ZINC WHITE.
Zinc oxide, known as a pigment under the name of zinc white, is one of
the most important white pigments. Although not absolutely permanent,
yet it has, in common with enamel white, the valuable property of
preserving its whiteness in air containing sulphuretted hydrogen. Its
low price has brought it into general use.
Although zinc white is a most important pigment, it is very seldom
made in colour works, because on account of its origin it is a product
of metallurgical processes. Zinc white in chemical composition is
pure zinc oxide; it is formed when zinc vapour burns in air. In
zinc-smelting works zinc white is obtained by putting zinc in tubes
which are heated to whiteness; the zinc vapours burn when they come
in contact with the air, and the zinc oxide is caught by special
arrangements.
The retorts used for this purpose are similar to those employed in the
manufacture of coal gas. From 8 to 18 of these retorts are arranged in
a furnace, in two rows, one above the other. In the lid of the retorts
is an opening, which serves for charging and for carrying off the zinc
vapours. When the operation is commenced, the retorts being heated
to a white heat, two zinc plates are brought into each, the metal is
soon volatilised, and the vapours pass through the above-mentioned
openings. A current of air, heated to 300° C., is blown in to meet the
zinc vapours, which take fire and burn with a blinding white flame,
producing a very fine white powder, which is carried by the current of
air through a series of chambers in which it deposits.
[Illustration: FIG. 24.]
C. Freitag recommends for the production of zinc white the use of
retorts of oval section, _A_, Fig. 24. These retorts, containing crude
zinc, are heated to a white heat, and then a mixture of generator gas
from coke and air is introduced by _B_ and the pipe running through
_B_. The zinc burns completely in the flame of the generator gas, which
contains excess of oxygen. A product of faultless nature is said to be
obtained in this way.
In zinc works zinc white is always made in the manner which has
been described. It may also be obtained as a by-product in another
metallurgical operation, the desilverisation of lead by Parkes’
process. In this process an alloy of silver and zinc is obtained. By
sending a current of superheated steam over the molten alloy the zinc
decomposes the steam, hydrogen and zinc oxide being formed. The zinc
white is carried by the current of gas into chambers, in which it
deposits.
The zinc white obtained by burning zinc is, as has been said, a very
fine pure white powder, which can at once be used for paint without
further preparation. The price of zinc white is rather higher than
that of white lead, but the difference is counterbalanced by the
greater covering power of the zinc white. Ten parts by weight of zinc
white completely cover a surface for which 13 parts of white lead are
required.
Whilst white lead cannot be mixed with many pigments, such as those
which contain sulphur, zinc white may be mixed with all without fear of
alteration. Zinc white is even better than enamel white for producing
pale pigments from lakes; it has a lower specific gravity than enamel
white, so that the mixture with the light lake can be more easily made.
Zinc grey, which is produced by some works, is zinc oxide discoloured
by metallic zinc. Pure zinc white always has a pure white colour; if
it is tinged with grey it is contaminated by metallic zinc, whilst a
brownish hue denotes the presence of cadmium oxide. The latter impurity
will be rarely met with in commercial samples, since cadmium is worth
much more than zinc. Zinc oxide is used by the colour maker in the
preparation of Rinmann’s green; it is also used, as stated above, as an
addition to other pigments.
=Griffith’s Zinc White.=—This pigment, which is equal in covering
power to white lead, consists of zinc oxysulphide. It is obtained by
precipitating a zinc solution with a solution of barium sulphide,
washing, drying, igniting and grinding the precipitate. As it contains
sulphur, it should not be mixed with copper or lead pigments.
=Tungsten White= (Lead Tungstate), PbWO₄, is obtained as a heavy powder
by precipitating a solution of sodium tungstate with lead acetate and
treating the precipitate, which consists of basic lead tungstate, with
dilute acetic acid, by which lead oxide is dissolved and a salt of the
above composition left. This white pigment is dearer than other lead
pigments, and has no special advantages over them; it is, therefore,
seldom used.
WHITE ANTIMONY PIGMENTS.
Antimony forms a number of white compounds which can be made by a
simple and inexpensive process, and might, therefore, be used as
pigments. Two antimony compounds in particular are so used—antimony
trioxide and oxychloride (powder of algaroth).
=Antimony Trioxide= occurs ready formed in nature as white antimony or
antimony bloom. It is very simply prepared by burning the metal in air,
when it forms soft needles with a silvery lustre. It is only necessary
to heat melted antimony to a little above its melting point in a
crucible placed in a slanting position, when the metal takes fire and
burns with a blue flame. Nitric acid converts metallic antimony very
quickly into antimony oxide with a copious evolution of brown fumes.
