OTHER RED MINERAL PIGMENTS.
=Chrome Red or Chrome Vermilion.=—Under the most varied names, chrome
orange, Persian red, Derby red, Chinese red, Indian red, chrome
vermilion, etc., there occur in commerce numerous pigments, orange to
dark red in colour, which can all be made from neutral lead chromate.
When this salt, _i.e._, ordinary chrome yellow, is treated with
smaller or larger quantities of a strong base, such as caustic potash
or caustic soda, “basic” lead chromates are formed, which exhibit a
more intense red shade the more lead they contain. These compounds can
be made either by adding caustic potash to the solution of potassium
chromate used for making the chrome, or the precipitated chrome yellow
may be treated with caustic potash. The latter process readily yields
good results, so that it is generally followed.
Recent researches have shown that differences in the shade of chrome
red are due to the varying size of the single microscopic crystals
of which it consists. When chrome reds of the most different shades
are ground the powder has almost the same colour. The larger are the
crystals the deeper is the shade of the red, so that the art of making
deep chrome red lies in working so as to produce large crystals.
As was said in describing the preparation of chrome yellow, the lead
liquors used in its manufacture contain varying quantities of acetic
acid, which affect the shade of the product. The quantities of caustic
potash necessary to turn a certain quantity of chrome yellow into
chrome red of a certain shade can, therefore, only be estimated from
time to time in an empirical manner. This estimation may be performed
by the method of Habich with great certainty. The precipitate of chrome
yellow is made in the ordinary way, and well washed 6 to 8 times.
Equal portions of the paste are brought into vessels of equal height
and diameter. To the first of these portions caustic soda solution of
a certain strength is added; to the second rather stronger caustic
soda solution; in each succeeding vessel the strength of the solution
is increased to a known extent. After the addition of caustic soda
the vessels are energetically shaken. They are then set aside for the
precipitate to settle in a place completely free from vibration. In
one of the vessels, an orange or red shade will be seen very similar
to that it is desired to make. Now, it is known how much caustic soda
solution of a certain strength has been added to this quantity of
colour, so that the quantity necessary for the whole of the chrome
yellow precipitate may be found by a simple calculation. In working on
the large scale, the process is exactly the same as in the preliminary
test; the calculated amount of caustic soda solution is added to the
precipitate in the tub, it is well stirred, and left at rest for some
hours. When the proper shade appears, the liquid above the precipitate,
which contains sodium chromate in proportion to the strength of the
caustic soda solution used, can be drawn off and used to precipitate
fresh lead acetate solution. The precipitate is washed first in the tub
and then on the strainers.
Since, as has been said, the depth of colour depends upon the size of
the single crystals, care should be taken in washing the precipitate
not to agitate it too vigorously, or the pigment will lose in depth by
the breakage of a large number of crystals.
Special formulæ for the manufacture of chrome red are abundant, but if
the above process be followed they are superfluous, and it is only for
the sake of completeness that one or two are given. Deep chrome red is
made by the action of 25 parts of caustic soda on 100 parts of pure
chrome yellow.
A noteworthy process is due to Liebig. Saltpetre is melted in a
crucible, and heated to a temperature below that at which oxygen is
evolved. Dry chrome yellow is introduced so long as effervescence
occurs; the fused mass appears, whilst liquid, deep black. As soon as
it is in quiet fusion it is poured on a cold plate. In consequence of
the rapid cooling it becomes brittle and can be easily powdered. The
hot mass is broken up and boiled with water, potassium chromate going
into solution. The chrome red made by this process is little inferior
in brilliance to vermilion.
According to Prinveault a handsome chrome red is obtained by treating
25 grammes of neutral lead carbonate with 10 grammes of yellow
potassium chromate dissolved in 5 litres of water, boiling for half
an hour, washing the violet precipitate, and finally treating with 1
gramme of sulphuric acid diluted with 100 grammes of water.
=Cobalt Red= consists of cobalt phosphate. It was first recommended
by Salvetat. It is obtained by precipitating a solution of a cobalt
salt by sodium phosphate. After drying at the ordinary temperature the
precipitate has a beautiful rose-red colour. By careful heating, the
shade becomes more violet according to the temperature used.
=Cobalt-Magnesia Red.=—When magnesia is moistened with the solution
of a cobalt salt and strongly heated, a rose-red mass is obtained
which can be used as an extremely durable artists’ colour. It probably
consists of a compound of cobalt oxide with magnesia. The method of
preparation is as follows: Magnesium carbonate is mixed to a thin paste
with a dilute solution of cobalt nitrate; this is heated and stirred
until quite dry, when the residue is strongly heated in a covered
crucible. In the manufacture of this and other cobalt pigments it is of
particular importance to prevent the entry of the fire gases into the
crucible. Unless this is done a product of good colour is not obtained.
=Cobalt Arsenate.=—By precipitating a cobalt salt with sodium arsenate,
a violet-red precipitate is obtained which leaves a fine durable red on
heating. The mineral erythrite has a similar composition and colour.
This pigment has a bright shade, is permanent, and although it contains
arsenic is not particularly poisonous, since after fusion it is little
soluble.
=Chromium Stannate.=—This artists’ pigment, which may be used in oil
and porcelain painting, is made in the following manner (Gentele): 1
kilogramme of tin is converted by strong nitric acid into metastannic
acid, 50 grammes of potassium chromate are dissolved in 1 litre of
water, the solution mixed with 2 kilogrammes of chalk and 1 kilogramme
of powdered quartz, and finally with the metastannic acid. The mixture
is dried and strongly heated. The temperature must be raised to a white
heat, at which the mass sinters and acquires a dark rose-red colour. It
is then completely extracted with boiling water, when a fine powder is
left.
This pigment, which is known in the market as “pink colour,” may be
used with advantage in oil painting in the place of rose madder lake,
over which it has the advantage in durability.
=Silver Chromate.=—A solution of silver nitrate gives, with potassium
chromate, a deep red precipitate of silver chromate, which is
occasionally used as an artists’ pigment under the name of “purple
red”. Not only is the price of this pigment high, but it has little
durability, since, like all silver compounds, it is acted upon by
sulphuretted hydrogen and turned dark, owing to the formation of black
silver sulphide.