THE RAW MATERIALS FOR EARTH COLOURS
The minerals constituting the raw materials for the preparation of the
earth colours occur under very divergent conditions in Nature. Some
of them, such as chalk, form immense deposits, even whole mountains,
whilst in other cases, _e. g._ the blue ferruginous earths, the
occurrence is connected with certain local conditions, and many are
found only in isolated deposits, as pockets or beds. This last is the
case, for instance, with the handsome brown iron pigments; and indeed
the names by which they are known indicate that they are only found in
well-defined localities, or that they are met with of special quality
there. The brown earth colour known to all painters as Terra di Siena,
is found at many other places as well as near Siena, but the product
from that city acquired aforetime a special reputation for beauty, and
therefore all similar earths, provided they are equal to that from
Siena, also bear the same name in commerce.
A number of raw materials for the preparation of earth colours are
found, it is true, in many deposits, but their utilisation depends, in
turn, on local conditions. For example, many copper mines contain, in
addition to the other cupriferous minerals, those used, in the powdered
state, as ultramarine or ultramarine green, and not infrequently lumps
of mineral are found containing both blue and green together. However,
it is only when these minerals occur in sufficient quantity to make the
necessary sorting profitable that their manufacture into pigments can
be regarded as practicable.
Before commencing to work a deposit it is essential to make sure
whether the raw material, or pigmentary earth, is actually suitable
for the manufacture of earth colour. Even the general character of the
material is important, those of soft, earthy consistency being much
easier to treat, and the cost of preparation smaller, than if the raw
material be hard, tough and crystalline.
The extent and thickness of the deposit, and the ease with which it can
be worked, also play an important, and even decisive part, since, other
conditions being equal, it will not pay to erect a colour works unless
the raw material is available in sufficient quantity and is cheap.
Generally, the deposit is not homogeneous throughout, the mineral being
purer in some places and more contaminated with gangue in others.
The percentage of moisture also varies, and in short, a number of
circumstances must be taken into consideration in forming a conclusion
as to whether a deposit is workable or not.
In order to arrive at a reliable opinion on all these conditions,
sampling is indispensable. If the samples are of uniform character,
they can be mixed together to make an average sample. But if they
differ considerably in appearance, general character, proportion
of gangue, etc., it is preferable to examine them separately, more
especially when the area which each represents is large.
The examination should extend, on the one hand, to the natural
percentage of moisture, and, on the other, to the purity of the
material. The water content is determined by thoroughly drying a
weighed sample, bearing, however, in mind the fact that pigmentary
earths of a clayey nature vary in water content according to the time
of year, besides changing in accordance with the weather when the won
material is stored in the open.
The purity can only be ascertained by an examination in which a
sample of the soft, clayey material is crushed and passed through a
narrow-mesh gauze sieve, the amount of the coarse particles--sand,
small stones, etc.--remaining on the sieve being determined. A more
accurate method, of course, is to separate the true pigmentary earth
from the gangue by levigation. For this purpose, a weighed quantity
of the crushed, air-dry sample is placed in a relatively narrow glass
vessel and thoroughly mixed with water, the turbid supernatant liquid
being poured off after a short interval. The residue is repeatedly
treated in the same way, until no more fine particles remain in
suspension, the residue then consisting of impurities, or gangue. Of
course, the washings can be collected, the suspended matter allowed to
settle, and finally weighed in an air-dry condition. By this means an
approximate idea of the yield of earth colour can be obtained at the
same time.
Raw materials which are not amorphous, soft and clayey must first be
crushed, an operation facilitated by heating to redness and quenching
in cold water. Oftentimes the heating causes a change of colour and
improves the covering power--a point to which reference will be made
later on.
In the following description of the various raw materials, the
chemical composition of the pure minerals will be given, together with
an enumeration of the most common impurities.
(A) WHITE RAW MATERIALS AND PIGMENTARY EARTHS
_Limestone_ (_Calcite_, _Limestone_, _Chalk_)
The number of materials furnishing white earth colours is comparatively
large, and these colours are particularly important, because, not
only are they extensively used by themselves, but they also serve as
adjuncts to other colours and for the production of special shades. The
chief raw material for the preparation of white earth colours is the
mineral calcite in its numerous modifications.
Calcite, or calc spar, occurs very frequently in Nature, and is one
of the most highly diversified minerals known. In its purest state
it appears as “double spar” (calcite), in the form of water-white
crystals, which are very remarkable for certain optical properties.
White marble is also a very pure variety of calcite, in which the
individual crystals are very small. The various coloured marbles owe
their appearance to certain admixtures of extraneous substances,
chiefly metallic oxides.
No sharp line of demarcation separates marble from ordinary limestone,
the difference between them really consisting only in the degree of
fineness of grain; and all limestones which grind and polish well
may be classed as marble. As is the case with marble, there are also
limestones of various colours, grey being, however, the most common.
This grey limestone forms huge mountain masses which, in Europe,
follow for example, the Alpine chain on its northern and southern
edges.
A few other examples of calcite may be mentioned which occur in
certain localities and, in part, are still in course of formation. To
these belong the stalactites and stalagmites, which sometimes consist
of extremely pure calcite. They are formed by the action of water,
containing carbonic acid in solution, which trickles through cracks and
cavities in limestone rock and dissolves out calcium carbonate from the
adjacent stone. On prolonged exposure to the air such water gives off
its free carbonic acid again; and as the calcium carbonate is insoluble
in pure water, it separates out in crystalline form. The masses formed
in this way usually resemble icicles in shape, and the finest examples
are to be found in the well-known stalactite grottoes at Krain, whilst
the grotto at Adelsberg is renowned for its beautiful stalactites.
Occasionally, stalactites have an opaque yellow or brownish tinge,
which they owe to the presence of iron oxide.
A formation similar in its origin to stalactites is the so-called
calc sinter and calcareous tuff. The former often occurs in cavities
as irregular masses which, in some places, enclose large quantities
of fossil animal bones, in which case they form “bone breccia”
(crag breccia). Calcareous tuff is deposited from numerous springs,
occasionally in very large quantities, enveloping plants and sometimes
forming thick deposits in which the structure of the plants can be
clearly recognised.
