BROWNS.
Brown colouring matters are obtained from all three kingdoms--the
animal, the vegetable, and the mineral--but in greatest abundance from
the last named. The natural mineral brown pigments afford almost every
variety of tint, and being largely composed of silica and metallic
oxides they are remarkably permanent.
ASPHALT OR BITUMEN.--These names are applied to a variety of
black or brown resinous matters found in many parts of the world in
a mineralised state, though derived originally from organic sources.
The “Bitumen of Judæa” is supposed to be found on and around the Dead
Sea, but the bulk of the product going by that name really comes from
Trinidad. All kinds of asphalt have a pungent and peculiar smell, melt
at a low temperature, are very combustible, and while dissolving in
turpentine, and more readily in coal tar naphtha, are insoluble in
water and in alcohol. Very little asphalt is now used as a pigment,
but it continues to find a limited application in varnish making,
notwithstanding the tendency of varnishes containing it to suffer from
minute cracks with the lapse of time.
BISTRE.--This pigment is used exclusively in water-colour
painting, for which purpose it affords a fine warm yellowish tinted
brown. It is of vegetable origin, being prepared from the soot which is
deposited in the flues leading from fireplaces which consume wood fuel.
Every wood, however, does not afford an equally good sample of bistre,
and beech occupies the foremost rank in this respect. The brightest
and blackest soot is selected, and after careful grinding and sifting
through a very fine sieve, it is repeatedly stirred up, for several
hours at a time, in a series of clean hot waters, the object of which
is to dissolve out all traces of tarry and other soluble matters, which
very seriously affect its permanence, being oxidised on exposure to
air and light, and thus weakening the tint. The washing is therefore a
matter of the very first importance. The solid pigment is allowed to
settle out of each wash water, and is collected and dried, being mixed
with a small proportion of gum water to give cohesion. The drying is
effected in a stove room.
BONE BROWN.--This unimportant pigment is simply underdone bone
black (see p. 6), and is obtained by stopping the calcination of the
bones at a point which falls short of thorough charring. In consequence
it contains a proportion of unaltered animal matters, which sooner or
later may undergo decomposition, and prejudicially affect the painting.
CAPPAGH BROWN.--A mineral pigment which is only a variety of
umber, and may best be described under that head (see p. 105).
CASSEL EARTH.--Another name for Cologne earth, _q. v._
CHICORY BROWN.--This vegetable pigment is rich-coloured but
lacks permanence. It is prepared by calcining roots, such as those
of chicory, in vessels to which air is not admitted, from which then
results a fine brown powder. This is boiled in water, and the solution
is evaporated to dryness, yielding a brown pigment, which, being
soluble in water, is sometimes employed by water-colour artists.
COLOGNE EARTH.--This material, which is also known as Cassel
earth or Rubens brown, is an earthy carbonaceous substance, probably
derived from the decomposition of lignite or brown coal, readily
undergoing combustion without emitting flame or smoke. Large deposits
of it are worked in the vicinity of Cologne, whence its name. It is
blackish-brown in colour, smooth and crumbling to the touch, and very
light. To remove soluble impurities it is subjected to several washings
in water, and then collected, mixed with a little gum-water, and dried
in small moulds. The colour is used by artists, but is very variable in
composition and uncertain in durability.
MANGANESE BROWN.--One of the most durable brown pigments used
by the Romans is found to be oxide of manganese, which discovery has
led to the proposal to prepare the binoxide of that metal as a brown
pigment. The method suggested is as follows:--
The protochloride of manganese, derived from the manufacture of
chlorine, or the protosulphate resulting from the calcination of the
protoxide with iron sulphate, is dissolved in warm water (85°-105°
F.); to this is added a sodium hypochlorite solution, or a solution
of potassium hypochlorite containing a small proportion of carbonate
of soda, the addition being continued until the precipitated
manganese binoxide ceases to change colour, marking the completion
of the oxidation. The supernatant clear liquor is drawn off, and the
precipitate is washed first with acidulated water (containing 2 per
cent. of sulphuric acid), and then with pure water till all trace of
the acid is removed. The dark-brown impalpable powder of manganese
binoxide is stove dried, and forms a permanent and safe pigment with
good covering power.
