YELLOW EARTH COLOURS
All the yellow earth colours, without exception, have ferric oxide as
their colouring principle, the differences in shade being entirely
due to the varying proportion in which that oxide is present. The
various names under which they are known date back to a period when
the chemical nature of these colours was still unknown, and have been
mostly derived from the locality of origin.
The yellow earths can therefore be divided into two groups, according
to their chemical character. The first group, in which the ferric oxide
is present as hydroxide, comprises all the ochres, Siena earth, and
a number of others which are obtained from native ochre by special
treatment. In the colours of the second group, ferric oxide is still
the colouring principle, but is combined with other substances in place
of water.
It is, as a matter of fact, incorrect to rank the ochres in general as
yellow earths, because they can be made to yield nearly every variety
of colour from the palest yellow to the deepest red, brown and violet.
These colours merit the particular attention of the colour-maker and
the painter, being distinguished by very low cost of production,
unusual permanence and beauty of tone. In the interests of that highly
important matter to the artist, namely the production of colours of
unlimited permanence, it is desirable that colour manufacturers should
bestow greater care on the manufacture of these colours than has
hitherto been the case. An extremely favourable point about nearly all
these pigments is that they can be very cheaply prepared by artificial
means, so that the manufacturer is in a position to turn out a large
number of the handsomest and most durable colours with a small amount
of expense and labour.
THE OCHRES
Ochres are found in many localities, most frequently in stratified
rock and rubble. The deposits are rarely extensive, mostly occurring
in pockets or beds. Wherever found, ochre may be termed a secondary
product, that is to say, one that has been formed through the
destruction of other minerals. The analysis of ochres from different
deposits shows great divergence in composition; and some consist almost
entirely of pure ferric hydroxide, that has already undergone natural
levigation and can be used as a pigment as soon as dug.
Such a form is, however, rare, and most ochres are intermixed with
smaller or larger amounts of extraneous minerals, the contamination
being sometimes so great as to preclude the use of the ochre as pigment
by reason of the high outlay required for extracting the colouring
constituents.
Occasionally, the ferric hydroxide is associated with a certain
proportion of clay, and as this increases, the ochre passes over into
ferruginous clay. This class can also be used as pigment, in certain
circumstances, that is to say when it is sufficiently rich in ferric
oxide to furnish a deep red mass on calcination. When, however, the
proportion of ferric oxide is low, its pigmentary power is no longer
sufficient, and the clay has not the requisite beauty of colour. The
ordinary earth used for making tiles is an example of this class, its
colour in the raw state being an ugly brownish-yellow, but turning a
dull “brick” red when fired.
In some deposits the ferric oxide is accompanied by lime. Unless the
latter exceeds a certain proportion, such ochres, too, are suitable as
pigments, the lime being easily removed by simple levigation; but when
the amount of lime is high, it is difficult to obtain certain highly
coloured shades of ochre from such material. These shades entail the
calcination of the ochre, and the temperature required is oftentimes
insufficient to transform the lime into the caustic state. Moreover,
the presence of caustic lime would be a drawback in some cases, it
being then impossible to mix the ochre with other colours without
endangering the shade through the action of the lime on these latter.
The following analyses will show the percentage composition of ochres
from various deposits:
Ochre from--
+-------------+---------+----------+--------------+
| | Comoal | Vierzen. | St. Georges. |
| |(Savoy). | | |
+-------------+---------+----------+--------------+
|Ferric oxide | 19 | 23·5 | 25 |
|Lime | 2 | -- | -- |
|Alumina | 20 | 69·5 | 70 |
|Magnesia | 1 | 69·5 | 70 |
|Silica | 44 | 69·5 | 70 |
|Water | 7 | 7 | 5 |
+-------------+---------+----------+--------------+
In the majority of cases the mineralogical characteristics of an ochre
enable conclusions to be formed as to its suitability as pigment. Good
ochre is more or less yellow to dark brown in colour, and can easily
be crushed between the fingers to a soft, fine powder which feels like
powdered steatite and does not produce a sensation of grittiness, this
latter indicating the presence of fine grains of sand in the ferric
oxide. The behaviour of the ochre in presence of water is specially
important. If it adheres firmly to the tongue, and forms a fairly
plastic paste when mixed with a little water, the mineral contains a
large percentage of ferric oxide, and as a rule will yield ochre of
good colour.