In some places a more or less pure white, extremely friable variety
of calcite is met with under the name “mountain milk” or “mountain
chalk” (earthy calcite), which seems to be a decomposition product,
and consists of a mixture of arragonite and chalk. Arragonite--which
will be referred to later--is completely identical, chemically, with
calcite--both being composed of calcium carbonate--the sole difference
being their crystalline form.
The most important for the colour-maker, however, is the variety known
as chalk. This is really a fossil product, _i. e._ it consists of the
microscopic shells of marine animals united into solid masses. Despite
the smallness of these animals, their epoch lasted long enough for
their shells to form entire mountains which are encountered all over
the world. A large part of the coast of England, the island of Rügen,
and many other localities, consist entirely of chalk.
In many cases, chalk is found interspersed with nodular masses of
flint, and in some places it also contains great quantities of the
remains of other marine animals, such as sea urchins, the spines of
which occur in such numbers in certain kinds of chalk as to unfit them
entirely for use as a pigment.
The foregoing varieties of calc spar are the most important, and also
occur in large quantities; but, to complete the tale, it is necessary
to mention also a few others which, however, are only found in small
amounts. To these belong, for example, anthracolite, a limestone
stained quite black by coal; the oolithic limestones or roe stones,
which are composed of granules resembling fish roe; muschelkalk,
which is also of fossil character and is almost entirely composed of
mussel shells cemented together with lime; the marls, which consist
of calc spar mixed with varying quantities of clay and consequently
often bear a great resemblance to loam in their properties. A few of
these varieties find extensive employment for certain purposes, some
marls for instance being used for making hydraulic lime, whilst all
modifications of calc spar that are sufficiently pure can be burned for
quick lime.
It has already been stated that the mineral arragonite is identical,
chemically, with calc spar, since both consist of calcium carbonate,
but differ in their crystalline habit. Thus, whereas the crystals of
calc spar belong to the rhombohedral or hexagonal system, those of
arragonite are always rhombic. This occurrence of one and the same
substance in two different crystalline forms is known as dimorphism,
and calcium carbonate is therefore dimorphous. Whether calcium
carbonate assumes the form of calcite or arragonite depends entirely on
physical causes. When the deposition of the carbonate takes place from
a cold solution the shape of the crystals is always one belonging to
the hexagonal or rhombohedral system; but when it is from hot solution,
rhombic crystals are invariably formed, calc spar resulting in the
former case and arragonite in the latter.
These different methods of formation which can be carried out in the
laboratory by producing the requisite conditions, occur on the large
scale in many parts of the world. Wherever a hot spring comes to the
surface, containing considerable amounts of lime in solution, this
separates out in the form of arragonite, which received its name from
the circumstance that specially handsome crystals of this mineral are
found in Arragon.
One of the best-known places where the formation of arragonite can be
observed at the present time is Carlsbad in Bohemia. The hot springs
there deposit a very large amount of lime, which is stained more or
less yellow or red by the presence of varying quantities of iron
oxide, and, under the name of “sprudelstein” is used for producing
various works of art. When the hot springs bring up particles of sand,
the lime substance incrusts these sand grains, forming globular masses
resembling peas, and consequently named pisolite.
In chemical composition, calcite and arragonite consist of a
combination of calcium oxide (lime) and carbonic acid, the formula
being expressed by CaCO{3}. Calcium carbonate is insoluble in pure
water, but dissolves somewhat freely in water charged with free
carbonic acid. It is assumed that a compound is formed, which is known
as calcium bi- (or acid) carbonate, is very unstable and can only exist
in a state of solution. When a solution of calcium bicarbonate--which
can be prepared by passing carbonic acid gas through water containing
finely divided calcium carbonate in suspension--is exposed for some
time to the air, it soon becomes cloudy, and a deposit of calcium
carbonate settles down at the bottom of the vessel, because, in the
air the dissolved calcium bicarbonate is decomposed into free carbonic
acid gas and calcium carbonate, which latter, as has been mentioned,
is quite insoluble in water. It has already been stated that this
phenomenon goes on in Nature in the formation of stalactites, lime
sinter and calcareous tuff.
Calcium carbonate is readily soluble in acids, the contained carbonic
acid being liberated (as carbon dioxide) with effervescence. When such
acids are employed for solution as form readily soluble salts with
lime, such as hydrochloric, nitric, acetic, etc. acids, a perfectly
clear solution is obtained; but if sulphuric acid is used, a white
pulpy mass is formed, consisting of calcium sulphate, or gypsum,
which, owing to its low solubility, separates out as small crystals.
Any sandy residue left when calcium carbonate is dissolved, mostly
consists of quartz sand. In dissolving dark-coloured limestones, grey,
or even black, flakes are left, which consist of organic material
very high in carbon. On limestone being subjected to fairly strong
calcination, all the carbonic acid is expelled, leaving behind the
so-called quick or burnt lime, which is, chemically, calcium oxide:--
CaCO{3} = CaO + CO{2}
Calcium carbonate = Quick lime + Carbon dioxide
If burnt lime be left exposed to the air for some time, it again
gradually absorbs carbon dioxide and is reconverted into calcium
carbonate. When burnt lime is sprinkled with water it takes up the
latter avidly, becoming very hot and finally crumbling down to a very
friable white powder, consisting of slaked or hydrated lime (calcium
hydroxide, Ca(OH){2}). The considerable rise of temperature in
quenching the lime is due to the chemical combination of the calcium
oxide and water.
Both quick and slaked lime dissolve to a certain extent in water, and
impart strongly alkaline properties thereto, lime being one of the
strongest of bases. On exposure to the air, the solution of quick lime
in water (lime-water) quickly forms an opalescent superficial film
of calcium carbonate, and in a short time no more lime is present in
solution, the whole having been transformed into calcium carbonate,
which settles down to the bottom of the vessel as a very fine powder.
