THE MANUFACTURE OF MINERAL PIGMENTS.
By mineral pigments we understand those which consist of compounds of
metals with elements such as sulphur, chlorine and iodine, or with
compound radicals, such as cyanogen, or of metallic salts.
Looking at the classification of mineral pigments from the chemical
standpoint, a grouping according to the constituents would appear
preferable, and we should have groups of colours consisting of metallic
oxides, sulphides, salts, etc. In such a division of pigments,
according to their constituents, no regard would be taken of the colour
of the pigment. The white zinc oxide would be placed in the same group
with yellow lead oxide and red lead oxide; and red mercury sulphide and
yellow cadmium sulphide would fall into the same group of the sulphur
compounds.
The chemical composition of a pigment is of less importance to the
colour manufacturer than its shade, and it therefore appears to us
more reasonable to prefer to classify mineral pigments according to
similarity of colour. We shall therefore place all those mineral
pigments together which possess the same colour.
Common usage differs from the scientific in the description of colours.
In the physical sense, yellow, red and blue are the so-called “simple
colours,” between which lie orange, green and violet as “mixed
colours”. Physics knows no white colour and no black colour, but
describes white as a mixture of all the simple colours and black as
the absence of colour. A grey or brown shade, produced by different
mixtures of simple colours, is just as little known in the scale of
colours as white or black.
The colour maker follows, as we have said above, the common manner of
speaking; to him white and black are equally as much colours as red
and green. Besides the pure principal colours (yellow, red and blue),
and the mixed colours obtained from them (orange, green and violet),
colour makers distinguish many shades of each colour—lemon yellow,
sulphur yellow, cherry red, blood red, violet blue, etc. For the
present purpose it is of great importance to accurately distinguish
the several shades, for the value of many colours is in proportion to
their beauty of shade. The colour maker is often required to produce a
colour of some particular shade, which he accomplishes in many cases by
a suitable alteration in the process by which the colour is made, in
other cases by mixing different colours, in which event chemistry is
of no help to him; he must depend on the sensitiveness of his eyes to
colour.
=White Mineral Pigments.=—We are acquainted with a great number of
white or, more properly, colourless mineral compounds; they possess the
property of reflecting, undecomposed, all the rays of light which fall
upon them, in consequence of which they produce that impression upon
the eye which we call white. According as a white substance reflects
every ray of light or absorbs a portion, we see it as a brilliant pure
white or, in the latter case, as a white with a grey tinge. If a white
body reflects the majority of the rays of light falling on it, but
decomposes a small number, we perceive a white which has a yellow, blue
or red tinge.
The most valuable white for the colour maker is evidently that which
reflects, unaltered, all the rays of light; it is the most brilliant,
and free from every tinge of colour. The physical condition of the
substance is most important. Solid substances are either crystalline,
that is, possess definite shapes formed according to a regular law,
or they are amorphous, that is, are composed of irregularly formed
particles. Snow and white lead may serve as representatives of these
two classes. Snow is composed of small colourless crystals of ice, the
flat surfaces of which reflect, undecomposed, the light falling on
them. The smaller are the crystals, the more pure appears to us the
whiteness of the snow, and the thinner is the layer of snow required
to produce the sensation of whiteness. But if the snow crystals are
larger, the white appears to have a bluish tinge, and only a thick
layer of snow is opaque. White lead, being an amorphous substance in a
condition of very fine division, reflects the light very regularly, so
that a thin layer of white lead appears quite opaque.
Among the artificial pigments, crystalline or amorphous, exactly the
same conditions hold good as between snow and white lead. Of amorphous
pigments a very thin layer is in most cases sufficient to make the
surface upon which they are spread invisible, or, as the technical
expression runs, “to cover,” whilst crystalline substances possess
a smaller covering power. A striking example of this is seen by a
comparison of two white pigments, white lead and “patent white” (lead
oxychloride). The former is amorphous, the latter crystalline. Both are
completely colourless and reflect white light, but in consequence of
its amorphous condition and finer particles, white lead possesses far
greater covering power than “patent white”.
Among all other colours the same rule holds. Amorphous pigments have
always a greater covering power than crystalline. The smaller the
crystals of the latter the greater is their covering power, so that in
preparing pigments of a crystalline character care must be taken to
make the crystals as small as possible.