In general it may be said that the value of an ochre varies directly
with its content of ferric hydroxide or oxide, because when this is
large the ochre will furnish a wide range of colours under suitable
treatment.
A simple test for quality consists in weighing out an exact small
quantity (10 grms.), and heating it to a temperature not exceeding 110°
C., until the weight remains constant. A simple calculation then gives
the amount of uncombined water in the sample. Since the proportion of
such water varies in different parts of one and the same deposit, the
test must be repeated, in order to obtain accurate results, on samples
taken from different points, or, preferably, on a properly prepared
average sample.
Even drying changes the colour of ochre considerably. To ascertain the
behaviour of an ochre on calcination, a large sample is dried at 110°
C. until the weight is constant, and divided up into a number of small
samples weighing, say, 10 grms. each. The samples are then heated to
different temperatures, one to the melting-point of lead, another to
that of zinc, and so on.
The higher the temperature employed, the more will the colour of the
ochre approximate to red; and specimens very rich in ferric oxide will
give bright red colours. Beyond this range, a further increase in
temperature will give violet shades, varying with the temperature and
the duration of heating. After this preliminary test, it is desirable
to make another on a larger scale, with quantities up to about 1 lb.
For this test, the different kinds of ochre frequently found in the
same deposit should be mixed together, in order to obtain an idea of
what the mean product, obtained in working on the large scale, will be
like.
On the whole, the results of this second test will be the same as in
the first series, the only object of the second test being to gain
information which may be particularly valuable in practical work. The
bottles in which the calcined samples are stored should be marked with
the temperature and length of heating, so that, when it is subsequently
desired to obtain an ochre corresponding to a particular sample, all
that is necessary will be to heat it to the same degree from the same
length of time. The performance of this simple test will be of great
assistance in standardising the work with a minimum loss of time.
When it is desired to ascertain the composition of an ochre
superficially its behaviour towards hydrochloric acid may be noted. A
weighed quantity of the freshly dug (undried) ochre is treated with
pure acid, free from iron, which will dissolve out the ferric oxide
and lime, leaving clay and quartz sand behind. The presence of lime is
indicated by effervescence on contact with the acid; and if there is no
effervescence, lime is absent. At the end of several hours the acid is
carefully decanted from the undissolved residue which is then stirred
up with water, left to subside, and weighed when dry. This method will
give the amount of substances, other than ferric oxide and lime, in
the sample. These substances usually consist of clay or sand.
For a quantitative determination, a small quantity--usually 1 grm.--is
weighed out, treated with a corresponding amount of hydrochloric acid,
and the solution filtered into a glass. The residue on the filter is
washed with distilled water, the washings being united to the acid
solution.
This solution is treated with ammonia so long as a precipitate of
ferric hydroxide continues to form, this being collected on a tared
filter and dried at 110° C. The precipitate may be regarded as pure
ferric hydroxide, and its weight will indicate the proportion of
hydroxide in the ochre with sufficient accuracy for technical purposes.
In reality, however, it is not pure ferric hydroxide, but contains in
addition all the oxides that are precipitable by ammonia, lime being
always carried down as well. It is therefore desirable to dissolve the
precipitate with a little hydrochloric acid, and reprecipitate with
ammonia.
CALCINING (BURNING) OCHRE
In many places ochre is only put through a very simple mechanical
preparation before being sold for pigment, namely left to dry in the
air so that most of the uncombined water evaporates. No matter how this
drying process is protracted, however, it is impossible to get rid
of all the water in this way, a certain proportion being retained by
the hygroscopic action of the ferric hydroxide, and to expel this the
mass must be heated to above 100° C. Drying is usually succeeded by
pulverising and sifting the loose earthy mass, which is then ready for
sale.