Limestone that consists entirely of calcium oxide and carbon dioxide
is of rare occurrence in Nature, foreign substances being nearly
always present. Since the nature of these admixtures is of the greatest
importance to the colour-maker, owing to the considerable influence
they exert on the suitability of the minerals for his purposes, it
is necessary that these extraneous substances occurring in limestone
should be more closely described.
Nearly all varieties of limestone contain certain proportions of
ferrous and ferric oxides. The presence of ferrous oxide, when the
relative amount is but small, cannot be detected by mere inspection;
and even many limestones containing really appreciable quantities of
ferrous oxide are pure white in colour so long as they are in large
lumps. If, however, such a limestone be reduced to powder and exposed
to the air for a short time, it gradually assumes a yellow tinge, the
depth of which increases with the length of exposure.
The cause of this change is due to the fact that ferrous oxide has a
great affinity for oxygen, by absorbing which it changes into ferric
oxide. (Ferrous oxide consists of FeO, ferric oxide of Fe{2}O{3}.)
Ferrous oxide and its compounds are of a pale green colour which is
not very noticeable, whereas ferric oxide has a very powerful yellow
colour, and consequently the limestone, when its superficial area has
been greatly increased by reduction to powder, assumes the yellow tinge
due to ferric oxide. A limestone exhibiting this property can evidently
not be used for making white earth colours, but is, at best, only
suitable for mixing with other colours.
Occasionally, limestone contains varying quantities of magnesia, and
when this oxide is present in large amount, changes into another
mineral known as dolomite. In many places this dolomite forms large
masses of rock, which, however, is not employed for making colours,
owing to the yellow shade imparted by the fairly large amount of ferric
oxide present.
_Gypsum_ (_Alabaster_)
This mineral occurs native in many places, and is frequently worked
for a number of purposes. Gypsum occurs in Nature in a great variety
of forms. The purest kind is met with either as water-clear crystals,
which cleave readily in two directions, or as transparent tabular
masses (selenite) which also cleave easily. Micro-crystalline
fine-grained gypsum is milk-white in colour, highly translucent and is
largely used, under the name of alabaster, in sculpture. Owing to its
low hardness, alabaster can be readily cut with a knife, and on this
account is frequently shaped by planing or lathe-turning.
Gypsum is generally met with in dense masses, which may be of any
colour, grey, blue and reddish shades being the most common, whilst
pure white is rarer. The dark-coloured varieties can only be used for
manurial purposes; but the white finds a twofold application as a
pigment, and, in the calcined state, for making plaster casts.
In point of chemical composition, gypsum consists of sulphate of lime,
or calcium sulphate (CaSO{4} + 2H{2}O). It is soluble in water, but
only in such small quantity that over 400 parts of the latter are
needed to dissolve one part of gypsum. On being heated to between 120°
and 130° C., gypsum parts with its two molecules of combined water and
becomes anhydrous calcium sulphate or burnt gypsum. When this latter
is stirred with water to a pulp, it takes up the water again, with
considerable evolution of heat, swelling up considerably and setting
quickly to a solid mass.
The number of substances exhibiting this property being small, burnt
gypsum is very frequently used for making casts of statuary, and for
stucco work in building. Finely ground white gypsum can also be used as
a pigment, but is inferior to calcium carbonate in covering power, and
is therefore seldom employed for this purpose, though frequently added
to other colours. The mineral known as muriacite or anhydrite consists
of anhydrous calcium sulphate; and is therefore similar in composition
to burnt gypsum; but it lacks the property of combining with water when
brought into contact therewith.
_Barytes, or Heavy Spar_
The mineral known as heavy spar occurs in very large quantities and in
numerous localities. It forms rhombic crystals, which are very often
extremely well developed and form flat plates of considerable size. A
remarkable peculiarity of this mineral is its high specific gravity,
which is due to the barium content. It is found native in all colours,
white being the most common.
Chemically, heavy spar is barium sulphate, BaSO{4}. It can be used as a
pigment _per se_, but only when prepared artificially, the trade name
for the product being permanent white, or blanc fixe. Powdered native
heavy spar, even when ground ever so fine, has not enough covering
power, this property being peculiar to the artificial product.
When it is desired to mix other pigments with a white substance, to
lighten the shade, permanent white can be specially recommended, since
it is quite insensitive to atmospheric influences and has no chemical
action on the colour, so that it can be used with even the most
delicate colours without risk. In this way, not only can the colours
be considerably cheapened, but over-dark colours can be shaded to the
desired extent. Another advantage of such mixtures is that a smaller
quantity of oil or varnish is required, barytes only needing about 8%
of its own weight of vehicle to form a workable mixture, whilst other
pigments take five times as much, or even more. In many cases the low
covering power of barytes enables large quantities to be added, and
this reacts favourably on the consumption of varnish.
Another barium mineral is witherite, or barium carbonate. This is not
used direct as a pigment, but--in contrast to heavy spar--is readily
soluble in hydrochloric acid, and therefore serves as raw material for
the preparation of artificial barytes and other barium compounds, the
first-named being obtained by treating a solution of barium chloride
with sulphuric acid, insoluble barium sulphate being precipitated.
_Talc_, _Soapstone_, _Steatite_
Talc occurs in Nature either as a pure white mass, of greasy lustre, or
occasionally as yellow, green or grey masses, all distinguished by a
peculiar greasy appearance and a soapy feel. This appearance is common
to all the minerals of the steatite group, and is the cause of their
generic name, soapstone. Although the steatites have a very low degree
of hardness--most of them can be scratched by the finger-nail--some
difficulty is encountered in reducing them to fine powder. Calcination
usually increases the hardness considerably, so that, in some cases,
the calcined mineral gives off sparks when struck with a steel
instrument.
Soapstone is composed of magnesium silicates, containing varying
proportions of magnesia and silica, together with a small quantity
of water, apparently in a state of chemical combination, a very high
temperature, approaching white heat, being required to effect its
complete expulsion, the residue then attaining the aforesaid high
degree of hardness. The composition of talc can be expressed by the
symbol H{2}Mg{2}(SiO{3}){4}, corresponding to 63·52% of silica,
31·72% of magnesia, and 4·76% of water. In some varieties of talc, a
portion (1–5%) of the magnesia is replaced by ferrous oxide. Talc is
quite unaffected by the action of dilute acids, boiling concentrated
sulphuric acid being required to decompose it, with separation of
silica.
