ENAMEL COLOURS.
An enamel is a glass distinguished from ordinary glazes by a much lower
melting point, and generally by opacity. Most enamels are coloured by
additions of metallic oxides.
Without going in detail into the art of enamelling, some observations
may here be given which appear indispensable for an understanding of
the manufacture of enamel colours. The enamels are glasses, but not
every glass can be used for an enamel, since the constituents of the
glass react with the substances by which the enamel is coloured, and
with certain colouring matters quite different shades to the intended
might be produced. Of equal importance with the composition of the
glass used in the preparation of the enamel is the temperature at which
the glass melts. Many colours bear but a low heat; they decompose at a
somewhat higher temperature and produce quite the wrong colour. This is
especially the case with the enamels coloured purple red by gold.
The enamel colours are fixed by mixing them with an easily fusible
glass (the flux), applying them to the object to be enamelled, whether
metal, glass, or porcelain, and heating the painted article in muffles
until the glass melts, and either dissolves the colouring matter or
encloses it unaltered. According as the enamel melts as a whole, or the
flux alone melts and encloses the colouring matter, so transparent or
opaque enamels are produced. The former are really coloured glasses,
the latter are glasses in which is enclosed the sintered colouring
substance.
Whilst enamels were formerly of importance only for artistic purposes,
they have recently attained great industrial employment; not only is
earthenware now covered with enamel, and thus made to resist the attack
of chemical reagents, but boiler tubes are lined with enamel to prevent
the formation of scale.
The colours used for enamels must in all cases be metallic oxides; for
yellow, silver and antimony oxides; for red, gold, copper oxide and
ferric oxide; for blue, cobalt compounds; for green, copper oxide or
chromium oxide. Two operations are required in the production of enamel
colours, the preparation of the flux and its fusion with the real
colouring substance. In most cases the latter operation takes place
simultaneously with the fixation on the enamelled article, but the
enamels may be melted and cast into lumps, which are then powdered.
=White Enamels.=—These are always ordinary crystal glass, to which tin
dioxide or potassium antimoniate has been added. Particular care must
be taken that only pure raw materials are used, for almost unweighable
quantities of iron compounds are sufficient to impart to the enamel a
yellow tinge. In almost all cases the glass must be decolourised by the
addition of pure pyrolusite (manganese dioxide), which at a red heat
gives up a portion of its oxygen to the ferrous oxide contained in the
glass, producing ferric oxide, which has much less colouring power. Too
large a quantity of pyrolusite imparts a blue tinge to the enamel.
The tin dioxide used in the glaze is made by direct oxidation of tin
in air. It has been found that tin burns much more easily in air when
it is mixed with lead; 20 to 40 parts of tin are melted with 100
parts of lead, and the alloy heated in shallow vessels in the air;
it takes fire at a certain temperature, and it is only necessary to
remove continually the layer of oxide in order to oxidise the whole
of the metal within a short space of time. The mixture of oxides
obtained in this manner is freed from particles of metal by grinding
and levigation; when it is fused with the glass, the lead oxide enters
into combination, whilst the tin oxide is embedded in the colourless
glass. When paste enamels are required the mass is melted in shallow
crucibles, poured into water and broken up to a coarse powder, which
is again fused. In some cases this operation must be repeated several
times to obtain a quite homogeneous product, for tin dioxide is very
heavy and sinks to the bottom of the fluid glass; this is readily
seen if the fused enamel is allowed to cool in the crucible. When the
crucible is broken the lower portions of the fused mass are dense
white, whilst the upper are merely milky white. By repeatedly fusing
the mass it is endeavoured to obtain it uniform. In fusing the enamel
care must be taken that it is heated in a vessel from which the fire
gases are completely excluded, since the smallest quantity of coal or
of ferruginous ashes coming into contact with the melted mass would
injure its colour. If the melting point of the enamel is too high it
can be lowered by the addition of a small quantity of pure quartz sand.
A fine white enamel is obtained by using litharge or red lead. The
mixture contains 60 parts of quartz sand, 30 of alum, 35 of common
salt and 100 of red lead. It is advisable to add a small quantity of
finely-powdered talc to the sand. In consequence of the considerable
proportion of alumina contained in this enamel it is difficult to melt,
and can be heated to very high temperatures without injury to the shade.
When antimony oxide is used a glass free from lead must be taken; lead
glass does not give a pure white with antimony oxide. A very good white
enamel is obtained by fusing 3 parts of crystal glass with 1 part of
sodium antimoniate.
=Coloured Enamels= owe their colour to metallic oxides. Enamels
coloured by metallic oxides, in which the oxygen is firmly united,
can be fired without great precaution, but if they contain oxides
easily decomposed, great care is required to obtain a fine colour. In
enamelling metals a white under-glaze is generally used beneath the
coloured enamel; it consists of a refractory white enamel. By this
means it is more easy to obtain pure colours.
=Yellow Enamels= are coloured by silver, antimony oxide combined with
litharge, or ferric oxide; from the latter a red can also be obtained.
To produce a yellow enamel by means of silver the article is first
enamelled white at a low temperature; silver oxide is then applied
where required, and the article again heated. It then frequently
happens that the surface has a metallic appearance owing to the
reduction of a part of the silver oxide to metallic silver. When this
coating is scraped off the enamel beneath is found to be coloured a
fine yellow.
