SMALTS.
This blue pigment is a very finely powdered glass, coloured by cobalt
oxide, according to the quantity of which products of very different
shades are obtained. If a very small amount of cobalt oxide is used
the smalts has a beautiful pale blue colour, but by the use of larger
quantities an almost black mass can be obtained.
Although the proportion of cobalt oxide contained in smalts is so
variable, yet it is probably present as a compound of constant
composition—the cobalt potassium double silicate of the formula CoO.
2SiO₂ + K₂O. 2SiO₂. The numerous published analyses of smalts lead to
this conclusion. The small quantities of other metallic oxides which
are found in smalts are to be regarded as chance impurities. Being a
glass free from lime smalts is easily decomposed; in this respect it
shows great similarity with water-glass, which is completely soluble in
boiling water.
Smalts comes into commerce in the form of powder. In the ”Blue Colour
Works” in which smalts is made a number of qualities are distinguished;
these are separated according to the fineness of the powder. Smalts of
good quality is composed of particles of the same size and has a pure
sky-blue colour. A shade inclining to red is generally to be ascribed
to the presence of iron; material of this nature is of little value.
Smalts is not easily attacked by chemical reagents. It has been above
stated to belong to the easily decomposed glasses, and it is attacked
by reagents which ordinary glass completely resists. Finely ground
smalts is strongly attacked by boiling sulphuric acid, and, after
fusion with soda, is soluble in acids. Water has some action upon
smalts; by the long continued action of a large quantity the shade is
injured, the colour acquires a greenish tinge.
In the manufacture of smalts there are three principal operations:—
1. Preparation of the charge.
2. Fusion of the charge.
3. Grinding the fused mass.
These processes are principally mechanical. Only as regards the
composition of the charge is chemical knowledge required.
=1. Preparation of the Charge.=—The cobalt is supplied by substances of
different origin—as a rule, roasted speiss cobalt and cobalt glance are
used; occasionally ores are employed without previous preparation. When
pure cobalt salts (sulphate and nitrate) are used, a product of very
fine shade is produced, but in practice these salts are not applicable
on account of their cost. There remain therefore only the compounds of
cobalt occurring in nature. On account of the foreign substances they
contain, these require a special treatment before they can be used in
the manufacture of smalts; the treatment varies according to the other
elements present in the cobalt ores. Ores containing bismuth are first
treated in order to obtain this valuable metal; ores containing arsenic
are roasted in furnaces of special construction, which are connected
with a series of stone or wooden passages in order to condense the
arsenic trioxide completely.
If ores are employed which contain only a small quantity of cobalt, the
same quantity of glass is melted time after time with fresh quantities
of the poor ore. At each operation cobalt is taken up. In place of
glass a mixture of potash and very fine quartz sand may be used with
advantage. It produces that species of glass which most easily takes
up cobalt oxide. The quartz sand is generally made by igniting flint
and quenching in water to facilitate the grinding; it is then powdered.
River sand is generally so impure that it would produce a brown or
green coloured glass, which, when fused with cobalt ores, would give
smalts of a bad shade.
=2. Fusion of the Charge.=—This process is very similar to the fusion
of ordinary glass. The materials, in very fine powder, are carefully
mixed and then ”fritted,” that is, heated until the mass sinters
together without melting. The real fusion is accomplished in glass
furnaces. The fritted mass is fused in small ”glass pots” made of
fire-clay, in a furnace similar in principle to the ordinary glass
furnace. The glass pots are made rather small, to hold about 50
kilogrammes, in order to obtain as homogeneous a product as possible.
They are placed to the number of six in a furnace heated by coal, in
which a very high temperature can be obtained. This is essential,
because the charge has a high melting point and must be made quite
fluid in order that impurities may settle.
The furnace must be slowly heated at first to prevent the pots from
cracking. When they are once heated the temperature is raised to
fuse the charge, which is repeatedly stirred so that the residues
may sink to the bottom. These residues consist chiefly of metallic
sulphides and arsenides. After six to seven hours’ fusion the stirring
is discontinued, and the temperature raised for a short time to the
highest it is possible to attain, so that the smalts may be as fluid
as possible. When this is the case it can be drawn out into long thin
threads. The melted glass is then taken out and poured into cold
water. The rapid cooling makes it extremely brittle, and considerably
facilitates the powdering.
When new pots are used for the fusion, their walls are considerably
attacked by the alkalis contained in the glass. In order to prevent
this a small quantity of common smalts is fused in them before the
introduction of the real charge; the inner surface then becomes covered
with a protective glaze.
=3. Grinding the Fused Mass.=—The smalts is next powdered under stamps.
These only differ from the ordinary construction in that both stamp and
trough are made of granite. The coarse powder from the stamps then goes
to mills of ordinary construction, in which it is ground as finely as
possible in a current of water. A blue liquid runs from the mills into
tubs. The coarser particles which settle in these vessels constitute
the article known commercially as strewing smalts, which is of small
value. The quantity of this quality is fairly considerable; it cannot
all be sold; part is fused in a later operation with a fresh quantity
of the fritted mixture. The liquid remaining in the tubs then gives
the different qualities of smalts. After having deposited the coarsest
particles in the first vessel it is brought into another, in which it
stays only a few minutes, when it is run off into another vessel, in
which also it remains but a short time. The liquid goes in this manner
through from three to five vessels, and in the process deposits all the
coarser particles. The oftener the liquid is brought into fresh vessels
the finer are the particles still suspended in it, and the longer it
should remain before being drawn off into another.