When the ochre contains sand or stones, this treatment is not
sufficient, and levigation is necessary. No particular trouble is
involved, the mineral being fairly heavy as the result of its content
of ferric hydroxide. A simple method of treatment suffices to improve
the value of the ochre considerably, and enables a grade that is not
particularly bright-coloured in its natural condition to be converted
into products of very handsome tone and various shades. This treatment
consists in heating the raw ochre to a definite temperature, during
which process the colour changes progressively, and any desired tone
can be obtained by suddenly cooling the hot mass.
The reason for this phenomenon is that the higher the temperature, the
larger the amount of water driven off from the ferric hydroxide, until
finally, when a very high temperature has been reached, the whole of
the water is expelled, and the ferric hydroxide is transformed into
ferric oxide. The hydroxide is brown, whereas the oxide, provided the
temperature has not been raised too high, exhibits the characteristic
colour known as “iron red.”
Consequently, the colour of moderately calcined ochre ranges through
a whole scale from brown to red; and the higher the temperature
employed, the redder the tone. If the heating be protracted after all
the hydroxide has become oxide, the latter undergoes molecular change,
increasing considerably in density and altering in colour; and after
very prolonged heating, the colour finally becomes violet.
The calcination, or burning, of ochre is ordinarily performed in a very
crude manner. The mineral is crushed to the size of peas, and spread
out on an iron plate which is made red-hot. As soon as the ochre has
reached the desired shade of colour, it is dropped into a tub of water
and then crushed to powder. The calcination requires great experience
on the part of the operator, because so long as the product is hot, it
has quite a different colour from that assumed on complete cooling.
Since only comparatively small quantities of ochre can be treated in
this way, and the operation unnecessarily increases the cost of the
product, owing to the large consumption of fuel, it is highly desirable
to employ a simple calcining apparatus capable of treating large
quantities.
Such an apparatus may consist of an iron drum, mounted with a gentle
slope inside a furnace, from which it projects at both ends. A shaft
carrying a sheet metal worm is rotated inside the drum; and the whole
apparatus is very similar to an Archimedean screw.
When the iron drum is raised to a strong red heat, and small quantities
of ochre are fed continuously into the upper end of the drum, the
rotation of the worm will push the material forward, and contact with
the glowing sides of the drum will produce the necessary calcination,
the degree of which can be modified by altering the speed at which the
worm is turned. The calcined product is discharged at the lower end of
the drum, either into a vessel of water, or, if only moderate heating
has been applied, direct into a collector.
[Illustration: FIG. 28.]
Fig. 28 represents an apparatus designed by Halliday for the dry
distillation of wood waste; but, with slight structural modifications,
it can also be used for calcining ochre. The material to be heated is
introduced, in small pieces, into the feed hopper _B_, and is carried
downward, by the worm _C_, into the red-hot drum _A_, through which
it is propelled by the worm _D_ until it drops out, at _F_, into the
tank _G_. The length of time the material is subjected to calcination
depends on the speed at which the worm _D_ is run. The pipe _E_ carries
off the water vapour expelled from the charge.
In order to obtain a uniform product when ochre is calcined in an
apparatus constructed on this principle, it is necessary that the
material introduced should be fairly regular in size, a condition which
is easily fulfilled by squeezing the freshly dug ochre between fluted
rollers, and then passing it over a series of screens, each grade being
then calcined separately.
Moreover, the apparatus is only suitable for calcining at medium
temperatures; and when highly calcined products are in question,
the operation is best performed in fire-clay cylinders, or in thick
cast-iron drums, similar to gas retorts, built into a furnace.
Other devices for calcining ochre will be described later.
