ANTIMONY VERMILION.
Antimony vermilion is a red pigment which will bear comparison in
fineness of shade with mercury vermilion, over which it has the
advantage of cheapness. In composition it is antimony trisulphide,
Sb₂S₃. This compound is obtained by precipitating a solution of
antimony trichloride with sulphuretted hydrogen. However, the
precipitate, which is a very fine red whilst wet, loses its colour in
drying, and the product is almost worthless as a pigment.
In another way it can be obtained in such a condition that it loses
nothing of its beauty in drying, but retains its brilliance. Böttger
gives the following process: a solution of antimony trichloride is
mixed with a solution of sodium hyposulphite (thiosulphate) and the
liquid heated so long as a precipitate forms, which is then washed on
a filter with water containing acetic acid. If pure water were used
for washing, the antimony chloride still present would be decomposed,
forming the white oxychloride, which would detract from the shade of
the antimony vermilion. In this process particular regard is to be paid
to the use of exact quantities of materials. The finest product is
obtained when 2 parts of a solution of antimony trichloride, which has
exactly the specific gravity 1·35, are mixed with a solution of 3 parts
of sodium hyposulphite in 6 parts of water.
According to R. Wagner, antimony vermilion is obtained by dissolving
4 parts of tartar emetic and 3 parts of tartaric acid in 18 parts of
water, heating to 60° C., mixing with a solution of sodium hyposulphite
(thiosulphate) and heating to 90° C. The precipitate is then carefully
washed and dried.
Pure antimony vermilion closely approaches, as we have said, ordinary
vermilion in shade, and for a sulphur compound shows a remarkable
resistance towards chemical reagents. By dilute acids, ammonia and
alkaline carbonates, it is attacked only on long continued contact, but
it is easily decomposed by very dilute hydrochloric acid and by caustic
alkalis. A mixture with white lead keeps for a long time, but there
can be no question of the permanence of such a mixture, in consequence
of the oft-repeated properties of lead pigments. Antimony vermilion is
well adapted for oil painting. When ground with oil it exhibits a red
of a brilliance in no way inferior to that of genuine vermilion. It may
also be used as a water colour, but is not adapted for fresco work,
since it is quickly decomposed by lime.
In spite of these favourable properties antimony vermilion has so
far been little used. Considering the small cost of its preparation,
especially when calcium hyposulphite, which gives an equally good
result, is used in place of the sodium salt, its use is to be
recommended in the place of mercury vermilion. It appears as if the
high price demanded for this substance by several manufacturers has
prevented its general employment.
=Appendix. Antimony Blue.=—This fine blue pigment can be prepared by
the addition of a dilute solution of potassium ferrocyanide to a clear
solution of antimony in _aqua regia_. According to Krauss, it contains
no antimony as colouring principle, but is a Prussian blue obtained
from the ferrocyanide, which is decomposed by the strong acid, with
evolution of hydrocyanic acid.