ENAMEL WHITE.
Barium sulphate, known as permanent white, enamel white, _blanc fixe_,
barytes white, is the only white pigment which is absolutely unaltered
by exposure to the atmosphere. Lead pigments are discoloured in the
course of time, and in the end turn black; bismuth white behaves in the
same manner; zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention
from the colour manufacturer, especially as it can be made by a very
simple and cheap method. When sulphuric acid or a soluble sulphate
is added to the solution of a barium salt, all the barium is at once
precipitated in the form of barium sulphate.
When quite pure, barium sulphate forms an extremely soft, brilliantly
white powder, which offers complete resistance to the action of the
atmosphere, and also of strong acids and alkalis. It is extremely
insoluble, and can be precipitated from the most dilute solutions,
and is then obtained in so fine a state of division that it cannot be
filtered from the liquid; it passes through the closest filter together
with the liquid. When the barium solution is heated to boiling before
precipitation, the precipitate is somewhat coarser, and can be filtered
off without difficulty.
Barium sulphate occurs ready formed in nature as the mineral barytes
or heavy spar. Finely ground barytes may be used alone as a pigment,
but more commonly is used for reducing white lead, for which purpose
it is particularly applicable on account of its high specific gravity.
This admixture must be regarded as diminishing the quality of the
pigment, because ground barytes has far less covering power than white
lead. Artificial barium sulphate is in a state of division which cannot
be reached by grinding barytes, consequently it considerably surpasses
the latter in covering power.
The raw material for the manufacture of enamel white is either barytes
or witherite (barium carbonate); the latter, however, occurs so rarely,
in comparison with barytes, that the greater quantity of all barium
compounds is obtained from barytes.
If witherite is obtainable in large quantity, enamel white can be
prepared from it by dissolving in hydrochloric acid and precipitating
the solution of barium chloride so obtained by sulphuric acid. If the
witherite is very pure, the process may be simplified by treating the
mineral directly with sulphuric acid, and separating the enamel white
by a process of levigation from the impurities. In this case it is,
however, necessary to add a small quantity of hydrochloric acid to the
sulphuric acid, for the latter forms on the surface of the witherite,
at the commencement of the reaction, a thin layer of barium sulphate,
which is quite sufficient to prevent the further action of the acid on
the witherite lying below. The hydrochloric acid forms barium chloride,
which is at once decomposed by the sulphuric acid into barium sulphate
and free hydrochloric acid; this again dissolves a fresh quantity of
witherite, and this process is repeated until the mineral is completely
and quickly dissolved.
Enamel white is, however, generally prepared from barytes, which is
ground into a very fine powder and converted into barium sulphide by
heating with coal (see pages 41 and 42). Hydrochloric acid acting on
the sulphide produces barium chloride and sulphuretted hydrogen.
The covering power of a pigment is greater the finer its state
of division, so that it would appear advisable to precipitate a
weak solution of barium chloride by sulphuric acid at the ordinary
temperature. When the barium sulphate has been completely precipitated,
a solution of pure hydrochloric acid remains, which ought to be
utilised; but when very dilute barium chloride solution is used, the
hydrochloric acid is so dilute as to be useless. The barium chloride
is, therefore, given in practice such a strength that it has a specific
gravity of about 1·198; when the barium sulphate has been precipitated
from this solution the residual hydrochloric acid has a specific
gravity of 1·043.
Water of considerable purity must be used to dissolve the barium
chloride. Experience has shown that water which contains appreciable
quantities of organic matter does not give a pure white product. The
presence of sulphate of lime in the water, which precipitates barium
sulphate, need not be regarded, because the barium sulphate is so
finely divided that it remains suspended in the liquid, and is carried
down on precipitation of the enamel white by sulphuric acid. Carbonate
of lime in the water causes the separation of barium carbonate; this
may be avoided by slightly acidifying the barium chloride solution,
thus converting the calcium carbonate into chloride.
According to C. A. F. Meissner, artificial barytes, suitable for use
in oil paints, is obtained by precipitating barium salts by soluble
sulphates in place of sulphuric acid, then quickly heating the washed
and dried precipitate in a muffle to a red heat and throwing into cold
water.
As has been already stated, enamel white is the most permanent pigment
that exists; it appears destined in course of time to replace white
lead and all other white pigments, especially as its cost is generally
lower than that of the other white pigments. It costs, for example,
only half as much as white lead. At present, the principal uses of
enamel white are found in paper staining; it is not used to any extent
in oil paints. On account of its permanence, it should be used in
the place of white lead and white zinc. It also appears particularly
suitable for obtaining pale shades; it can be mixed with any other
pigment in any quantity without altering it in the least. This is, of
course, only true when the enamel white is completely pure, and when it
has been freed from every trace of hydrochloric acid by careful washing.
LITHOPONE.
A white pigment is obtained, according to Orr’s process, by lixiviating
crude barium sulphide, obtained by igniting barytes with coal, with
water and dividing the solution into two parts. Zinc chloride is added
to the first portion, then zinc sulphate, and finally the second
portion of barium sulphide solution. The white precipitate obtained
by this process contains one equivalent of barium sulphate to two
equivalents of zinc sulphide. It is collected, quickly dried, heated
in retorts to redness, and, whilst still hot, thrown into cold water,
by which its density and therefore covering power are increased. The
pigment is finally washed and ground. It is a good white, but when
mixed with lead pigments discolours them by reason of the sulphide it
contains.