Owing to its low specific gravity and chemical indifference, talc is
suitable for lightening the shade of certain lake pigments. It can also
be used as a pigment by itself, and also as a gloss on wall-paper, for
mixing with paper pulp, and for various other purposes.
_Clay_
The mineral known as clay is, in all cases, a product of the
decomposition of other minerals, mainly felspar. This substance is a
double silicate of alumina and potash, K{2}O.Al{2}O{3}.(SiO{2}){6}.
Pure kaolin is Al{2}O{3}(SiO{2}){2} + 2H{2}O, or 46·50% silica, 39·56%
alumina, 13·9% water.
Clay may be supposed to have been formed by the conversion of felspar,
under the action of air and water, into silicate of alumina, the
silicate of potash being dissolved out. Being insoluble, the silicate
of alumina would be transported by the water, in a very fine state of
division, and finally deposited as a sediment, which in course of time
became a solid mass. This, when again brought into contact with water,
forms a very plastic pulp which, when dried and baked, forms a solid
mass, brick, which is no longer affected by water. Perfectly pure clay
forms a white mass, which, under the name of China clay or kaolin, is
used for making porcelain, and is only occasionally met with in large
quantities.
Pure kaolin is characterised by its great chemical indifference, being
decomposed only by strong alkalis and sulphuric acid. At the high
temperature of the pottery kiln, kaolin sinters to a very compact
mass, but cannot be fused, except when small quantities are subjected
to the intense heat of the oxyhydrogen flame, whereupon it fuses to a
colourless glass of great hardness.
In an impure state, silicate of alumina occurs frequently in Nature,
and then forms the minerals known under the generic names of clay,
loam, marl, etc. These impure clays contain varying proportions of
extraneous minerals which produce changes in the physical and chemical
properties. They are grey, blue or yellow in colour, the grey and blue
varieties mostly containing appreciable quantities of ferrous oxide,
whilst the yellow kinds contain ferric oxide. When fired, all of them
become yellow or red, the ferrous oxide being transformed into ferric
oxide by the heat. Some fairly white clays are high in lime, which
makes them fusible at high temperatures. In some very impure kinds,
even the comparatively low heat of the brick-kiln is sufficient to
cause partial fusion. For colour-making, the white clays, especially
kaolin and pipeclay, form a highly important material, being
procurable at very low prices and fairly easy to prepare.
The white clays are either used as pigments by themselves, or for
mixing with other colours of low specific gravity.
(B) YELLOW EARTHS
The number of yellow earths is large, but most of them exhibit a
certain similarity in chemical composition, the pigmentary principle in
the majority being either ferric oxide or ferric hydroxide. The former
is yellow, the latter brown, and the colour of the minerals resembles
that of the preponderating iron compound.
_Brown Ironstone_
The mineral known as brown ironstone consists of ferric hydroxide, and
usually forms compact masses, no decided crystals having, so far, been
observed. The lumps have an irregular or earthy fracture, a hardness
of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in
the different varieties, from yellowish (rusty) brown, through cinnamon
to blackish-brown. The chemical composition of the pure lumps may be
expressed by the symbol 2Fe{2}O{3} + 3H{2}O; but a little manganese
oxide and silica is generally present even in the pure kinds.
The chief varieties of this mineral are:--
(_a_) Fibrous brown iron ore, or brown hematite, mostly forming
reniform or stalactitic masses.
(_b_) Compact brown ironstone, usually in dense masses, and not
infrequently also appearing in pseudo-morphs of other minerals.
(_c_) Ochreous brown ironstone. This variety is the most important to
the colour-maker, for whose purposes it is preferably used. It nearly
always forms very loose, earthy masses, yellow or brown in colour.
(_d_) Clay ironstone. This consists of a mixture of the above-mentioned
varieties with variable proportions of other minerals, clay being the
most common ingredient. Nodular iron ore, oölitic, bog and siliceous
ore belong to this class, as also the minette ores that are found in
great abundance in Alsace-Lorraine, Belgium and Luxemburg, and are
classed with the oölitic brown ironstones.
In most cases, the varieties enumerated are found together, and are
used for the production of iron. The ochre constituting the most
interesting member to the colour-maker often occurs as deposits
embedded in dense masses of brown ironstone, though in many places it
is found by itself.
CHEMICAL COMPOSITION OF VARIOUS BROWN IRONSTONES
The following analyses of brown ironstone from different deposits will
give an idea of the composition of these minerals.
_Ordinary Brown Ironstone_
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| | 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. | 10.|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|Ferric oxide |76·76|73·75|77·54|78·50|78·42|48·25| -- | -- | -- | -- |
|Manganese | | | | | | | | | | |
| oxide |16·56|10·50| 2·70| 1·95| 1·30|24·73| -- | -- | -- | -- |
|Iron | -- | -- | -- | -- | -- | -- |33·9 |37·88|54·80|55·04|
|Manganese | -- | -- | -- | -- | -- | -- | 0·15| 0·17| 0·57| 0·20|
|Alumina | -- | -- | -- | -- | 1·13| 2·33|10·03| 0·88| 1·15| 2·50|
|Lime | 0·60| 2·75| 0·48| 5·08| 3·55| 2·85| 0·41| 0·32| 0·50| 0·34|
|Magnesia | 6·44| -- | 1·25| 4·50| 0·18| 0·90| 0·67| -- | 0·02| 0·38|
|Silica | -- | -- | 3·55| 0·85 5·48|11·35|28·29|33·38| 0·02| 0·38|
|SO{3} | -- | -- | -- | -- | -- | 0·09| -- | -- | -- | -- |
|P{2}O{5} | -- | -- | -- | -- | 0·09| 0·08| -- | -- | -- | -- |
|Sulphur | -- | -- | -- | -- | -- | -- | 0·33| 0·06| 0·04|Trace|
|Phosphorus | -- | -- | -- | -- | -- | -- | 0·04| 0·56| 0·02| 0·06|
|Loss on | | | | | | | | | | |
| incineration| 5·65|14·00|14·51| 9·12| 9·10| 9·80| 9·88| 7·77|10·55|10·71|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deposits: (1) Hamm; (2) Schmalkalden; (3) Hüttenberg (Carynthia); (4)
Styria; (5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8)
Elbingerode (Harz); (10) Pennsylvania.
