RED EARTH COLOURS
The number of minerals that can be directly used as red earth pigments
is comparatively small, and by far the greater proportion consist of
ferruginous colours, a few of which are obtained by the mechanical
treatment of native iron ores or clays coloured red by ferric oxide,
the majority, however, being formed by burning certain materials of
another colour. To these belong nearly all the materials mentioned in
connection with the ochres and the brown iron colours, together with a
few by-products of the chemical industry.
In addition to the foregoing, which have ferric oxide for their
pigmentary principle, is the native mercury sulphide, occurring, as
scarlet, crystalline masses, under the name of cinnabar (vermilion).
The only reason for including natural vermilion with the earth colours
is to make the list complete, the largest proportion of this pigment
being prepared by artificial methods. The product sold as “Chinese”
vermilion may, in former times, have really been introduced from
China into Europe, and prepared there by grinding and levigating the
best-coloured lumps of the natural cinnabar; but, at the present
time, all the vermilion made--in Europe at least--is from sulphur and
mercury, by artificial processes, and the name Chinese vermilion is
merely retained to designate a particularly fine grade.
On the basis of occurrence and chemical properties, the red earths
can be classified into several groups. The first comprises natural
products requiring only mechanical preparation, such as the minerals
known as hematite, micaceous iron ore, Elbaite, etc., and the special
modification of red ironstone termed raddle. All these minerals consist
almost entirely of ferric oxide in a pure state. The mineral, bole (red
chalk, terra sigillata, Lemnos earth), is chemically allied to the
ochres, being, like them, composed of alumina, frequently accompanied
by lime and small quantities of magnesia, but differing in that ferric
oxide is always present in bole, whereas the ochres always contain
ferric hydroxide.
The second group consists of the artificial reds obtained by burning
or calcining raw materials, whose ferric hydroxide is more or less
transformed by heat into ferric oxide, such as vitriol ochre, pit ochre
and alum sludge.
Of late years the artificial earth colours have attained a high degree
of importance. They are obtained in large quantities in the manufacture
of sulphuric acid from green vitriol. Formerly, it is true, they were
also used as pigments under the name of caput mortuum or colcothar, but
were not held in much esteem; and it is only within recent times that
it has been discovered that these inferior by-products can be converted
into very handsome and brilliant colours, which now form important
articles of commerce.
BOLE
Bole, Lemnos earth, terra sigillata, etc., is, for the most part, a
product of the decomposition of highly ferruginous minerals, and
occurs, in the form of lumps, having a conchoidal fracture, in pockets
or detritus. The lumps have a sp. gr. of 2·2–2·5, are Isabella brown to
dark brown in colour, and give a slightly greasy-looking streak. There
are two distinct varieties of bole: the one adhering firmly to the
tongue, whilst the other lacks this property and, when placed in water,
crumbles down to powder in emitting a peculiar noise.
The composition of the boles varies, but all of them may be regarded
as alumino ferric silicates combined with water. Most of the specimens
examined from different deposits contain 24–25% of water, 41–42% of
silica, and 20–25% of alumina, the remainder consisting of ferric oxide
with small traces of manganese oxide.
Some varieties, however, are exceptional and contain only 30–31% of
silica and 17–21% of water, _e. g._ those from Orawitza and Sinope.
Lemnos earth, the true terra sigillata, is mostly silica (66%) with 8%
of water, and contains a smaller percentage of ferric oxide than the
others. It is also of a distinct colour, lighter than the true boles
and having a greyish or yellowish tinge.
The behaviour of the different kinds on burning is just as diverse as
their chemical composition. Whilst some kinds are infusible at even the
highest temperatures, and merely change into hard, red masses; others,
again, fuse at a moderate heat. This difference is due to chemical
composition, those high in silica being generally less refractory than
those in which alumina preponderates.
In order to render the boles suitable for painting, they are put
through a somewhat different treatment than the other earth colours.
The freshly dug material is first sorted, the uniformly coloured lumps
of fine texture being set apart and suffused with water, with which
they form a pasty mass of low plasticity, which is kneaded by hand
to make it homogeneous, and is then stirred up with more water. When
the lumps have distributed in the water, the latter is drawn off into
a second tub, and the residue is stirred up with fresh water, the
treatment being repeated until the effluent no longer shows any signs
of colour.
