If we were treating the subject purely from a practical point of view,
with the glazing and firing of a piece of porcelain the manufacture
might be held to be terminated. This would be strictly true, for
instance, of the white porcelain of Berlin, so largely used in the
chemical laboratory; a great deal, too, of the china in domestic use
receives no decoration of any kind. But for us there remains still to
examine the element of colour and the way in which it is applied to the
decoration of porcelain.
This is effected in three different ways: by the employment of coloured
glazes; by painting on the surface of the paste before the glaze is
applied (this is the decoration _sous couverte_); and finally by
coloured enamels applied to the surface of the glaze. These methods may
be combined, but as this is rarely the case, such a division forms the
basis of a convenient classification, more especially for the wares of
China and Japan.
In the case of both the paste and of the glaze, we have been dealing
with a restricted group of elements, with alumina, lime, potash and
soda; and apart from impurities unintentionally introduced, all the
combinations of these bodies are colourless. We have now to consider the
effect of introducing certain of the heavy metallic bases which combine
with the excess of silica to form coloured silicates.
The metals that give to Oriental porcelain its brilliant hues are few in
number. Indeed, in all lands and at all times, iron, copper, cobalt, and
manganese have been the principal sources of colour in the decoration
not only of porcelain, but of most other kinds of pottery. As equal to
these four metals in importance, but not strictly to be classed as
colouring materials, we may place tin, the source of most opaque whites,
and lead, which is the main fluxing element for our enamels. Next in
importance to these metals come antimony, long known to the Chinese as a
source of yellow, and finally, but this last only since the beginning of
the eighteenth century, gold, as the source of a red pigment.[19] This
exhausts the list, not only for the Far East, but for all the pottery of
Europe up to the end of the eighteenth century.
It was in a period of artistic decline that the advance of chemical
knowledge led to the introduction of other colours, derived both from
new metallic bases and from fresh combinations of those already known.
By far the most important of these new colours are those derived from
the salts of chromium, but uranium and other rare metals have also been
called into use. As with the sister art of painting, the beauty and
harmony of the effects produced have not kept pace with the enlargement
of the palette–the result was rather to accentuate the decline that had
already set in from other causes.
There are two metals, iron and copper, that have always been of
pre-eminent importance as sources of colour. Each of them forms two
series of combinations differing entirely in hue, so that were we
confined to the use of these two metals, our palette would still be a
fairly complete one.
The protoxide of copper, especially when a certain amount of lime and
of soda is present, forms a series of beautiful blue and green
silicates. When the proportion of oxygen is decreased, as happens when
the surface of the ware is exposed in the kiln to a reducing flame, a
suboxide of copper is formed, which gives a deep and more or less opaque
red hue to the glaze. So in the case of iron, the so-called sesqui-oxide
is perhaps the most abundant source of colouring matter in the mineral
kingdom: the colours produced by it range from pale yellow to orange,
brown, and full red. When, however, the iron is present as a protoxide,
the colour given to the glaze is entirely altered; it ranges from a pale
sea-green to a deep olive.
The remaining two elements that have long played an important part in
the decoration of pottery are cobalt and manganese. These metals, in the
form of silicates, yield the well-known series of blues and purples. One
important source of the famous underglaze blue of China and Japan is a
black mineral known to us as wad, which occurs in earthy to stony
concretions. This wad contains oxides of both cobalt and manganese, and
the quality of the blue obtained from it depends in great measure upon
the proportion in which the two metals occur.
The employment of antimony is comparatively rare, but, generally in
combination with iron, it is an important source of yellow. In spite of
the volatile nature of most of its salts, in the presence of silica this
metal is able to withstand a high temperature.