When the liquid has run through three or four vessels, as the case
may be, it still retains the finest particles in suspension. It is
now run into a tank, in which it remains until it is perfectly clear
and has deposited all the blue. The deposit in this tank has a dirty
bluish-grey colour; it is utilised by being added to a later fusion.
It would be expected that this last deposit would have the best shade,
since it consists of smalts in the finest state of division, which
would no doubt be the case if the smalts remained unchanged. Smalts
is, however, a glass which is rather easily decomposed. In grinding,
the smalts comes in contact with large quantities of water, and in the
succeeding levigation the water decomposes it; the final residue is
thus produced. In view of the easy decomposition of smalts by water, it
is necessary to carry out all processes in which it is in contact with
water as rapidly as possible, in order to obtain the largest yield of
smalts.
The smalts deposited in the successive vessels have a different
degree of fineness, and are sold under particular marks. The Saxony
works, which produce the greater part of the smalts used in Germany,
distinguish the qualities by letters. The better qualities are
distinguished by the same letter, the fineness of the quality being
indicated by the repetition of this letter. Thus FFFE, FFE, and FE
indicate qualities which are of lower value the less often the letter
F occurs in the mark. The qualities following those marked F are
distinguished by M; the lowest quality is marked OE. F indicates fine;
M, medium; and O, ordinary.
The qualities generally designated smalts are those deposited in the
third and fourth vessels; they are also known as king’s blue or azure
blue.
The processes used in different works apparently vary. The essentials
of the processes are, however, as they have just been described; the
time during which the liquid is left in the various depositing vessels
is different.
Smalts, being a glass, well resists the action of the weather, although
on account of the absence of lime it has not the great chemical
indifference of ordinary glass. Smalts is not altered by atmospheric
influences, but from analogy with the behaviour of water-glass, to
which smalts is closely related in chemical composition, we should
conclude that, although smalts resists the action of the air for a
tolerably long time, it cannot be regarded as absolutely permanent; in
the course of time potassium carbonate and silica will be formed.
Since the introduction of the manufacture of ultramarine, the use of
smalts has considerably decreased. Ultramarine is incomparably the
more handsome pigment, but it cannot be exposed to high temperatures
without undergoing change of shade. For purposes in which the pigment
is subjected to a high temperature, as in porcelain painting or
earthware glazing, smalts has maintained its position. Smalts is also
used in fresco painting, for colouring wax and tinting writing paper.
For the latter purpose it should be rejected. It has no influence on
the durability of the paper, but it spoils steel pens. Any one who
has written for a long time on paper coloured with smalts must have
observed this; the best steel pens are soon worn out by such paper. The
very fine splinters of glass present in large number in the paper act
upon the steel as a very fine but very hard file, and quickly wear it
down.
When smalts are used for painting earthenware, the outlines of the
design are quite obliterated, the cause of which is that smalts, being
itself a glass, fuses with the glaze (also a glass) of the vessel.
When clean outlines are required smalts should not be used; some other
cobalt pigment resisting heat should be substituted.
In addition to the above blue pigments which are in common use, there
are two others to be mentioned, rarely used and very costly—tungsten
blue and molybdenum blue.
=Tungsten Blue= is made by adding excess of ammonium chloride solution
to a hot solution of potassium tungstate, collecting the precipitate
resulting when the liquid has completely cooled, drying and heating
to redness in a crucible, through the cover of which a porcelain tube
reaches to the bottom. When the crucible begins to glow, hydrogen is
passed through the tube; the crucible is then heated to a full red
heat for fifteen minutes, the current of hydrogen being continued.
The pigment obtained in this way is a deep blue powder of velvety
appearance.
=Tessié du Motay’s Blue.=—The following is the method of preparation:
10 parts of sodium tungstate, 8 parts of tin crystals, 5 parts
of yellow prussiate, and 1 part of ferric chloride are dissolved
separately and the solutions mixed. The precipitate is washed and
exposed to light in thin layers. The blue colour develops in the course
of several days. According to the discoverer, this pigment consists of
a compound of tungsten oxide with a double cyanide of iron and tin;
in physical properties it is similar to good Prussian blue, but is
distinguished by greater durability on exposure to light and also by
a much higher cost. Up to the present this pigment has not been made
commercially.
=Molybdenum Blue.=—This fine but costly pigment is obtained by mixing
a solution of sodium molybdate with a solution of stannous chloride.
A blue precipitate is formed, which, after washing and drying, may be
used as an artists’ colour. It is distinguished by great durability. A
good shade of molybdenum blue can be obtained with greater certainty
by adding finely-powdered tin and a few drops of hydrochloric acid to
a solution of pure molybdic acid. In composition it is a mixture of
stannic molybdate and the blue modification of molybdenum oxide.