The liquid in which the finely divided bole is suspended is left to
settle, and the bole subsides as a fine powder, which is dried to the
condition of paste, pressed into moulds and dried completely.
Owing to its low content of ferric oxide, the colour of bole is not
particularly bright, but is very permanent--a property equally shared
by all the other ferric oxide pigments.
NATIVE FERRIC OXIDE AS A PIGMENT
In nature, ferric oxide forms extensive deposits, which, by reason of
the light red colour characteristic of certain varieties of ferric
oxide, are largely employed in painting. These colours may be classed
among the oldest known to mankind, ferric oxide pigments having been
used frequently in the most ancient paintings.
The most important varieties of ferric oxide for our purpose are: iron
glance, with its modifications, micaceous iron ore and frothy hematite;
red hematite, and raddle.
IRON GLANCE
This substance forms handsome black crystals of very high lustre,
which, when small and scaly, constitute micaceous iron ore. Both,
when rubbed down, furnish a dark red powder of no particular beauty.
Micaceous iron ore forms the transition stage into frothy hematite, or
iron cream, the sole difference being that the crystals of the latter
are much smaller, and the scales finer, the iron-black colour passing
gradually into cherry red. At the same time, the lustre, though still
high, loses most of the metallic sheen exhibited by micaceous iron ore.
HEMATITE
The variety known as hematite or bloodstone, sometimes occurring as
shiny nodules, is distinguished by its handsome red colour. Some of
the lumps are composed of long, thin crystals grouped about a common
centre so as to form a globular mass. Despite its bright colour, the
hardness of hematite (between 3 and 5) prevents it from being used as a
pigment, the value of the product not being commensurate with the cost
of reduction.
RADDLE
There are numerous deposits of red ironstone, in the state of fine
earth, where the operations of grinding and levigation have, to a
considerable degree, already been carried out by Nature. These deposits
form the mineral which, under the name of raddle, is often used as a
pigment for ordinary paints. It may be considered to have originated
in the transformation of red ironstone, by the natural forces that can
everywhere be seen disintegrating rocks, namely water and frost, into a
fine powder, which has been transported, often over long distances, by
water, and has finally settled down.
In places where the process has been carried out in this manner, the
raddle will be in a condition, as regards fineness of division and
beauty of colour, that leaves nothing to be desired, and the material
itself is ready for use as a very valuable pigment. Large deposits of
this kind, however, are of rare occurrence; but there are plenty in
which the ferric oxide is associated with varying quantities of clay,
sand, and sometimes lime.
The conditions here are on all fours with those of clay, which, too,
has been formed in a similar way. Pure clay, the so-called kaolin, is
a highly valuable material, whereas ordinary loam--highly contaminated
clay--is only of low value. In judging the quality of raddle as a
pigment, the presence of impurities is of less account than their
nature; and in some cases a very highly contaminated raddle may be
worth far more, as a pigment, than one containing only very small
admixtures of extraneous substances.
As stated above, the ordinary impurities in raddle are clay, lime and
quartz sand. An admixture of clay, even if fairly large, is no great
drawback, since the material can be used in its natural state, and also
be toned by burning. Lime is less favourable, for though a calcareous
raddle can be used as it is, the lime parts with its carbon dioxide
on calcination, becoming changed into caustic lime and imparting to
the product qualities which preclude its employment for a number of
purposes, especially for mixing with delicate organic colours.
The presence of quartz sand is immaterial when the raddle is to
be burned, inasmuch as sand is unaltered by calcination. But it
constitutes a drawback because it makes the fine raddle gritty and
unsuitable for fine paint work. The only way to eliminate this impurity
is by levigation--an expensive operation which should, as far as
possible, be avoided for these native ferric oxides, because they must
be sold very cheaply, and have to compete with the large quantities of
oxide obtained as a by-product of the chemical industry.
