BROWN EARTH COLOURS
In point of chemical composition, the majority of the brown earth
colours are closely allied to the reds, both kinds containing ferric
oxide. The main difference consists in that, in the brown earths, the
ferric oxide is combined with water to form ferric hydroxide.
Many of the brown earth colours, however, are of entirely different
chemical composition, and either consist mainly of organic matter
derived from the decomposition of plants--and therefore very similar
to brown-coal or peat--or else contain varying quantities of inorganic
substances mixed with these dark-coloured organic decomposition
products.
The brown earth colours form a highly important group, some of the
members of which are used in the finest paintings, and, for certain
purposes, cannot be replaced by other pigments. Those containing ferric
hydroxide are found--though not very frequently--in natural deposits,
the most celebrated being the terra di Siena, occurring in the vicinity
of that city.
TERRA DI SIENA
This highly renowned pigmentary earth is found in deposits, and, in
the crude state, forms dark brown masses which are devoid of lustre,
crumble readily between the fingers, have a smooth conchoidal fracture
and absorb water with avidity, in consequence of which property they
adhere to the tongue. Their chief chemical constituent is ferric
hydroxide, with which, however, variable quantities of sand, clay
and ferric oxide are admixed. These admixtures cause a considerable
divergence in the colour of the earth, ranging from pure brown to
reddish-brown, and, in the case of very impure lumps, to an unsightly
yellow-brown.
Mineralogically, terra di Siena is often regarded as a distinct species
which, according to the results of analysis, must be considered, not
as ferric hydroxide, but as ferric silicate combined with water.
Sometimes, a portion of the ferric oxide is replaced by alumina, so
that the percentage composition of the mineral becomes approximately:
ferric oxide, 66%; silica, 11%; alumina, 10%; and water, 13%. The
hardness of this mineral is 2·5, and the sp. gr. 3·46.
The method of formation of terra di Siena was probably on the same
lines as that already described in the case of ochre, namely by the
breaking down of minerals--in this case brown ironstone--and natural
levigation, the powder being deposited in places where the water
containing the ferric hydroxide in suspension came to rest and allowed
the solid particles to settle down.
The best lumps of terra di Siena in point of purity and colour can be
used as pigments without any preparation; but in most cases the earth
is lightly calcined, in order to improve the colour. This treatment
enables a whole series of tones, from pure brown to the brightest red,
to be obtained. The stronger the heating, the more water expelled from
the hydroxide, and consequently the closer the approximation of the
colour to that of ferric oxide.
The pigments met with in commerce as terra di Siena can also be
prepared artificially, by making ferric hydroxide and heating this,
when dried, until the requisite tone is attained. For this purpose,
ferrous oxide is precipitated from green vitriol and exposed to
the air, under which conditions it is rapidly transformed into
ferric oxide, and the greyish-green colour of the mass changes to
brown. Lighter tones can be obtained by the addition of inert white
substances; and, in other respects, the method of preparation is the
same as that of artificial ochre.
These pigments are sold under various names, the dark shades, between
pure brown and red brown, being usually called terra di Siena or
mahogany brown, whilst the paler sorts are sold as satinober--more
correctly satin ochre, golden ochre, etc. Other pigments, chemically
allied to the ferric oxide or ochre pigments, are sometimes found on
the market under various and entirely arbitrary names.
It may be pointed out that the greatest confusion exists in the
nomenclature of pigments, to such an extent that, in many cases,
neither the chemist nor the manufacturer knows precisely what pigment
is implied by a given name. The confusion is still further increased by
the use of names taken from different languages.
TRUE UMBER
Umber, properly so called--also known as Turkish, Cyprian or Sicilian
umber, from the country of origin--derives its name, according to
some authorities, from the province of Umbria (Italy), where a brown
earth is found, though others ascribe it to the Latin “umbra” (shade)
because of the pigment being used for painting shadows.
True umber is an earthy mass of fine texture and liver-brown colour,
merging into chestnut in some of the lumps. Chemically, it consists of
a double silicate of iron and manganese combined with water, a portion
of these metals being usually replaced by alumina. The greater hardness
(1·5) and higher specific gravity (2·2) of true umber in comparison
with Cologne earth (which is quite arbitrarily termed “umber”), form a
ready means of differentiation between the two.
According to Viktor Merz, the umber found in Cyprus consists of: ferric
oxide, 52%; manganese oxide, 14·5%; and alumina, 3%; and is, possibly,
merely a mixture of clay with hydroxide of iron or manganese. An umber
examined by Klaproth contained 13% of silica, 5% of alumina, 48% of
ferric oxide, 20% of manganese oxide and 14% of water.
The tone of umber can be modified, in the direction of red, by
calcination, but this process is seldom employed, the dark brown shade
of this colour being the one most appreciated.
In some parts of northern Germany, Thuringia in particular, the iron
mines contain smaller or larger pockets of ferric hydroxide, of a
fine earthy texture, from which umber is prepared, by levigation and
calcination. The product is sold under various names: chestnut brown,
wood brown, mahogany brown, bistre flea brown, roe brown, according to
the shade of the calcined pigment.