MARS BROWN.--One of the products obtained by the calcination
of Mars yellow (_q. v._) at various temperatures and under different
conditions is a full-tinted and durable brown due to sesquioxide of
iron. Another method of preparing it is from alum, sulphate of iron,
and chloride of manganese. In either case the pigment is not superior
to umber or oxide of iron, while it cannot be produced as cheaply.
PRUSSIAN BROWN.--An artists’ colour known by this name is
prepared from Prussian blue, but as it has no superiority over Vandyke
brown or umber, and is higher priced, it is not in general use. It
consists essentially of carbon and ferric oxide, resembles bistre in
tone, and possesses durability and good covering powers. The operation
of calcining the Prussian blue should be conducted slowly, and is
best performed in a closed vessel, though it may also be done in the
open. The pieces of blue should not be larger than a hazel nut. They
soon split, scale off, and become red, when the heating should cease.
On breaking the cooled particles they will show a patchy coloration
varying from yellowish-brown to black. On grinding, the mass assumes
the desired brown hue.
RUBENS BROWN.--Another name for Cassel brown or Cologne earth
(_q. v._).
SEPIA.--This is one of the few pigments derived from the
animal kingdom. It is produced by several sea-inhabiting creatures
belonging to the class called _Cephalopoda_, and more particularly
by two members of the genus _Sepia_, known respectively as _Sepia
officinalis_ and _Sepia loligo_. A peculiarity of these cephalopods
is that they are provided with what is commonly called an ink bag, in
other words a gland or sac filled with a blackish-brown liquid, which
possesses intense colouring power. The object of the secretion is the
protection of the creature from pursuit by its enemies, a portion of
fluid being discharged at will, and so obscuring the surrounding water
that escape is facilitated.
For the sake of this pigment the cuttle-fish are sought after by
fishermen in the localities frequented by the animals, notably in the
warm waters of the Mediterranean. When the creatures are captured,
their glands are carefully extracted and sun-dried so as to solidify
the contents. In this state ink bags are sent into commerce. The
colourman subjects the sacs to boiling in a solution of soda or
potash, whereby the colour is dissolved out of the receptacle, and
being filtered clear of all fragments of the animal tissue, is next
precipitated by the addition of acid, collected on a filter, washed,
and dried. It then forms an exceedingly useful pigment, having,
according to Prout, the following average composition:--
Per cent.
Black pigment (melanin) about 78
Calcium carbonate 10½
Magnesium carbonate 7
Alkaline chlorides and sulphates 2
Organic matter 1
It is remarkably permanent for an organic substance, suffering no
alteration on being combined with other pigments, and withstanding the
effects of exposure to air and light. Though slightly transparent, and
not quite constant in tint, it possesses very great colouring power.
Being of extremely fine texture it can be worked up equally well as an
oil colour or as a water colour, but it is especially in the latter
capacity that it forms an indispensable artists’ colour, and permits
the production of a great range of shades and tints.
ULMIN.--The pigments grouped under this name are also of
organic origin; but though they possess good colour, mix well, and flow
readily from the brush, they lack the durability which is essential to
their successful use. The following methods have been employed in their
preparation:--(1) Fused caustic potash is digested in alcohol, and the
liquor filtered and heated till a brown powder is thrown down, which is
filtered and washed with acidulated water; (2) Waste cotton, peat, or
brown coal, heated with an alkali; (3) Farinaceous matters carbonised
by mineral acids.
UMBERS.--These form a large class of natural earths of a
brown colour, differing widely in the proportions of their chief
constituents, but closely allied to the ochres and siennas in general
composition, and owing their colour mainly to the presence of hydrated
oxides of iron and manganese, the latter prevailing in the umbers to
a greater degree than in the ochres and siennas, which consequently
belong to the yellow group (_q. v._).
Beds or veins of umber of varying thickness and extent are found
in many places, especially in connection with magnesian limestone
(dolomite). Apparently they are often derived from decomposition of
this rock, perhaps due to the infiltration of carbonated water, which
has acted upon the calcium and magnesium carbonates in the dolomite,
and left the silica and the iron and manganese as oxides, forming the
bulk of the umber. Usually these beds of umber are near the surface,
though covered by an overburden of vegetable soil, and the operation
of working them may be called quarrying rather than mining, being of a
superficial and simple character, often only amounting to small pits.
As no umber is a definite body, but rather a mixture of various
substances, so the composition of every kind is peculiar to itself,
and very wide differences are noticeable. Even the same bed will not
necessarily produce always the same class of umber. The following
figures show the extent to which the proportions of the several
ingredients may vary:--
Per cent.