The suitability of a given specimen of raddle for use as a pigment may
be easily ascertained by weighing out exactly 100 grams and heating
to about 120° C. The loss of weight will give the amount of water in
mechanical retention. The residue is suffused with strong vinegar,
and left for several days, being stirred at frequent intervals. The
carbonates of lime and magnesia present will dissolve in the acid,
the ferric oxide remaining untouched. The liquid is decanted, and the
residue washed several times with water and dried, the diminution in
weight being a measure of the carbonates in the sample. If the vinegar
has turned a yellow colour, the presence of ferric hydroxide in the
mineral is indicated, this hydroxide being readily soluble in acetic
acid. If the residue feels gritty, it contains quartz sand, the amount
of which can be found with sufficient accuracy by levigating the mass
and weighing the sandy residue after drying.
Deposits occur, in many places, of a mineral similar to raddle, but
formed under peculiar conditions. Thus, there are found, in the
vicinity of brown-coal deposits that are rich in pyrites, earthy
masses which are occasionally of a handsome red colour and consist of
a variety of minerals admixed with a considerable proportion of ferric
oxide.
These masses probably originated in fires in the coal seams, whereby
the pyrites became transformed into ferric oxide and basic ferric
sulphate; and where the deposits are of sufficient size, they may be
advantageously utilised in the production of cheap reds. In most cases,
however, the minerals must be levigated, owing to the frequency with
which they contain large proportions of extraneous minerals in a gritty
condition.
BURNT FERRIC OXIDE AND OCHRES
It has already been stated, in dealing with the yellow ochres, that
these colours can be toned by burning, part of the ferric hydroxide
losing its water and changing into red ferric oxide. The more severe
the burning, the larger the amount of ferric oxide formed and the
nearer the colour of the product approximates to red. According,
however, as the original ochre was yellow or brown, the tone of the
burnt colour will lie between orange and brownish red. If the heating
be pushed so far as to transform all the ferric hydroxide into oxide,
the red will come more and more into prominence in proportion to the
amount of hydroxide in the original material. If the product consists
entirely of ferric oxide, as is the case with that obtained, as a
by-product, in the manufacture of English sulphuric acid, a pure
red ferric oxide (caput mortuum, colcothar, English red, etc.) will
be obtained. If the heating be increased above a certain point, the
pure ferric oxide will change colour, assuming a brown to violet tone
according to the temperature employed.
(_a_) _Burning in the Muffle_
Since, as a rule, the quantity of material treated in the preparation
of these brown, violet to black ferric oxide pigments for the
purposes of the painter on porcelain is not large, the same kind of
muffle furnace (Fig. 29) as serves for making enamels can be used. The
fire-clay muffle _M_ is inserted in a reverberatory furnace _O_, with
a good draught, and is raised to a white heat. The finely powdered
material to be burned is spread out evenly on plates of sheet-iron or
fire-clay, and introduced into the white-hot muffle, where it is left
for a period corresponding to the colour desired. To save time, the
plates may be pre-heated in a second muffle arranged above the first.
[Illustration: FIG. 29.]
By this means a large range of tones can be obtained from one and
the same material, by heating it to different temperatures; and the
colours so produced are distinguished, not only by their warmth of
tone, but also by very high stability. In fact, they may be regarded
as permanent, because very strongly calcined ferric oxide only passes
very slowly into solution even under prolonged boiling in the strongest
acids. Owing to this excellent property, which is equalled by very
few other pigments, and the low cost of preparation, these colours
deserve the most careful consideration by all manufacturers who are in
a position to obtain suitable material in sufficient quantities.
(_b_) _Caput Mortuum, Colcothar_
Previous to the English method of making sulphuric acid by the
oxidation of sulphur dioxide with nitric acid, this acid was
manufactured by heating dehydrated ferrous sulphate (green vitriol);
and even now, fuming sulphuric acid--oil of vitriol, or Nordhausen
sulphuric acid--is largely obtained by the same process.
When anhydrous ferrous sulphate, FeSO{4}, is exposed to a very high
temperature--strong white heat--it is decomposed into sulphur trioxide,
SO{3}, sulphur dioxide, SO{2}, and a residue, mainly composed of ferric
oxide and a little basic ferric sulphate, which remains behind in
the heating-pan. In fact, even at the highest possible temperatures
obtainable in the furnaces used for the distillation of the green
vitriol, it is impossible to recover the whole of the sulphuric acid, a
small portion being tenaciously retained by the iron.