A mineral (“siderosilicate,” according to Von Walterhausen) composed of
ferric silicate, and approximating in this respect to terra di Siena,
is found in the neighbourhood of Passaro (Sicily) in deposits of tuff.
It forms masses which are transparent at the edges and are usually
liver-brown to chestnut in colour. The hardness of the mineral is 2·5,
the sp. gr. 2·713, and the average chemical composition: silica, 34%;
ferric oxide, 48·5%; alumina, 7·5%; and water, 10%.
The foregoing are only a few examples of brown or red-brown earth
colours. In all these minerals the pigmentary principle is iron, in
combination either with oxygen alone (ferric oxide), with oxygen and
water (ferric hydroxide), or silica compounds (ferric silicate), and
always associated with certain quantities of other metallic oxides,
especially alumina and manganese oxide. Although but few of these
minerals have gained any special reputation as pigments, there is
no doubt that similar minerals, which are certain to occur in or
near many deposits of iron ores, could equally well be used for that
purpose. There is no need to emphasise that the discovery of such a
mineral would be a very valuable find, and that the products obtainable
therefrom could be utilised to great advantage.
The testing of a mineral for its suitability as pigment is a very
simple matter, all that is required being to subject a small quantity
to the same treatment that is applied to the earth colours on a large
scale. For this purpose a few pounds of the mineral are levigated, and
the residue is dried. To ascertain the tones obtainable by calcination,
small samples--of about 100 grms.--are placed in crucibles, and
gradually heated in a furnace. When the masses have attained a
sufficient temperature, the samples are taken out of the furnace,
at intervals of ten minutes, and left to cool. It will then not be
difficult to decide whether the mineral is at all suitable for the
purposes of the colour-maker; and if so, these tests afford at once an
indication of the temperature and time the mineral must be heated in
order to obtain pigments of definite tones.
COLOGNE EARTH (COLOGNE UMBER)
The application of the term “umber” to this earth can only have been
based on a certain similarity in colour to true umber. In chemical
composition, however, the two are quite different, Cologne earth really
consisting of a mixture of humic substances. It is well known that the
rotted wood found in the interior of decaying trees is often a handsome
brown colour; and all woody matter, after lying a very long time,
finally acquires this colour, owing to the transformation of the wood
into dark-coloured compounds richer in carbon. This effect can be seen
on the large scale, in Nature, in the case of coal, brown coal and peat.
Now Cologne earth consists of a brown-coal mould, dark brown in colour,
of earthy character and of such low cohesive power that it crumbles
with ease. Owing to this character, Cologne earth can be easily ignited
by the flame of a candle, and then burns with a strong, smoky flame,
leaving very little ash and disseminating the peculiar bituminous smell
given off when brown coal is burned.
The geological characteristics of Cologne earth enable one to conclude
that, where similar conditions prevail, materials of analogous nature
may be discovered. This earth is found embedded in a deposit of brown
coal, in which it forms pockets, and occasionally large bodies. Now,
brown-coal deposits of enormous extent occur in very many localities,
as for instance in Upper Austria and in Bohemia; and many of these
mines are sure to contain pockets of brown-coal mould, which have
perhaps been overlooked, but might very well be utilised in the
preparation of colours of very similar character to Cologne earth.
The preparation of this material is very simple. The earth coming from
the deposits is put through a simple levigation treatment which leaves,
as residue, lumps of semi-decomposed wood, mineral admixtures, sand,
etc. The levigated earth is sold in the form of cubes.
Cologne earth comes into the market under various other names, such as:
umber, Cassel brown, Spanish brown, etc.
The fiery brown which was so greatly preferred by the famous painter
Van Dyck, and named Vandyke brown after him, was of very similar
composition to Cologne earth, and is said to have been obtained from a
deep brown peat earth. The Vandyke brown of the present day, however,
is almost invariably a ferric oxide pigment, toned to the proper shade
by suitable calcination.
ASPHALTUM BROWN (BITUMEN)
As a natural product, which can be used as a painters’ colour without
any special preparation, asphaltum (bistre, bitumen) may also be
classed among the earth colours. Chemically, it is composed of
hydrocarbons of various kinds, and is thus similar to tar; in fact,
asphaltum may also be regarded as a natural tar resulting from the
decomposition of various organic substances. Many deposits of this
mineral are known, and two of them are particularly celebrated: those
of the Dead Sea, in Syria, and the Lake of Asphalt, in Trinidad. Both
deposits consist of craters filled with water on which the asphaltum
floats in large cakes.
Several kinds of asphaltum are met with in commerce, ranging in colour
from brown to black. The preparation of the material as a pigment
is confined to grinding the mass, which is always of a low degree
of hardness. Being readily soluble in oil of turpentine and then
furnishing the most beautiful brown tones when laid on thinly, the
pigment is usually sold in this condition, although it is also ground
in oil for the same purpose.
Finally, it may be mentioned that various useless materials can be
transformed, by suitable treatment, into brown pigments closely
resembling Cologne earth and applicable to the same uses. Such pigments
can be prepared from brown-coal slack (from inferior brown coal) or
bituminised wood--a variety of brown coal looking like charred wood--by
treating these materials with a lye made from wood ashes and lime, and
washing and drying the residue.