_Argillaceous Brown Ironstone_
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| |_a._ |_b._ |_c._ |_d._ |_e._ |_f._ |_g._ |_h._ |_i._ |_k._ |
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|Ferric oxide |80·76|19·4 |55·39|66·33|57·32|52·50|39·50|75·67| -- | -- |
|Iron | -- | -- | -- | -- | -- | -- | -- | -- |40·90|21·69|
|Manganese | | | | | | | | | | |
| oxide | -- | 8·2 | -- | 6·42| -- | 5·49| 6·12| 0·72| -- | -- |
|Manganese | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
|Zinc oxide | 0·92| 1·6 | -- | -- | 0·47| -- | -- | -- | -- | -- |
|Alumina | 2·36|11·0 |12·80| 7·74| 1·68| 5·23| 9·89| 3·10| 4·95| 3·88|
|Lime | -- | 2·6 |Trace| 0·41| 0·13| 3·36|20·34| 5·01| 5·59|21·25|
|Magnesia | -- | 0·2 | -- | 0·37| -- | 0·36| -- | -- | 0·49| 0·30|
|Silica |4·58 |48·61|22·73|12·97|30·64| 8·64| 5·22| 8·70|16·63|14·71|
|P{2}O{5} | -- | -- | -- | -- | 0·32| 3·86| 2·19| 3·68| -- | -- |
|Phosphorus | -- | -- | -- | 0·02| -- | -- | -- | -- | 1·13| 0·48|
|SO{3} | -- | -- | -- | 0·03|Trace| -- | -- | -- | -- | -- |
|Sulphur | -- | -- | -- | -- | -- | -- | -- | -- | 0·10| 0·05|
|Loss on | | | | | | | | | | |
| incineration|12·71| 9·1 | 8·50|11·77|12·70|20·55|25·74| -- |16·04|28·70|
+-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
(_a_) Oölitic (pea) ore from Elligserbrick (Brunswick); (_b_) from
Durlach (Baden); (_c_) and (_d_) Ore from Esslingen; (_e_) Oölitic ore
from Siptingen (Baden); (_f_) from Adenstedt, nr. Pirna (argillaceous);
(_g_) _Ibid._ (calcareous); (_h_) Minette from Esch; (_i_) Red minette
from Dolvaux; (_k_) Brown minette from Redange.
_Limonite (Bog Iron Ore)_
+----------------+-----+-----+-----+-----+-----+-----+
| | 1. | 2. | 3. | 4. | 5. | 6. |
+----------------+-----+-----+-----+-----+-----+-----+
|Ferric oxide |61·00|67·46|65·66|67·59|70·05|62·20|
|Manganese oxide | 7·00| 3·19| 3·87| 1·45| 1·78| 7·60|
|P{2}O{5} | 2·00| 0·67| 1·13| 0·18| 0·34| 1·60|
|SO{3} -- | 3·07|Trace| 0·21|Trace|Trace| |
|Silica | 6·00| 7·00| 7·15| 7·89| 8·03|16·60|
|Alumina | -- | -- | 5·09| 4·18| 1·50| 2·20|
|Lime | -- | 0·90| 0·82| 0·47| 2·31| 1·60|
|Magnesia | -- | -- | 0·15| 0·23| 0·12| 3·73|
|Water and | | | | | | |
| organic acids |19·00|17·00| 6·22|17·81|15·87|19·90|
+----------------+-----+-----+-----+-----+-----+-----+
(1) Limonite from Lausitz; (2) Limonite from Auer, nr. Morizburg; (3 to
6) Swedish limonite.
_Ochre_
Ochre, or yellow Terra di Siena, forms earthy-looking masses, fawn,
reddish-yellow to brownish-red in colour. Whilst not infrequent in
Nature, ochre is only found in small quantities, as pockets, and not
as extensive deposits. The discovery of a bed of good coloured ochre
is, however, always a very valuable find, bright natural ochres being
somewhat rare, and most kinds requiring special preparation before they
can be used as painters’ colours. Owing to the comparative scarcity of
good coloured ochres, they are often called after the place of origin,
such as Thuringian, Italian (Siena), English, etc., ochre.
In nearly every case, ochre is a decomposition product of various
ferruginous minerals, which has been transported by water, often in
admixture with other minerals, and finally deposited in the places
where it is now found. Most ochres consist of varying mixtures of clay,
ferric hydroxide and lime; and, as a rule, the higher the proportion of
ferric hydroxide, the deeper the colour. Thus, for example, the ferric
hydroxide may amount, in the dark grades, to 25% of the entire mass,
whilst in the lighter kinds it may be as low as 3%. It is very rare
that ochre is put on the market in its native condition, being mostly
subjected to chemical treatment enabling a definite shade of colour to
be obtained. This will be gone into more fully later.
_Yellow Earth_
Yellow earth is found in many places as compact masses, and less
frequently as schistous deposits. It has a fine earthy fracture, and is
mostly devoid of lustre, except for a faint shimmer on the surface of
fracture; slightly greasy feel; and a tendency to crumble, in water,
to a non-plastic powder. It contains silica, ferric oxide and water
in varying proportions, and the yellow earths from different deposits
always vary slightly in percentage composition. These differences are
clearly shown in the following analyses of two varieties from the
vicinity of Amberg (Bavaria):--
I. II.
Silica 33·23% 35·10%
Alumina 14·21 14·40
Magnesia 1·38 --
Ferric oxide 37·76 36·80
Water 13·24 13·60
When heated, the colour changes gradually to red, and the earth becomes
extremely hard. There are several recognised commercial grades,
the price of which varies mainly in accordance with the colour and
fineness. The Amberg variety is specially esteemed, the Hungarian and
Moravian kinds being less valuable.