OCHRES FROM VARIOUS DEPOSITS
As previously stated, ochres are frequently met with in Nature, both
in the immediate vicinity of iron ore, and also at considerable
distances from such deposits. In the latter case, the ochre must be
assumed to be the decomposition products of ferruginous minerals and
to have been carried off by water until the latter became stagnant and
allowed the ochre to settle down. In their method of deposition these
ochres are therefore analogous to clay, and they, too, often contain
large quantities of extraneous minerals, which have given rise to the
diversified substances grouped under the name of ochre.
Although ochres are so widespread in Nature, only certain kinds, found
in certain localities, have acquired a high reputation. For the most
part, these ochres are such as have already been prepared in a high
degree, by Nature, for the purpose for which they are employed.
Thus, we find that all the ochres which have acquired a high repute
among painters for particular beauty of tone and permanence, are
distinguished by two properties: a high content of ferric hydroxide and
great purity.
The former of these properties imparts brightness of colour; and such
products will furnish, on calcination, a wide range of colour shades.
When, as is the case with the finer qualities of ochre, the mineral
contains only a very small proportion of impurities, there is no
difficulty in bringing it, by simple grinding or levigation, into a
condition in which it is at once fit for use as a pigment.
The Italian ochres have, for long ages, enjoyed a high reputation
for their beauty of colour and permanence. This category includes,
for example, the renowned Siena earth, Roman earth, Italian umber,
and other ochre colours. This high renown is probably due less to
the inherent properties of the mineral than to the circumstance that
the art of painting attained a high state of development at an early
period, and that the artists paid special attention to the use of
bright and permanent colours for their work.
Although, at present, many deposits of ochre are known that are
quite able to compete, on the score of beauty, with the best Italian
products, the good name of these latter has nevertheless been
maintained. It is true that the name of Italian ochre is often merely
borrowed, for application to a product originating in some other
country, varieties of terra di Siena, for instance, being put on the
market that have actually been derived from deposits in Germany.
As a result of this custom, certain names, such as terra di Siena,
umbra di Roma, have become generic terms, and their use denotes, not
an intention to suggest that the earth colours in question really come
from Siena or the vicinity of Rome, but that the properties of the
article are equal to those of the old-established colours of Siena or
Rome.
It would occupy too much space to go into an exhaustive description of
all the native varieties of ochre, and would inevitably lead to a good
deal of repetition. It will therefore be sufficient, for our purpose,
to deal with only a few of them.
The best-known ochres are those of Rome and Siena, the latter being
frequently called, in commerce, by its Italian name, terra di Siena.
Roman ochre forms yellowish-brown masses, of fairly fine texture and
composed of ferric hydroxide and clay. They are put on the market both
in the raw and calcined state. On calcination, the colour soon changes
to red, and if carefully performed, the resulting colours have a very
warm, fiery tone.
Closely approaching Roman earth is the English ochre, which is worked
more particularly in Surrey, and is not infrequently sold as Roman. In
many deposits this English ochre occurs in such a high state of purity
that the best pieces are picked out and sold without being even crushed
or ground. The pieces of lower quality are very carefully ground and
levigated, for the purpose of being calcined for the production of
different shades, and then furnish highly prized colours.
In point of chemical composition, the ochre family also includes terra
di Siena, bole, umber and Cassel brown. These minerals, however, are
not yellow like ochre, but brown, and will therefore be dealt with
along with the brown earth colours.
ARTIFICIAL OCHRES
Products very similar, both in chemical composition and colour,
to the native ochres can also be very simply and cheaply made by
artificial means. Their preparation may be particularly recommended to
colour-makers who desire to turn out a wider range of iron pigments,
but are not in a position to obtain natural ochres at a low price.
In the manufacture of artificial ochre, an endeavour is made to imitate
the natural processes which have led to the formation of ochre,
and, of course, to avoid anything likely to hinder the production
of a suitable colour earth, for example the presence of sand or a
considerable admixture of extraneous minerals.
As already mentioned, the chief impurities in natural ochres are clay
and sand, both of which can be easily excluded during the manufacture
of artificial ochre, or their amount controlled in such a manner that
paler or darker products can be obtained at will, and the tone varied,
in any desired manner, by calcination, as in the case of the native
article.