This red residue is sold under various names--colcothar, caput mortuum,
English red, Indian red, etc.--and is used as a low-grade pigment,
and also as a polishing agent. The name caput mortuum is a survival
from the time of the alchemists, and was probably applied to indicate
a dead-burned product, from which all the active ingredients had been
removed.
Although, in former ages, this substance was held in low estimation as
a pigment, attempts have been made in recent times to convert it, by
suitable treatment, into a more valuable product; and these attempts
have been crowned with success, affording another instance of how a
high commercial value can be imparted to a waste product by proper
manipulation.
(_c_) _Calcining Ferric Oxide_
In order to obtain a series of tones of colcothar, it is subjected to
repeated calcination, but not by itself, since it would require an
extremely large quantity of fuel to effect any change of tone in view
of the very high temperature the material has already been exposed
to in the sulphuric acid plant. If, however, salt be added, then a
variety of tones can be obtained without recourse to any particularly
high temperature. It is frequently stated that the only effect of
the presence of salt is to keep the calcining temperature uniform,
inasmuch as the salt volatilises at a strong red heat, and when that
temperature is reached, the whole mass cannot get any hotter until the
whole of the salt has passed off, all the heat applied being consumed
in transforming the salt into the state of vapour.
As a rule, however, the amount of salt added does not exceed 6% of the
weight of the charge to be calcined; and this quantity does not seem
to be sufficient to keep the temperature at a uniform level through
the several hours required for the calcining process. The author is
therefore of opinion that the salt also has a chemical action on the
material during the calcination.
As already mentioned, colcothar is by no means pure ferric oxide, but
always contains basic ferric sulphate. Now, it is feasible that some
reaction may take place between the basic sulphate and the sodium
chloride at calcination temperature, with the formation of caustic
soda, which, being a far more powerful base than ferric oxide, deprives
the latter of sulphuric acid, sodium sulphate being formed. The
chlorine of the salt combines with the iron to form ferric chloride,
which volatilises at a glowing heat.
According to this hypothesis, therefore, the addition of common salt
in the calcination of colcothar is less for the purpose of maintaining
a uniform temperature within certain limits than for decomposing the
basic ferric sulphate present and inducing the formation of a product
consisting entirely of pure ferric oxide. The various tones obtained
are due to the varying length of exposure to the heat.
The following method is pursued in the conversion of colcothar into
iron pigments on a manufacturing scale. The crude colcothar from the
sulphuric acid plant is ground, as finely as possible, in ordinary
mills, and the resulting soft powder is intimately mixed with salt,
2, 4 or 6% being the usual proportions added. The calcination is
ordinarily continued for six hours in the case of the mixture
containing the largest amount of salt; but only two hours, or even one,
for the other mixtures.
The operation is carried on in earthenware pipes, a large number of
which (up to sixty) are built into a furnace. The latter is fired very
carefully, the temperature being raised only very gradually, since
experience has shown that much better coloured products are obtained in
this way than by raising the mass quickly to a high temperature.
When incandescent ferric oxide is allowed to cool down with
unrestricted access of air, the colour is not nearly so bright as when
air is excluded during the cooling. Since air has no action on ferric
oxide, this remarkable phenomenon cannot be due to the presence of
the air, but probably to the influence exerted by the rapid change
of temperature on the arrangement of the finest particles of the
oxide. Nevertheless, some manufacturers hold that rapid cooling, with
restricted access of air, improves the colour.
To exclude air from the ferric oxide during calcination, the open ends
of the pipes are flanged and covered with close-fitting plates, which
are luted with clay. The expansion of the internal air as it grows hot
would burst the pipes unless a means of escape were provided, which
consists in leaving small vent holes in the cover plates.