But before considering the application of colour to the glaze, we must
mention briefly a method of decoration which was in great favour at
Sèvres some years ago–I mean the application of colour to the paste
itself. This was done long ago by Wedgwood, sometimes to the whole mass
of the paste, as was the case with his jasper ware, which some
authorities class as a true porcelain. At Sèvres these coloured pastes
have been generally applied to the surface only, in thin layers, or even
as mere coats of paint. When laid on in successive coats, as in the
so-called _pâte-sur-pâte_, the amount of colouring matter need not be
large, from 2 to 5 per cent. When larger proportions of coloured oxides
are mixed with the _pâte_, and this is painted on with a brush, the
process differs little from the ordinary decoration under the glaze,
into which it indeed may be said to pass. Coloured pastes of this
description have never been employed by the Chinese, and it is not
possible to obtain much brilliancy or decorative effect by their use.
They are, indeed, foreign to the nature of porcelain, sacrificing the
brilliant white ground which should be the basis of all decorative
schemes.
When the colouring matter is subjacent to the glaze it must be of a
nature to withstand the full heat of the subsequent firing; we are
restricted therefore to colours ‘_à grand feu_.’ This practically
confines us to cobalt and to certain combinations of iron and copper, as
far as the ‘old palette’ is concerned. At Sèvres and elsewhere other
metals have been made use of whose silicates withstand the extreme
temperature of the kiln. By the use of chromium we have command of many
shades of green. If to an oxide of tin we add a minute quantity of the
sesqui-oxide of chromium, we can obtain, in the presence of lime, many
shades from rose to purple; and a mixture of cobalt and chromium
produces a fine black. There is, however, as yet no satisfactory yellow
pigment known that will withstand the _grand feu_. At the best we can
get a straw colour from certain ores of tungsten and titanium, and from
uranium a yellow deeper in tint but uncertain in application.
The majority of the colours we have mentioned require a more or less
oxidising flame for their full development. There are, however, two most
important groups of coloured glazes, long the monopoly of the Chinese,
but now successfully imitated in France and elsewhere, which require,
for a term at least, to be subjected to a reducing flame.
The first of these glazes is the well-known CELADON, using that term in
its proper and restricted sense, for certain shades of greyish green.
The celadon of the Chinese is produced by the presence of a small
quantity, about two per cent., of protoxide of iron in the glaze. An
oxidising flame would change this protoxide to the yellow sesqui-oxide.
We may note that a celadon of good tint can only be produced when a
considerable quantity of lime is present in the glaze.
The other group, depending also upon a reducing flame, is constituted by
the famous SANG DE BŒUF and FLAMBÉ glazes.
The colour of the first is given by the red sub-oxide of copper, chiefly
suspended in the glaze. In the case of the _flambé_ or ‘transmutation’
glazes, the strange caprices of colour have their origin, in part at
least, in the contrast of the red sub-oxide and the green silicate of
copper. In the case of both these glazes everything depends on the
regulation of the draught of the furnace in which they are fired. The
French have lately been at great pains to master the difficulties
attendant upon the development of the effects sought after, and some
success has been attained not only on a porcelain ground as at Sèvres,
but these glazes have also been applied to fayence at the Golfe St. Juan
and elsewhere. It has been proved by some experiments made at Sèvres,
that in the firing, the critical period, during which so much depends
upon the regulation of the draught, is _just before_ the melting of the
glaze. Once melted the glaze not only forms an impervious cover which
prevents the smoky flame from discolouring the paste below, but the
glaze itself is no longer sensitive to the action of the gases which
surround it. It is therefore only during a short period preceding the
moment when the glaze begins to melt, that it is necessary to promote a
smoky and reducing flame. This is a point of considerable practical
importance.[20]
The application of the DECORATION UNDER THE GLAZE is essentially a
Chinese method. To it we owe the important family of ‘blue and white’
ware. The superiority of the Chinese in the management of the blue
colour has been attributed to various causes. The result is no doubt
influenced not only by the constitution of both paste and glaze, but
also by the fact that the colour is painted upon the _raw_ paste.
An important factor also is the care exercised by the Chinese in the
selection and preparation of the blue pigment, by which not only the
desired intensity but the richness of hue is secured. The quality of the
blue depends in great measure upon the presence of a small quantity of
manganese in the cobalt ore employed.