Antimony trioxide may be more cheaply prepared from antimony sulphide,
artificial or natural (stibnite), by finely powdering, moistening with
water, and gently warming on plates. Oxidation takes place, the sulphur
is converted into sulphur dioxide and the antimony to trioxide. The
heating should not be carried too far, or the antimony takes up further
oxygen and forms the tetroxide.
=Antimony Oxychloride, or Powder of Algaroth=, is obtained by
dissolving stibnite in strong hydrochloric acid, the operation being
performed under a chimney to carry off the sulphuretted hydrogen. The
solution of antimony trichloride, when the impurities have settled, is
poured into a large vessel of water. At once a pure white precipitate
is formed, which quickly sinks; it is washed with water until the
washings have no acid reaction. Washing should not be continued
too long or the oxychloride will be further decomposed; by washing
with hot water it is almost entirely changed to antimony trioxide.
The precipitate, after washing with cold water, has generally the
composition expressed by the formula SbOCl.
Antimony oxide and oxychloride are both very crystalline powders, and
have in consequence small covering power.
=Bismuth White= is not used as a painters’ pigment; it has no advantage
over the white pigments previously described, and is much more
expensive. Its only use as a pigment is for the preparation of white
cosmetics, and even for this purpose zinc white is now frequently used;
it is cheaper and quite as satisfactory.
Bismuth white is prepared by treating metallic bismuth with fuming
nitric acid. The white precipitate at first formed completely dissolves
in an excess of acid, and when the solution is poured into a large
quantity of water, basic bismuth nitrate—bismuth white—separates.
Pure bismuth white is a soft, heavy powder, brilliantly white; it must
be preserved in air-tight vessels as soon as it is dry, otherwise it
acquires a yellowish tinge. Bismuth compounds are, if possible, more
susceptible to the action of sulphuretted hydrogen than lead compounds.
The yellow colouration, turning to black in the course of time, is due
to black bismuth sulphide.
=Tin White= is used for earthenware enamels. It is obtained by treating
granulated metallic tin with very strong fuming nitric acid. The heavy,
white powder which is formed is separated from the undissolved tin by
floating. Tin white has no application as a pigment; when mixed with
glazes it gives them a handsome, milky appearance.
=Manganese White.=—When large quantities of a solution of impure
manganese chloride are at hand, such as are produced in the preparation
of chlorine, manganese carbonate may be obtained. A small quantity of
soda solution is first added and the liquid left for several days so
that the oxide of iron may separate. When this is the case an addition
of soda gives a pure white precipitate.
=Magnesia White or Mineral White= is obtained, according to T. H.
Cobley, by mixing a solution of magnesium sulphate with calcium
chloride, adding 10 per cent. of aluminium chloride to the mixture and
stirring in slaked lime so long as a precipitate is formed. A cheaper
process is to precipitate mixed solutions of magnesium and aluminium
sulphates by slaked lime.
=Annaline.=—A white pigment is recommended for use under this name; it
consists of dead-burnt gypsum, which has been converted into a fine
powder by grinding and levigating. (Dead-burnt gypsum has been so
strongly heated that it is not able to again unite with water.)
To obtain paler shades of certain colours, additions are made of
natural pigments, such as chalk, which has been converted into a very
fine powder by levigation. An addition of chalk to a heavy mineral
pigment, such as chrome yellow, is not advisable; it would, besides,
seriously diminish its covering power.
* * * * *
In the preceding pages, a large number of white pigments has been
enumerated. It would be easily possible to increase the list, but the
result would be of no practical interest, for other pigments, neither
in respect of quality nor price, can compete with the cheaper white
pigments. Although white lead has at the present time an enormous
use, it is to be hoped that this pigment, of good colour but little
permanence, may be replaced entirely in the course of time by zinc
white, and for some purposes by enamel white.
White pigments, in addition to their use alone, are employed to produce
tints, which are obtained by mixing deeper colours with the white
pigment. By adding the proper quantity of white to a colour it is
possible to produce all paler shades of that colour. For example, the
different varieties of the red lakes which are found in commerce are
obtained by mixing white pigments with the deep red lakes.
The particular white pigment to be employed in these mixtures depends
on the nature of the colour and on its specific gravity. It should
always be remembered that white lead will not increase the permanence
of a colour, since it will be discoloured in a short time by the action
of the atmosphere. In the manufacture of fine colours for artists lead
pigments should be absolutely excluded.
To produce paler shades of colours which contain lead and therefore
have a high specific gravity, _e.g._, chrome yellow, white lead may be
used; for other colours, barytes or zinc white should be employed. In
the case of lakes enamel white is too heavy; zinc white or magnesia is
suitable.