Water given off at 212° F. 4 to 65
Water in combination 5 to 11½
Silica 4½ to 29½
Manganese dioxide 7 to 27
Ferric oxide 6 to 36
Calcium carbonate is sometimes present to the extent of 2½ to 6 per
cent., and at other times is quite absent, its place being taken by
½ to 1 per cent. of lime (calcium oxide); some of the English umbers
contain about 2 per cent. of calcium sulphate (gypsum) in addition to
the carbonate. Alumina may occur to the amount of 2½ to 12½ per cent.,
or may be wanting altogether. In a sample of Derbyshire umber analysed
by Hurst there appears to have been over 30 per cent. of barium
sulphate (barytes), which looks suspiciously like adulteration.
Almost every variety of shade may be found in umbers. The darkest and
richest in colour--a warm violet-brown--is the so-called Turkey umber,
mined in Cyprus, and formerly shipped viâ Constantinople; this is
of very fine quality and commands the higher price in the market. A
reddish-brown Irish umber, known as Cappagh brown, obtained from the
Cappagh mines in Cork county, is much esteemed among artists, both
for water-colour and oil painting, and especially for the latter when
it has been subjected to a preliminary desiccation at a temperature
of about 170° F. Heated to the boiling point its colour changes to a
rich red, resembling burnt sienna. Cornish umbers are of fairly good
quality. Derbyshire umbers are poor, and incline to a reddish tint,
besides being gritty. Sometimes they are adulterated with a little lamp
black, which renders the tone more like that of Turkey umber, and thus
deceives the unwary buyer.
There are three conditions in which umbers come into commerce: (1) as
raw lump, being the mineral just as it is mined; (2) as raw powdered,
when it has been ground very fine and levigated or washed in flowing
water, whereby the particles get assorted according to their several
degrees of fineness; and (3) as burnt, being the powder after it has
been subjected to calcination in a closed furnace. Some umbers are so
soft that they can be washed without any previous grinding, but this is
not generally the case. The apparatus used in grinding and levigating
is common to all pigments where these processes are employed, and will
therefore be described once for all in a later chapter. The calcination
is conducted at a red heat, and by this process the tint is made darker
and warmer, but it must not be pushed too far or the pigment will
blacken.
While different samples of umber present differences of tone and shade,
from a yellowish to a violet brown, they are alike in being very
durable and proof against the injurious influences of air, light, and
impure atmospheres; ordinary acids and caustic soda have no appreciable
effect. They mix well with other pigments without provoking any change,
and are equally satisfactory as oil or water colours. They do not
admit of much adulteration, except in the substitution of an inferior
grade for a superior one, and possibly the addition of barytes as a
make-weight.
VANDYKE BROWN.--What the original brown used so much by
the great Van Dyke was no one can tell. The pigments now sold under
the name of Vandyke brown are of varying composition, some being
simply mixtures of red oxide of iron and lamp black, others are
natural earthy substances after the character of Cologne earth, and
others again are artificial products of the partial carbonisation of
vegetable matters, such as cork waste. As it is uncertain what was the
composition of the original Vandyke brown, no standard of chemical
purity can be established.
Probably the most general sources of Vandyke brown are red oxide and
lamp black, and the quality of such a pigment will chiefly depend on
securing a good black, as any traces of unburned oily matter will
make the paint difficult to dry. Almost any variety of shade can be
produced by adjusting the proportions of lamp black and red oxide, with
sometimes the addition of a little ochre. The pigment made in this
manner forms the staple brown paint for industrial application, mixing
well with oil, and being of a durable character, but it does not mix so
well with water.
Vandyke browns of the Cologne earth type, from earthy lignites and
peaty matter, are much used in and around the localities where they are
produced, and entail nothing more than grinding and levigation to fit
them for the market. They are in general best adapted to water-colour
painting.
Warm and slightly reddish tints of Vandyke brown are obtained by
the partial carbonisation of ligneous material, in other words by
subjecting cork and bark waste to moderate calcination in closed
retorts. These mix equally well in water or oil.
All varieties of Vandyke brown are stable pigments, without any
disturbing influence when used in admixture with other colours, and
quite proof against any change on exposure to air and light. Next to
the umbers they are the most generally useful browns.