The only other colour that the Chinese have succeeded in using under the
glaze is the red derived from the sub-oxide of copper. The full
development of this colour has for long been a lost art, but a less
brilliant red from this source, often little better than a buff colour,
is sometimes found in later examples combined with the blue.
In the application of colours under the glaze there is one difficulty
that the Chinese have surmounted even in their commonest ware, and this
is the tendency of the cobalt blue to dissolve and ‘run’ in the glaze,
giving to the design a blurred and indistinct appearance. It would seem
that the sharpness of outline depends upon the consistency of the glaze
at the moment when it first melts. At that point the glaze should be
viscous and not inclined to flow, and this is what occurs in the case
of the highly calcareous glazes of the Chinese.
Before passing to the enamel colours, we must say something of a class
of glazes which may be looked upon as to some extent of an intermediate
character. These are the glazes associated with the ‘San tsai,’ the
‘three colours’ first used in combination by the Chinese.
These coloured glazes were applied, not, as is usually the case in
China, to the raw paste, but they were, it would seem, painted on the
surface after a preliminary firing. Being applied with a brush, the
whole surface of the biscuit was not necessarily covered, and glazes of
all these colours could be used upon the same piece of porcelain. Glazes
of this class were rendered more fusible by the addition of a certain
quantity of lead, and on this ground, and still more in their historical
relation, as we shall see later on, these ‘painted glazes’ may be
considered as a link connecting the old refractory glazes of the
monochrome and ‘blue and white’ wares on the one hand, with the fusible
enamels which were at a later time _superimposed_ upon the glaze on the
other.
The three colours which are applied in this way by the Chinese are: (1)
A turquoise blue derived from copper with the addition of some soda or
potash. (2) The manganese purple, often described as aubergine. (3) A
yellow prepared from an iron ore containing some amount of antimony.
None of these colours would stand the full heat of the furnace, and for
a reason which will be explained further on, they are known as the
colours of the _demi grand feu_.[21]
COLOURED ENAMELS. We have now to describe
[Illustration: _PLATE II._ CHINESE MING PORCELAIN, BLACK GROUND]
the decoration that is applied to the surface of the glaze. In these
coloured enamels the colouring matter is dissolved in a flux which
contains a large quantity of lead. The comparatively gentle heat at
which such enamels fuse allows of the use of a much larger palette than
is available for the decoration under the glaze.
It is well to point out at the outset the marked distinction in
composition and in appearance between the brilliant enamels of the
Chinese and the dull tints of the ‘porcelain colours’ found in the hard
pastes of Meissen and Sèvres. To make clear the cause of this difference
it will be necessary to enter into some little detail.
The colouring matter in the European enamels may amount to as much as a
third part of the total amount of the flux with which they are
incorporated. As there is not enough of this flux to dissolve the whole
of the oxides, the enamel remains dull and opaque after firing. The
flux, in fact, is only used as a vehicle to attach the colour to the
surface of the porcelain. The effect in consequence is inferior in
brilliancy to that obtained by the Chinese with their transparent
enamels in which the metallic oxides, present in much smaller quantity,
are thoroughly dissolved to form a glass. There is, unfortunately, a
practical obstacle to the application of these glassy enamels to the
hard pastes and glazes of Europe. It is impossible to ensure their firm
adhesion to the subjacent glaze. The Chinese, however, do not appear to
find any difficulty in effecting this. The following explanation has
been given to account for the difference of behaviour:–the tendency of
the enamel to split off in cooling, as has been proved by experiment,
arises from the small amount of contraction at that time of the highly
kaolinic paste, compared with that of the superimposed glassy enamel.
The more silicious paste used by the Chinese contracts, on the contrary,
at the same rate approximately as the enamels that it carries, and
these enamels may therefore be laid on in sufficient thickness without
any risk of their subsequently splitting off.[22] To appreciate the
difference in the decorative value of these two classes of enamels it is
only necessary to compare the brilliant effect, say, of a piece of
Chinese egg-shell of the time of Kien-lung with the tame surface of a
contemporary Meissen plate, elaborately painted with landscapes or
flowers.
