transparent, is
of so small covering power that this process has no application in the
manufacture of white lead; but a far more troublesome method is used by
which a non-crystalline product, amorphous lead carbonate, is obtained.
Many pigments are formed by burning (oxidising) metals, as, for
example, zinc white; others are prepared by melting salts together, as
Naples yellow; others again are formed by very complicated processes
still partially unexplained, as is the case with ultramarine. In the
manufacture of colours we find all chemical processes in use.
It may be here remarked that it is quite possible to manufacture some
colours, indeed a large number, according to fixed directions, without
any particular chemical knowledge being necessary to carry on the
processes. Indeed, in works we find most processes being carried out
by ordinary labourers who are quite destitute of any knowledge of
chemistry. We must, however, add that we are convinced that any colour
maker who works simply in a purely empirical manner, according to a
stereotyped recipe, will never be in the position to raise himself
above the position of a workman; he will not be able, when a slight
mishap occasions a change in the ordinary course of the process, to
devise a means of overcoming the defect, but will be compelled to
dispose of the faulty product in the condition in which it exists. Such
a manufacturer is in a condition of blind dependence on the chemical
works and dealers from whom he receives the raw materials requisite
for the preparation of his colours. If he should receive materials
which contain impurities not to be detected by empirical methods, the
inevitable result will be that the colours produced from them will not
be equal to the standard. If, in making a colour which is the outcome
of several processes, a workman once makes a mistake, the product will
not be of the required quality.
On the contrary, if the manufacturer possesses a certain amount of
chemical knowledge, it will not be difficult for him to ascertain the
causes of a failure in a process, and, at the same time, to devise
means by which the defects may be removed. The manufacturer is more and
more in the habit of buying the chemicals which he requires for his
manufactures rather than of making them himself. He should, therefore,
be in a position to form an opinion as to the usability and purity of
these substances, which will only be possible when he has the knowledge
requisite for subjecting them to a chemical examination.
Although we shall presuppose, as we have said, that those who intend to
concern themselves with colour manufacturing possess an acquaintance
with the principles of chemistry, yet this book has been so planned
that it may be of use (we hope) to the practical man who is innocent
of chemical knowledge. On this account, we have devoted care to the
description of those raw materials which are bought in large quantity,
and to the simple investigation of their purity.
When the manufacturer has the advantage of a chemical education, apart
from his endeavours to produce colours lacking nothing in beauty or
depth of shade, he will direct his endeavours in two directions, in
respect of which great advances are yet to be made—the permanence and
harmlessness of his colours.
Many pigments possess the undesirable property of losing their
brightness under atmospheric influences; many, indeed, fade away
completely in the course of time. We have only to examine a picture
some centuries old; in spite of the care bestowed on its preservation,
we can say with certainty that, in the course of time, it will be so
completely altered that nothing will remain of the original colours.
It is the endeavour of the sensible manufacturer of colours to make
only such as remain unaltered by atmospheric action, and also undergo
no change when they are mixed with other pigments. Although it may be
highly desirable that the painter should possess a knowledge of the
chemical properties of the colours he uses, still it should be the
first object of the maker to take care that he places on the market
only colours which will remain as much as possible unaltered when used
alone, and will remain undecomposed when mixed. This is, unfortunately,
not the case with many colours now in use. We shall return later to
this point, of such extraordinary importance to the artist.
The second point to be observed, is to produce only harmless colours.
The advances of chemistry have made known to us a series of colours
which have the advantage over others known for a longer period that
they are non-poisonous. Unluckily, these harmless colours frequently
fall behind the poisonous colours in brilliance, and generally they are
more expensive. Here, too, is opened to the manufacturer a wide field
of activity. The more completely poisonous substances disappear from
the colours in use, the more widespread will be the use of colours. We
should remark that the expression “poisonous colours” is to be used
with a certain reserve. Many pigments which contain lead, copper,
antimony, mercury, etc., are poisonous, because they contain poisonous
metals; but poisoning with them will not readily take place on account
of their insolubility. It is different with the very poisonous arsenic
compounds, which should be removed from the list of colours in common
use; many a misfortune caused by them would then be avoided.
Endeavours to produce innocuous colours have been more successful than
the efforts after permanence. There are now very few commonly used
colours which can be accounted very poisonous compounds, and which
cannot be replaced by other colours of equal beauty. On the whole, we
are now in the position to prepare harmless colours suited to most
purposes. Special endeavours should be made to sell these, so that
such cases of poisoning should not occur as, for example, caused by
gingerbread which had been wrapped in paper coloured by emerald green.