The colour not being particularly good, this earth is never used for
fine work, but is largely employed as a yellow wash for houses and as
ordinary distemper. It may also be used as an oil paint.
Red Ochre is a less important, cheap variety of ochre, chiefly used in
cheap paints and for low-priced wall-papers. It occurs in the deposits
as clayey masses.
_Terra di Siena_
Terra di Siena is a very pure form of ferric hydroxide. When ground,
the light to dark brown lumps furnish a pale to dark yellow powder,
which can be transformed into a number of gradations by burning. In
spite of its handsome colour, this pigment is deficient in covering
power, in addition to which it darkens when mixed with varnish, and
dries slowly.
(C) THE RED EARTHS
Apart from the small quantities of native vermilion handsome enough
for direct use as painters’ colours when reduced to powder, the red
earths, with practically no exception, consist of ferruginous minerals,
and it is only within a recent period that red painters’ colours have
been prepared from certain chemical waste products from manufacturing
processes. In all cases, however, compounds of iron and oxygen
constitute the bulk of the red earths. In addition to ferric oxide,
which is the chief material used for making the important red colours,
are compounds of ferric oxide and water, _i. e._ ferric hydroxides. The
ferric oxide pigments are among the most important in the entire series
of earth colours, being on the one hand very cheap, and on the other
so handsome in colour that ferric oxide can be used for the finest
paintings.
Ferric oxide can also be shaded very extensively by a fairly simple
treatment, so as to furnish a whole range of very handsome shades.
In nature, ferric oxide occurs in numerous varieties of one and the
same mineral, red iron ore, which is also known as hematite, blood
stone, raddle, etc.
_Red Ironstone_
Red hematite occurs native as rhombohedral crystals, which mostly
consist solely of ferric oxide, and may be considered as pure oxide
for the purposes of the colour-maker. The difference between the
several varieties is due, not to any chemical variation, but entirely
to changes in physical structure. The varieties with a radial, fibrous
structure are known as red hematite, the colour of which ranges from
blood red to dark brown and is frequently accompanied by metallic
lustre. The scaly modification of this mineral forms micaceous iron
ore, and is usually a deep iron black. In the neighbourhood of
volcanoes it is frequently found as particularly handsome crystals.
Iron cream (frosty hematite) is the name given to a beautiful cherry
red variety, which easily rubs off, has a greasy feel and is composed
of extremely fine scales.
The so-called raddle occurs in Nature as a readily pulverulent earthy
mass of ferric oxide contaminated more or less with extraneous
substances. On account of its abundance and low market price, it is
largely used in painting.
Although mixed with numerous foreign substances, certain clay
ironstones, oölitic ironstones and siliceous ironstones may be regarded
as ferric oxide in the sense understood by the colour-maker, all these
minerals having a deep red to deep brown colour and being capable of
finding advantageous employment as pigments.
Ferric oxide is distinguished by two properties which render it
specially valuable to the colour-maker. When combined with water,
its colour is no longer red, but a handsome brown; and, on the other
hand, when heated, the colour passes through brown into a permanent
dark violet. By suitable treatment of such minerals as consist mainly
of ferric hydroxide, mixtures can be obtained which contain the oxide
and hydroxide in variable proportions and give a whole range of shades
between brown and red.
The preparation of these colours is easy when very pure red ironstone
is available. The somewhat expensive pigment, Indian red, is--when
pure--really nothing but a very pure ferric oxide of Indian origin.
Ferric oxide, however, often contains impurities which considerably
influence the colour of the product. Owing to the fact that large
quantities of ferric oxide are formed as by-products in certain
chemical processes which are carried out on a very extensive scale,
this oxide, which is very pure, can be advantageously used for making
iron pigments, especially as its application for other purposes is very
restricted, and it can therefore be had at a very low price.
The following analyses show the composition of a number of red
ironstones, Nos. 1, 2 and 3 being hematite from Froment, or Wetzlar,
No. 4 from Wetzlar, Nos. 5 and 6 hematite from Whitehaven, No. 7
from Thuringia, No. 8 from Bohemia, No. 9 from Spain, No. 10 from N.
America, and No. 11 from England.
+-----+--------+-----------+---------+-------------+-----------+--------+
| | Ferric | Manganese | | | Alumina, | |
| No. | oxide. | oxide. | Silica. | P{2}O{5}. | lime and | Water. |
| | | | | | magnesia. | |
+-----+--------+-----------+---------+-------------+-----------+--------+
| 1 | 94·00 | Trace | 2·00 | -- | Trace | 2·00 |
| 2 | 80·95 | -- | 16·74 | 0·51 | 0·97 | 0·83 |
| 3 | 73·77 | -- | 23·16 | 0·45 | 1·41 | 1·21 |
| 4 | 92·45 | -- | 5·63 | 0·19 | 0·65 | 1·08 |
| 5 | 96·27 | -- | 4·20 | -- | 0·59 | 4·40 |
| 6 | 63·05 | 5·29 | 22·76 | -- | 0·49 | 4·40 |
| 7 | 85·00 | 1·601 | 3·304 | 0·457 | 8·795 | 0·633 |
+-----+--------+-----------+---------+-------------+-----------+--------+
+---+------+-------+-----+-----+------+-------+--------+-------+--------+
| | |Man- |Alu- | |Mag- | | |Phos- |Loss on |
|No.|Iron. |ganese.|mina.|Lime.|nesia.|Silica.|Sulphur.|phorus.|calcina-|
| | | | | | | | | |tion. |
+---+------+-------+-----+-----+------+-------+--------+-------+--------+
| 8| 33·64| 0·10 | 7·58| 8·10| 0·82 | 17·80 | Trace | 0·19 | 9·82 |
| 9| 31·38| 0·19 | 0·06|29·95| 0·35 | 0·87 | -- | 0·09 | 23·68 |
| 10| 62·54| 1·93 | 1·71| -- | 0·50 | 3·80 | 0·02 | 0·04 | 2·23 |
| 11| 62·91| Trace | 1·39| 0·70| 0·42 | 5·89 | 0·05 | 0·11 | -- |
+---+------+-------+-----+-----+------+-------+--------+-------+--------+
There are certain other minerals closely allied, both chemically and
mineralogically, to red ironstone, namely, the brown hematites or
ironstones used in the manufacture of iron. Brown hematite consists
of ferric hydroxide, Fe{2}O{3}H{2}O, and occurs in a variety of forms
in Nature, the most frequent being pea (oölitic) ore, which owes its
name to the spherical shape of the grains. Some brown hematites are
decomposition products of other minerals, and contain sulphur and
phosphorus in addition to ferric hydroxide. Like the pure hydroxide,
they are brown in colour, but differ therefrom considerably in their
chemical behaviour when heated. This is particularly the case with the
so-called bog ore, which is mostly found, as spongy yellow-brown to
black masses, in swamps, and owes its origin to the decomposition of
various ferruginous minerals. It varies greatly in chemical composition
and occasionally contains up to about 50% of sand. The amount of ferric
oxide in bog ore varies between 20 and 60%, and it also contains 7–30%
of water, up to 4% of P{2}O{5}, small quantities of ferrous oxide and
manganese hydroxide, together with, in most cases, mechanically admixed
organic residues.