The raw material for artificial ochre is always a ferrous salt,
which can be purchased in large quantities and at very low prices,
namely green vitriol, which, in the pure state, consists of ferrous
sulphate, FeSO{4} + 7H{2}O. This substance forms sea-green crystals,
which are readily soluble in water and impart an objectionable inky
flavour thereto. On exposure to the air, green vitriol turns an ugly
brown colour, and is no longer completely soluble in water, passing
gradually into the condition of basic ferrous sulphate. This is because
ferrous oxide is a highly unstable substance, which attracts oxidation
and changes into ferric oxide. This latter, however, requires for
the production of soluble salts a larger quantity of acids than does
ferrous oxide, and therefore the oxidation of ferrous sulphate in the
air leads only to the formation of salts that are imperfectly saturated
with acid, namely basic salts.
When a solution of green vitriol is left exposed to the air, basic
ferric sulphate is also formed, which settles down to the bottom of
the vessel as a rusty powder. If, however, a corresponding quantity of
sulphuric acid be added to the solution at the outset, the resulting
ferric sulphate remains in solution.
On treating the green vitriol solution with one of caustic potash,
caustic soda or quick lime, the ferrous oxide is thrown down as
the corresponding hydroxide, forming a voluminous greyish-green
precipitate. This hydroxide still possesses a great affinity for
oxygen, and when the precipitate is brought into contact with air, its
colour rapidly changes to a rusty red, through the transformation of
the ferrous hydroxide into ferric oxide. The ferrous hydroxide can also
be precipitated by alkali carbonates, the deposits behaving in exactly
the same manner as that thrown down by the caustic alkalis.
Various methods can be adopted in the preparation of artificial ochre,
the selection depending on the properties desired in the finished
product. To obtain an ochre with particularly good covering power, the
method must be different from that employed to furnish a cheap product,
in which low price is more important than covering power.
In the former case, the ferrous hydroxide is mixed with substances
which, in themselves, possess fairly high covering power, such as chalk
or white clay; in the second, gypsum, which is of low covering power,
is used.
The preparation of the cheapest kinds of artificial ochre will be
described first, followed by that of the higher grades which belong to
the most valued artists’ colours.
For cheap artificial ochres, the ferrous hydroxide is thrown down by
caustic lime from a solution of green vitriol. According as a lighter
or darker shade is required, two to three parts of ferrous sulphate
are dissolved in water, care being taken to select crystals of a pure
green colour, since those that have a rusty look are only imperfectly
soluble, because they contain basic ferric sulphate.
The solution will always be cloudy, owing to the partial precipitation
of the hydroxide by the lime in the water; but this is immaterial.
For the precipitation, a milk of lime is prepared by slaking one to
two parts of quicklime (according to the quantity of ferrous sulphate
to be treated) in water, and stirring this up in enough water to make
a thin milk. Care must be taken to exclude any large particles of
lime, since these would find their way into the finished product and
make the colour uneven. On this account, the milk of lime should be
carefully strained through a loosely woven cloth or fine sieve, into
the precipitation vessel.
The ferrous sulphate solution is then poured in, the mixture being kept
stirred, and an ugly, grey-green precipitate is produced, consisting of
a mixture of ferrous hydroxide and calcium sulphate, the reaction being
explained by the equation:--
FeSO{4} + Ca(OH){2} = Fe(OH){2} + CaSO{4}.
The larger the amount of ferrous sulphate solution added to the milk
of lime, the darker the resulting ochre. As soon as all the ferrous
sulphate is in, the stirring is suspended, and the liquid is left until
quite clear. The water is drawn off through tapholes in the side of
the vessel, care being taken not to disturb the fine precipitate, and
fresh water is added, in which the deposit is stirred up and again left
to settle down. This operation, which is once or twice repeated, is to
wash the precipitate.