As previously mentioned, calcined ferric oxide is very inert,
chemically, so that, when the calcination has been strong, prolonged
boiling with the most powerful acids is needed to bring the oxide into
solution. If the heating has been continued up to the strongest white
heat, and the ferric oxide maintained in that condition for several
hours, even hot sulphuric acid will have only a slight effect on the
oxide, and the only way to make it more readily soluble is by fusion
with potassium bisulphate.
Now indifference to chemical action is just the property required of a
pigment for fine work; and in this respect, the ferric oxide colours
are superior to all others. The gradations of tone that can be obtained
from ferric oxide by varying the calcination are very numerous,
comprising all between iron red, red-brown and pure violet.
The author has tried heating ferric oxide for a considerable time at
a very high temperature, equivalent to the strongest white heat, and
obtained a product which was no longer pure violet, but had a decidedly
blackish colour. Perhaps, by greatly prolonging the heating, it might
be possible to get a pure black; but, even if this were so, the matter
would be of no special interest, because black pigments for paints can
be prepared in a much cheaper manner. All that would be accomplished
would be the proof that ferric oxide actually undergoes an extensive
molecular modification when heated.
FERRIC OXIDE PIGMENTS FROM ALUM SLUDGE
Alum is manufactured from alum shale and alum earth, the former being
a carbonaceous clay shale interspersed with pyrites, and the latter
a clay charged with pyrites and bitumen. The raw materials are left
in heaps for several years, the pyrites being thereby oxidised with
formation of free sulphuric acid and ferrous sulphate. This free acid
reacts further on the clay, which it transforms into sulphate of
alumina; and by leaching the heaps with water, a solution is obtained
which contains the sulphate of alumina and the ferrous sulphate. On the
liquor being concentrated, a basic ferric sulphate is deposited, which
is worked up into red pigment.
For this purpose it is first levigated in a special manner, the sludge
from the pans being placed in a large vat, suffused with water, and
kept in slow circulation by stirrers, which distribute the particles
in the water, forming a turbid liquid. This liquid is conducted into a
gently sloping shute, the sides of which are perforated with openings
at certain intervals, to allow part of the liquid to run off into
large collecting vessels underneath.
The heaviest of the suspended particles settle down first and are
flushed out by the water escaping through the first opening. The finer
the particles, the longer they remain in suspension, so that the liquid
escaping through the last holes carries off only a very fine powder.
The liquid collected in the different vessels is allowed to subside and
is then drawn off from the firm deposit. The operation is repeated with
fresh quantities of sludge until sufficient sludge has been collected
for further treatment. The collecting vessel furthest away from the
intake of the shute contains the finest levigated material, and this is
used for making the best ochres.
The levigated mass is dried in a very simple manner, being usually
spread out on boards, which are exposed to the air in open sheds,
covered with a roof to keep out the rain. Here the sludge is left until
it forms a pasty or earthy mass, and is then calcined.
The best calcining furnace is of the type used for colcothar; but the
pipes must be connected to an exhaust pipe for carrying off the vapours
disengaged during calcination.
[Illustration: FIG. 30.]
However, since alum manufacturers do not usually go in for making the
highest-grade pigments, simpler calcining furnaces are used, consisting
of reverberatory furnaces in which the heating-gases are allowed to
act directly on the materials of the charge. A front elevation and
section of such a furnace are shown in Figs. 30 and 31. The furnace
is constructed with several arches, one above another, marked _c_,
_k_, _d_. The charge is introduced through the openings _b_ and _b’_.
The furnace chamber is at _a_, and the ashpit at _g_. The gases of
combustion flow over the charge on the hearths of the several arches
and escape, at the top, into the stack, along with the acid vapours
liberated from the glowing mass.
[Illustration: FIG. 31.]
The further the hot gases get away from the fire, the cooler they
become, and therefore the less strongly heated the charge on the upper
hearths. Consequently, the resulting product (ferric oxide) from
the different stages of the furnace differs in colour; and a number
of gradations can be obtained by blending. The ferric oxide pigments
prepared in this way are not pure oxide, but also contain small
quantities of sulphuric acid and metallic oxides which were present in
the original crude sludge. However, by reason of the simple process of
preparation employed, these pigments are usually sold at lower prices
than those from colcothar; and for less fine work they are excellent.