From the above definition of white pigments it follows that an immense
number must exist, since every colourless substance in a state of fine
division appears white. Generally only those bodies are used which
are insoluble in water, or almost insoluble, and which possess great
covering power. The following may be mentioned as white pigments, only
a few of which are in use: white lead, white zinc, permanent white,
lead oxychloride, lead sulphate and sulphite, zinc oxychloride, lead
antimoniate, antimony white, tin white, tungsten white, and in addition
certain earths, pipe clay, china clay, etc. Several of these pigments
are far too expensive for ordinary use, and have no advantage over much
cheaper pigments except for very special purposes, such, for example,
as bismuth white for cosmetics.
In general use we find very few artificial white pigments; these are
lead, zinc and barium compounds. Circumstances may arise which make it
expedient for the colour maker to manufacture other white pigments, for
example, a demand for them, or favourable opportunities for obtaining
the requisite raw materials.
The white lead pigments, of which there is a large number, as we have
indicated, all have the great disadvantage that they are not permanent,
that is, are changed by atmospheric influences. It is well known that
lead is a very delicate reagent for sulphuretted hydrogen, with the
sulphur of which it forms a black compound. Now the air, especially
in towns, contains sulphur in the form of sulphuretted hydrogen or
ammonium sulphide; though the quantity is very small, the fate of
every white or coloured lead pigment is decided by it; after a longer
or shorter time it will be discoloured, will gradually darken, and
finally be turned black. In spite of this great changeableness of lead
pigments they are used by artists and painters, although the majority
could be replaced by more permanent pigments entirely unaltered by the
atmosphere.
WHITE LEAD.
This pigment, which in addition to other good properties has remarkable
covering power, was amongst the earliest known artificial pigments.
Already in the fourth century before Christ, Dioscorides described the
preparation of white lead, which was obtained by exposing lead to the
action of the vapours of vinegar, removing the white layer and treating
it with water. The Roman writers describe a similar method; they use
the name _cerussa_, under which white lead is known to-day in commerce.
Although white lead has been so long known, it was left to Bergmann,
in 1774, to show that it contained carbonic acid; before that it was
believed to be lead acetate. The development of analytical chemistry
was followed by a knowledge of its constitution and the use of more
rational methods of manufacture. Whilst in the middle ages the
manufacture of white lead was almost exclusively in the hands of the
Dutch and Venetians, in later years it gradually spread, and now
many works are concerned in the manufacture of this pigment. That
adulterations of white lead were not rare in former times appears from
the writings of Basil Valentine, an alchemist of the fifteenth century.
Commercially white lead is known under most varied titles, of which the
following are the principal: White lead, Venetian white, Dutch white,
Krems white, Kremnitz white, flake white, etc.
According to its chemical composition, white lead is a compound of
lead carbonate and lead hydroxide, that is, a basic lead carbonate.
Commercial white lead, apart from intentional admixtures of other white
substances, contains lead carbonate and lead hydroxide in varying
proportions, as is shown by the following analyses by Mulder, who found
that all the samples examined by him were composed according to one or
other of the following formulæ:—
2 PbCO₃.Pb(OH)₂, containing 86·27 per cent. of PbO.
5 PbCO₃.2Pb(OH)₂ ” 85·86 ” ”
3 PbCO₃.Pb(OH)₂ ” 85·45 ” ”
4 PbCO₃.Pb(OH)₂ ” 85·00 ” ”
According to Hochstetter, the manufacture of white lead must be
directed to obtaining the compound 2 PbCO₃.(OH)₂, which possesses the
following percentage composition:—
PbO 86·32
CO₂ 11·36
H₂O 2·32
The compound of this composition is distinguished by being completely
amorphous, and so possesses the greatest covering power. Commercial
white leads, as is seen from the formulæ of Mulder, may differ
appreciably from this composition, when they will have a smaller
covering power, since they will contain some quantity of neutral
lead carbonate, PbCO₃ (containing 83·46 per cent. of PbO), which is
crystalline.
White lead is prepared according to very different methods, the
principle of which consists in subjecting a solution of tribasic lead
acetate to the action of carbonic acid, by which the basic carbonate
is produced and the neutral acetate formed, the latter being then
reconverted into the basic acetate, which again serves to produce white
lead and so on.