When this object has been sufficiently accomplished, the mass is
shovelled out of the vessel and spread thinly on boards, where it is
left until the desired shade of colour has been attained, the colour
changing quickly on exposure to air, owing to the oxidation of the
ferrous hydroxide into ferric hydroxide. To ascertain whether oxidation
is complete, a large lump of the mass is broken across; and if it is
of a uniform yellow-brown colour throughout, without being darker on
the outside than in the middle, all the ferrous hydroxide will have
been transformed into the ferric state. The product can now be dried at
once, and when ground will be ready for sale.
To obtain different varieties from the product, it is carefully heated
(in a finely powdered condition) in shallow pans; but the operation
needs caution, or the water in the gypsum present will be expelled,
giving rise to drawbacks that are manifested when the colour is used.
For instance, in mixing such a colour with water, the gypsum would
again absorb water and cause the whole mass to set as a useless
solid lump. Since gypsum parts with its water at a comparatively low
temperature, it is better not to heat these cheap ochres at all, but
to obtain the various shades by modifying the proportion of ferrous
sulphate employed.
Another defect of the ochres prepared by this method resides in the
excess of lime present, it being impracticable to measure out the
quantity of lime used with such accuracy that only just enough is taken
to precipitate the ferrous hydroxide, there being always a slight
excess. This lime is transformed into calcium carbonate on the mass
being exposed to the air, just as in the preparation of Vienna white;
but as the saturation with carbon dioxide takes a considerable time,
some of the lime remains in the caustic state and is liable to affect
other colours that may be mixed with the ochre.
An artificial ochre uniting in itself all the qualities of the natural
product, and also capable of being shaded by burning, can be prepared
in the following manner. An accurately weighed quantity of pure
crystallised ferrous sulphate is dissolved in a definite amount of
water, and the solution is treated with successive small portions of
crude nitric acid, until all the ferrous oxide has been changed into
the ferric state. The change can be detected by a very decisive test.
If a liquid containing ferric oxide in solution is brought into contact
with a solution of red prussiate of potash (potassium ferricyanide), no
precipitate is formed in the absence of ferrous oxide, but only a brown
coloration; whereas, if ferrous oxide is present, a beautiful blue
precipitate is formed at once, the colour of which is so intense that
very small quantities of ferrous oxide can be detected by this means.
For the purpose now under consideration, the presence of small amounts
of ferrous oxide in the solution is immaterial, because they are soon
changed into ferric oxide on exposure to the air. It might, therefore,
be asked, why take the trouble to oxidise the ferrous oxide by means
of an agent involving expense, which could be saved by allowing the
oxidation to take place in the air?
The advantage, however, of the direct employment of a solution of
ferric oxide is that it gives at once a colour that can be dried
straight away; whilst at the same time the colour undergoes no change
in drying, whereas it does when ferrous oxide solution is used.
The method of producing ochres from this ferric solution varies
according as the product is to be used without any further treatment
than drying, or is to be modified by firing.
In the former event, caustic lime is again used as the precipitant, but
in only just sufficient quantity to throw down all the ferric oxide in
the solution. This amount can be calculated exactly, 36·84 parts by
weight of pure burnt lime being required for every 100 parts of pure
ferrous sulphate taken. The actual quantity, whether larger or smaller,
will depend on the relative purity of the sulphate and lime; and this
can readily be ascertained by a simple trial.
The lime is used in the form of milk of lime, as already described. If
lime alone is employed, the precipitate will consist of pure ferric
hydroxide and the calcium sulphate thrown down at the same time. The
resulting colour, when dried, will be an intensely brown mass, which
can be used in place of the very dark natural ochres.
In order to obviate entirely the disadvantages resulting from the
presence of a large amount of caustic lime in the precipitate, fine
levigated chalk or white clay is added in the preparation of the
lighter shades of ochre, the addition being made as soon as the
two ingredients have been brought into contact; and the mixture is
thoroughly stirred, to ensure uniform admixture with the ferric
hydroxide. The colour of the settled deposit will be lighter or darker
in proportion to the amount of chalk or clay employed; and in this
way the whole range of shades from pale yellow to bright brown can be
obtained without the application of heat.