The glassy enamels used by the Chinese resemble the pastes used for
artificial jewellery. They are essentially silicates of lead and an
alkali. The composition of the flux has to be modified to ensure the
full development of the colour of the different metallic oxides which
are either made up with it or added subsequently. But in a general way
we may say that the colourless fluxes which form the basis of the
coloured enamels are prepared by melting in a crucible a mixture of pure
quartz sand and red lead, and adding more or less alkali. In certain
cases the lead predominates, as when it is proposed to make an emerald
green enamel by means of copper, or when the flux is to serve as a basis
for the ruby colour given by a minute quantity of gold. On the other
hand, if copper be added to a flux containing an excess of either soda
or potash, we obtain a turquoise blue. A fine purple, again, can only be
obtained from manganese with an alkaline flux; if too much lead is
present only a brown tint is obtainable.
To melt these enamels and to ensure their adherence to the subjacent
glaze another firing at a gentler temperature is necessary; indeed in
many cases more than one such firing has to be resorted to. The
comparatively high temperature required to develop the colour of one
enamel may be sufficient to decompose or otherwise damage another part
of the decoration. The lowest temperature of all is that of the
muffle-fire in which the gilding is fixed. This is therefore the last
decoration to be added.
The oven in which these enamels are melted on to the surface of the
already glazed porcelain is called a muffle. The ware in this case is
protected from the direct action of the flame by the closed rectangular
box of fireclay in which it is placed, like bread in a baker’s oven. The
muffle is placed over the fireplace of a rectangular furnace, and the
flame plays round the sides in such a way as to ensure the uniform
distribution of the heat. For the sake of greater cleanliness and the
avoidance of dust, the pieces to be fired are placed upon tiles of
porcelain rather than upon biscuit or fireclay supports. The temperature
may vary from a dull to a full red heat (600° to 1000°C.), and the
firing lasts from four to twelve hours.
We have already mentioned incidentally many of the so-called
‘muffle-colours’ or enamels. Those used in China were carefully studied
some years ago by Ebelmen and Salvétat at Sèvres. It would appear that
the opaque white of the Chinese is obtained from arsenic–the merits of
the use of tin for this purpose appear to be unknown to them. The blacks
are made from the already mentioned cobalt-manganese ore (wad), mixed
with white lead–when oxide of copper is added a more lustrous black is
obtained.[23] For the blue enamel, a very small quantity of cobalt
suffices to give a brilliant colour. The various tints of the greens and
blues derived from copper depend on the nature of the flux; of this we
have already given an instance. Antimony in combination with lead gives
a bright yellow, which tends to orange when a little iron is present; by
the addition of more iron the colour of old bronze is imitated. Iron in
the state of the sesqui-oxide is the source of many shades of red, but
as this iron oxide will not readily combine with silica to form a
transparent glass, it has to be applied as a more or less opaque paint,
and thus differs from the other colours in being in perceptible relief.
Hence the importance of the ruby red derived from gold, which was first
introduced into China in the early part of the eighteenth century, and
soon became the predominating colour in the decoration of the time (the
_famille rose_).
The palette of the European enameller is a more extensive one, and each
large porcelain manufactory has its book of recipes. The composition of
the enamels and the relation of the metallic oxides to the fluxes
employed have been systematically studied in more than one laboratory.
It is only at Sèvres, however, that the results obtained have been made
public. It has been the pride of successive generations of chemists–of
Brongniart, of Salvétat, of Ebelmen, not to mention living men–to
devise fresh sources of colour for the decoration of porcelain. First
chromium, then nickel, cadmium, uranium, iridium, and platinum have been
added to the list of metals from which enamel pigments have been
derived. Among the colours of the muffle-stove the chief gain has
perhaps been the discovery of the quality possessed by the oxide of zinc
of altering the tints of other metallic oxides with which it is mixed.