The phosphorus content makes bog iron a very inferior material for
smelting, the resulting iron being of low quality. Nevertheless, it can
sometimes be advantageously used in making earth colours, though the
products cannot lay much claim to beauty of colour.
_Bole_
The native earth pigments known by this name form masses of the colour
of leather to dark brown, with a conchoidal fracture and an earthy
appearance. Bole chiefly consists of iron silicate combined with
water, some varieties containing small quantities of alumina. The
composition fluctuates very considerably, most varieties containing
41–42% of silica, 20–25% of alumina, and 24–25% of water, the remainder
consisting of ferric oxide. Some kinds, such as Oravicza and Sinope
bole, contain only 31–32% of silica and 17–21% of water.
Bole is used as a paint for walls, clapboards, etc., and is only
mentioned here because of its relationship to the ferric oxide pigments.
_Alum Sludge_
Large quantities of clarification sludge are produced, in alum works,
as the sediment from the red liquors. This sludge consists mainly of
ferric oxide, with small quantities of other oxides and sulphuric acid
(basic ferric sulphate), and would be an entirely worthless by-product
except for the fact that it can be manufactured into pigments, some of
them of great beauty.
All alum makers should treat this residue and convert it into
pigments, which they could put on the market at a low rate, the cost
of preparation being small. Since the material is chiefly composed of
ferric oxide, the resulting colours are very similar to those obtained
from iron ores; and all shades, from yellow-brown, through red, to the
darkest brown, are represented.
_Mine Sludge_
The water frequently present in iron mines sometimes contains large
quantities of sediment, which consist mainly of iron ochre and can be
advantageously worked up into pigments. There is scarcely any need to
mention that all substances containing ferric oxide can be used for
making any of the pigments obtainable from the oxide itself, the only
difference between the various raw materials being their degree of
purity, so that it is not always so easy to obtain a certain desired
shade from a given material in such beauty as is furnished by another
material, the small quantities of impurities associated with the ferric
oxide having, in many instances, an important influence on the colour.
(D) BLUE EARTHS
Only two minerals are known which are capable of direct use as blue
pigments, viz. vivianite (native Prussian blue) and copper carbonate
(azurite, ultramarine), and as neither of them is particularly
handsome, they are only used for unimportant work. Lapis lazuli is no
longer employed.
_Azurite, or Ultramarine_
This mineral, which is of frequent occurrence with malachite and
other cupriferous minerals, forms small crystals of a beautiful deep
azure blue consisting of cupric oxide in combination with carbon
dioxide and water, expressed by the formula 2CuCO{3}, Cu(OH){2}, or
Cu{3}(OH){2}(CO{3}){2}, and containing 69·19% of cupric oxide, 25·58%
of CO{2} and 5·23% of water. The colour of the powdered mineral is much
paler than that of the crystals. The pigment, which is used for cheap
paints, is not particularly stable, and loses much of its beauty when
applied to plaster.
_Vivianite_
This mineral occurs in many places as crystalline masses, but also
forms earthy deposits, some of which, especially in certain bogs,
attain considerable thickness. The colour is between indigo and
blackish blue; and the freshly won mineral often has an unsightly
whitish appearance, which, however, soon changes into the pure blue.
The cause of this peculiarity is due to the fact that vivianite
originally consisted of hydrated ferrous phosphate, which is white,
this compound being transformed, under the influence of the air, into
the blue ferric phosphate.
Vivianite contains ferric oxide, phosphoric acid and water, but
in variable proportions. The original composition, expressed by
Fe{2}(PO{4}){2} + 8H{2}O{2}, corresponds to 43·03% of ferrous oxide,
28·29% of P{2}O{5} and 28·68% of water; but, in the air, part of the
ferrous phosphate is oxidised to basic ferric phosphate, so that the
content of ferrous oxide may range from 9·75 to 42·71%, and that of
ferric oxide between 1·12 and 38·20%. Vivianite is also sold as blue
ochre, and is now seldom used as a painters’ colour, owing to the
introduction of a large number of artificially prepared blues, which
are superior to vivianite in colour and are cheaply made. However, it
can still find application in localities where it is obtainable in
quantity.
(E) GREEN EARTH PIGMENTS
The green earth pigments comprise green earth (Verona green) and
malachite. Like the blue earths, they cannot lay any particular claim
to beauty, but they are very cheap, and consequently are largely used
where low price is the chief consideration.
_Green Earth_
In Nature, green occurs as an entirely non-crystalline earthy mass,
which is probably a decomposition product of augite. It has a close,
earthy fracture, a colour between seladon and olive green, and a
slightly greasy appearance. In point of chemical composition it
consists of silica, alumina, magnesia, sodium, potassium, ferrous oxide
and water, the usual representative formula being ROS{1}O{2} H{2}O, in
which RO symbolises a metallic oxide.
The colour is due to ferrous oxide; and if left exposed to the air for
a long time, or subjected to powerful calcination, the great affinity
of ferrous oxide for oxygen causes the colour to turn red and red-brown.