Ochre that has been made with chalk is unsuitable for toning by heat,
because this treatment would causticise the lime, and the ochre could
not be mixed with other colours, since these would be affected by that
substance. On the other hand, when white clay is used in preparing
the ochre, the latter can be more easily toned by firing, provided
care be exercised in the process. The ochre must be dried completely
in the air, and either spread out in thin layers on iron plates, for
the burning process, or else put into a drum, of the kind already
described, in which the mass is moved onward by a worm.
The clay remains unaltered in firing, but the gypsum parts with its
water of crystallisation. In order to restore the latter, the ochre
issuing from the drum is discharged direct into a vessel of water, in
which it can be kept in constant motion by a stirrer. The water is soon
warmed by the heat of the mass, and absorption by the gypsum proceeds
at a rapid rate. When the whole charge has been fired and collected in
the vessel of water, the stirrer is stopped and the precipitate dried,
being then ready for use.
In certain circumstances, ochre can be made by other methods. In large
towns, ammonium salts are sometimes obtainable at a moderate price,
being manufactured in large quantities as a by-product in gasworks. For
our purpose, crude gas liquor might be used, since it contains ammonia
for the precipitation of the ferric hydroxide. In most cases, however,
this gas liquor contains only very small quantities of ammonia, and,
therefore, in a works of any size, very large vessels would be needed
for the production of a comparatively small quantity of ochre. On this
account, preference is given to crude carbonate of ammonia, which is
also obtainable at low prices.
On bringing a solution of this salt into contact with one of ferric
oxide, ferric hydroxide is precipitated, and the sulphate of ammonia
resulting from the reaction remains in solution. By stirring white clay
into the liquid at the same time, the ochre can be correspondingly
lightened in shade.
The precipitates obtained in this way can be dried at once, and
converted into any shade obtainable with natural ochre, from brown
to red, by strong firing. The sulphate of ammonia still remaining in
the air-dried product is completely volatilised by the heat, and the
resulting ochres are even superior to the natural varieties in beauty
and permanence.
OCHRES AS BY-PRODUCTS
In the manufacture of certain chemicals, substances of divergent
composition are obtained which are sold under the name of ochre and are
used as painters’ colours. Whereas ochre, properly so-called, consists
of either ferric hydroxide or ferric oxide in association with clay,
lime, etc., the products now under consideration are basic ferric salts
composed of varying quantities of ferric oxide in combination with
certain proportions of sulphuric acid.
These ochres are obtained as by-products in the manufacture of green
vitriol from pyrites, and in alum manufacture; and, according to their
origin, they are classed as vitriol ochre, so-called alum sludge, and
pit ochre. All the basic ferric sulphates of which they are composed
form fairly large crystals, and, therefore, in most cases, the covering
power is small. On this account the products are of low grade and are
put on the market at low prices, for which reason they are largely used
in making cheap paints.
_Vitriol Ochre._--Commercial green vitriol is, for the most part,
manufactured from native sulphides of iron. When many of these
sulphides are piled in heaps and left to the action of the air, oxygen
is gradually absorbed and green vitriol is formed which is dissolved
out by rain and is collected in large clarifying tanks.
In the case of pyrites, however, the mineral must first be roasted in
a current of air, since otherwise its conversion into green vitriol
would only proceed in a very sluggish manner. In any event, the aqueous
solution of ferrous sulphate has to be concentrated, by evaporation, to
the point at which the green vitriol crystallises out.
Both in the clarifying-tanks and--still more so--in the
evaporating-pans, a rusty-looking sediment forms at the bottom,
consisting of basic ferric sulphate. This originates in the partial
oxidation of the ferrous oxide (first formed) while the pyrites is
exposed to the air, and since the quantity of sulphuric acid present is
insufficient to saturate all the ferric oxide, basic salts are produced.