The antimony yellow is obtained by mixing 1 part of antimony oxide, 1
part of alum and 1 to 3 parts of white lead, according to the depth of
colour required. These finely-powdered materials are intimately mixed
with 1 part of sal ammoniac, and heated in an open vessel with stirring
until the yellow colour appears. The vapours of the ammonium chloride
indicate the proper temperature for the operation. When this substance
is completely volatilised the temperature should not be further raised,
or the mixture would fuse.
By means of ferric oxide a fine and durable yellow is produced; the
quantity employed must not be too large, or a red colour will result. A
very high temperature is required for burning in this colour. The alum
used in making the yellow enamel colours serves to prevent the oxides
from fusing.
=Red Enamel.=—Ferric oxide is generally used. When a purple red is
required it is obtained either directly from metallic gold or from
purple of Cassius.
Red iron enamel is made by slowly heating 20 to 25 parts of pure
ferrous sulphate with 10 parts of aluminium sulphate until all the
water of crystallisation is expelled, when the temperature is gradually
raised to an intense heat. The shade depends upon the temperature: the
higher it is the darker is the colour. Tests are taken from time to
time of the mixture, and rapidly cooled. Hot ferric oxide is black,
consequently the shade can only be judged with complete certainty in a
cooled portion. Although the temperature required for the preparation
of this colour is very high, yet it ought not to rise so far that the
magnetic oxide is produced, a small quantity of which would cause the
enamel to appear dirty red, since it gives to the flux a blackish
green colour. The red ferric oxide colour should not be dissolved in
the glass of the enamel, but should be embedded in it. Thus, when the
enamel is burnt on, the temperature should not rise so high that ferric
silicate is formed, otherwise a yellow or even completely black enamel
results.
Purple red is obtained by means of gold; formerly purple of Cassius
was exclusively employed for this purpose, but an equally bright red
can be obtained from gold chloride. The colour obtained from gold will
bear only a very low heat; it must be mixed with a very fusible glass,
brought on to the article to be enamelled, and heated just sufficiently
to melt the mass. The following mixture is used for gold red and other
delicate colours: 3 parts of calcined borax, 3 parts of quartz sand and
1 part of chalk. A very small quantity of gold is sufficient to produce
a deep red, the amount of gold preparation used for a pale red or a
medium purple must be carefully weighed.
=Blue Enamels= are always coloured by cobalt oxide. Any cobalt
compound can be used, since at a red heat the silica of the glass
displaces other acids, and produces cobalt silicate. It is most simple
to take cobalt nitrate. This salt is readily obtained completely pure,
and absolute purity is of great importance, for only thus is a pure
blue produced.
Cobalt oxide produces a more or less deep blue when fused with varying
quantities of glass. In order to produce shades similar to turquoise
and forget-me-not, a white enamel must be used beneath the blue, or
bone ash must be mixed with the blue enamel; this produces a paler
colour, which is also opaque.
=Green Enamel= is coloured by copper oxide or chromium oxide. In the
former case 1 part of copper oxide is used to 30 to 50 parts of glass,
according to the depth of shade required. The enamel made with copper
oxide alone has never a pure green colour; it exhibits a blue tinge. A
pure green is obtained by adding a very small amount of ferric oxide,
which produces a yellow and compensates for the blue shade, so that a
pure green is formed.
Chromium oxide produces a beautiful emerald green without any addition.
The enamel may be exposed to very high temperatures without injury.
Green enamels may also be made by mixing blue cobalt enamel with a
yellow enamel: an unexceptional shade results, but this method is
seldom used, since green enamels are obtained in a simpler manner from
copper and chromium oxides.
=Violet Enamel.=—Manganese dioxide is used exclusively to produce
violet enamel; an extremely small quantity is sufficient to colour a
considerable quantity of glass. A pure violet shade is only obtained
when very pure manganese dioxide is used; the artificially prepared
substance should be used to produce the finest violet: the cost is
considerably greater than that of the mineral pyrolusite, but equally
good results can rarely be obtained from pyrolusite.
=Black Enamel.=—When a large quantity of ferric oxide, copper oxide or
cobalt oxide is fused with a glass, a deep black enamel is obtained.
Generally mixtures of these oxides are used; experience has shown that
a much deeper black is thus obtained than from any one alone.
Now that enamels are not used exclusively for artistic purposes, but in
considerable quantities for earthenware and other technical purposes,
the manufacture of enamel colours has attained considerable importance;
works already exist occupied almost exclusively with this special
branch of manufacture.
By Lacroix’ process enamel colours are made which can be applied to
porcelain without further admixture with a flux; for example, a very
fine blue is made by dissolving 300 parts of pure alumina and 100
parts of cobalt carbonate in nitric acid, evaporating the solution to
dryness, igniting the residue and fusing it with 300 parts of quartz
sand free from iron, 900 parts of crystallised boric acid and 1,800
parts of red lead. A blue glass is obtained which easily melts, but
is difficult to powder; but if poured when fluid in a thin stream
into cold water, it forms thin threads, which are very brittle in
consequence of the rapid cooling, and may be easily converted into a
soft powder.