Green earth is found in many localities, _e. g._ Bohemia, Hungary, the
Tyrol and Cyprus, the finest, however, occurring near Verona, on which
account it is known as Veronese earth.
_Malachite_
The commercial pigment consists of powdered malachite, a mineral
which usually occurs in compact masses of a handsome emerald green
colour, though isolated lumps exhibit considerable variation in shade,
some of them being dark green and others very pale. In chemical
composition, malachite is closely allied to azurite, consisting of
cupric oxide, carbon dioxide and water, and the difference is entirely
one of percentage proportions. The formula is CuCo{3}, Cu(OH){2}, or
Cu{2}(OH){2}CO{3}, corresponding to 71·90% of cupric oxide, 19·94% of
carbon dioxide and 8·16% of water.
Powdered malachite (even the dark green varieties) is always rather
light in colour, and for this reason is not much used. Furthermore, the
mineral is rather hard (3·5), and is consequently difficult to grind;
in addition to which the mineral is fairly expensive, on account of its
employment as a source of copper, particularly fine pieces being also
used as ornaments or for making works of art. Moreover, like all copper
compounds, it is very sensitive to the action of sulphuretted hydrogen,
and liable to discoloration in course of time.
(F) BROWN EARTH PIGMENTS
Numerous minerals are adapted for the manufacture of brown pigments. On
the basis of chemical composition, they may be classed in two groups;
those consisting of ferric hydroxide, and those in which the brown
colour is due to organic substances.
The first group comprises the minerals which have already been
mentioned in connection with the red earth pigments, bole and brown
ochre (umber), Terra di Siena, Cologne earth and a number of other
earths rich in ferric hydroxide belonging to this category. The second,
or organic group, includes compounds that are very rich in carbon and
are therefore of a very dark colour, the shades ranging from light
brown to black, _e. g._ the true umbers and asphaltum.
_Umber_
As already mentioned, the term “umber” was formerly applied to brown
varieties of ochre, whereas at present it is extended to certain
masses of brown-coal character, often interspersed with iron ochre and
sometimes containing manganese. Umber generally consists of fairly
dense, earthy masses, which are dried and ground--after crushing and
levigation, if necessary.
Valuable varieties are Cappagh brown and Caledonian brown, both with a
reddish tinge.
It is thus evident that “umber” now implies two different kinds of
materials, organic masses and iron-manganese compounds, which can also
be used as oil paints. These umbers can also be extensively shaded by
burning, the final colour being particularly influenced by the amount
of manganese compounds present.
The carbonaceous umbers (Cassel brown, Carbon brown) are combustible,
and mostly leave behind a merely small residue of ash. An important
property of these umbers is their partial solubility in alkalis, a
peculiarity which is utilised for the preparation of brown wood stains.
_Asphaltum_
Asphaltum forms very friable dark brown to black masses, which, in
contact with a light, easily ignite and burn with a bright, but very
smoky, flame, disengaging a peculiar, “bituminous” smell, and leaving
only a very small quantity of ash.
Extensive deposits of asphaltum are found at the Dead Sea, the Pitch
Lake on the island of Trinidad, in Dalmatia, and many other places,
where, however, it is in an impure condition and frequently contains
large quantities of sand. In many localities the rock is impregnated
with asphaltum, which makes it dark brown to black in colour and gives
rise to a bituminous odour when rubbed.
Peat beds sometimes contain pockets of a mass with a handsome brown
colour and consisting of a mixture of humic acids and other organic
substances which may be ranked with the humin bodies that are always
formed when organic matter decomposes in presence of an insufficient
supply of oxygen. These bodies are dark coloured, mostly deep brown,
rich in carbon, and, to some extent, similar to brown coal or peat in
chemical composition.
Their high carbon content renders these substances very inert towards
chemical reagents, and therefore particularly adapted for the
preparation of painters’ colours. Genuine Vandyke brown, which is the
handsomest brown known, is an earth rich in humin compounds; and Cassel
brown also belongs to this group.
(G) BLACK EARTH
The colour of these earths is entirely due to carbon, and pure carbon,
a certain form of which occurs native, is itself used as a pigment.
Actually, there are only two minerals that require to be mentioned in
this connection: black schist and graphite.
_Black Schist_
In most cases this is a clay shale, so rich in carbon as to appear
deep black. In commerce, this mineral is also erroneously called
“black chalk”; but at present it is seldom used as a pigment or
drawing-material, black chalks being produced far more cheaply than the
expense of preparing the natural article.
Grey clay shales are used for making grey earth pigments (stone grey,
and mineral grey).
_Graphite_
This mineral is found, in a very pure state, in many localities,
celebrated deposits occurring in England, Siberia, Bohemia and Bavaria,
whilst North American graphite has lately come into prominence.
Graphite is a modification of pure carbon, and is met with in the form
of hexagonal (rhombohedral) crystals, usually occurring as hexagonal
plates with a lustrous, iron-black colour. It rubs off easily, and
readily burns away, leaving a very small amount of ash, when subjected
to a very high temperature in presence of air.
The principal uses of graphite are as an anticorrosive paint for iron,
and for making lead pencils.
As already mentioned, the term “earth colours” has been considerably
broadened of late. Whereas, formerly, it was restricted to colours
prepared exclusively from minerals by a simple treatment, limited to
crushing, levigation or calcination, it now includes the pigments
obtainable from large by-products of certain chemical processes. This
latter class is especially important as affording an opportunity of
utilising products formerly considered worthless and whose removal
often entailed heavy expense.
By drawing on these materials the industry of the earth colours has
greatly enlarged its scope. At present, many colours of this kind are
on the market, and it is to the interest of many manufacturers to
endeavour to utilise certain waste products in the same direction.
The advantage of such a course hardly needs emphasising; but, to give
only a single example, it may be mentioned that the manufacture of
fuming sulphuric acid from green vitriol, by the old process, produces
residues which were formerly looked upon as quite worthless, and sold
at very low prices, but are now worked up, in a number of factories,
into very handsome and durable pigments.