The yellow-brown sludge deposited in the pans during the concentration
of crude green vitriol liquor, constitutes the product termed vitriol
ochre, which contains varying amounts of ferric oxide, sulphuric acid
and water, according to the quantity of ferric oxide resulting from the
oxidation of the pyrites and the character of the latter, _e. g._:--
Ferric oxide 65–70%
Sulphuric acid 14–16%
Water 13–16%
Although the colour of these ochres is not particularly handsome, they
can be transformed, by firing, into colours of fairly good quality. As
this subject will be more thoroughly gone into when dealing with the
preparation of the red iron pigments, the applicability of these ochres
will only be casually referred to here. During the burning process,
these ochres, of course, part with the whole of their contained water;
and by protracted, high calcination, the whole of the sulphuric acid
can also be expelled, so that finally nothing but pure ferric oxide is
left.
_Alum Sludge._--Solutions of crude alum always contain a certain amount
of ferric oxide which settles down at the bottom of the pans during
concentration. This sludge, too, consists of basic ferric sulphate,
but is inferior in covering power to vitriol ochre, the crystals being
of coarser grain. On the other hand, the ochreous sediment from the
alum concentrating-pans has the valuable property of being readily
transformable into red-brown to pure red tones by burning. For this
reason, particular attention has been devoted to this sludge in a
number of alum works.
Since the products are only of value when burned, and the shades
thereby obtained are always red, they will be dealt with more fully
along with the red earth colours.
_Pit Ochre._--Springs containing small quantities of ferrous sulphate
and other salts are met with in many iron mines, but, in most
cases, the amounts are too small for their recovery by artificial
concentration to be contemplated. If, however, the conditions allow of
the springs being easily diverted, they may often be utilised for the
preparation of low-grade ochre.
The chemical composition of these pit ochres varies considerably, and
depends on the geological character of the locality. Water can only
dissolve such minerals as occur in the form of fairly readily soluble
compounds; and for this reason pit waters are always solutions of the
metals which are found in the mine.
The variety of compounds that may be present in an ochre can be seen
from the subjoined analyses of ochres deposited from pit waters at
Rammelsberg. As elsewhere, two distinct classes of ochre are met with,
having a conchoid and an earthy fracture respectively. The latter
usually contain rather more ferric oxide, and, in particular, a higher
content of foreign substances, the most important of which is quartz
sand. In the Table, the ochres with conchoid fracture are marked A, and
those with an earthy fracture, B.
A. B.
Ferric oxide 68·75 63·85
Zinc oxide 1·29 1·23
Copper oxide 0·50 0·88
Sulphuric acid 9·80 13·59
Water 15·52 18·45
Clay and Quartz 4·14 2·00
The preparation of the ochre is a simple matter, consisting in
collecting the mass and sorting out the loose, earthy portions of a
pure yellow colour from the denser and darker parts. The former are
dealt with separately, usually by a simple process of levigation, for
the sole purpose of getting rid of the earthy matter, quartz sand in
particular.
The denser varieties require much more work, but yield a far superior
product, which, by suitable treatment, can be converted into the
finest grades of ochre. The first operation consists in a very careful
crushing, and as the pieces are often very hard, they are treated
in ordinary or stamp-mills, edge-runners being also employed with
advantage.
The product reduced by any of these means is passed through a number
of sieves, to separate the fine particles from the coarse; and the
finest dust is burnt. This last treatment causes a considerable loss
in weight, both the accompanying water and most of the sulphuric acid
being volatilised. However, since, as already stated, all varieties of
ochre can be obtained, the process is consequently very remunerative
notwithstanding the loss in weight it involves.
_Yellow Earth._--From the particulars given in the general description
of the earth colours, yellow earth may also be regarded, to some
extent, as an ochre, but one containing a large proportion of foreign
substances. It might, however, be more accurately termed a clay
contaminated with a considerable amount of quartz sand and a certain
proportion of ferric oxide. The method of preparation is on the
same lines as for ochre, but burning is never practised, nor is the
treatment so careful as for the better grades of ochre, the low price
of the colour making this unremunerative.