It is with a comparatively small branch of the art of the potter that we
are concerned in this book. Porcelain or china, in all countries except
the one where it was slowly brought to perfection, has always remained
something of an exotic, and even in China we shall see that it was the
immediate Imperial patronage and the constant demand for the court at
Pekin that brought about the great development of the art under the
present dynasty. In Japan, the first independent country to which the
new art spread, it was under the eye of the greater and smaller feudal
lords, often in the very garden of their palaces, that the kilns were
erected, while the ware produced was reserved for the use of the prince
and his household. Both in China and Japan we shall find the decline of
the art to go hand in hand with the advance of the demand for the
Western market, so that by the beginning of the nineteenth century we
lose all interest in the manufacture.
This dependence upon royal or princely support is equally prominent in
the history of the shortlived porcelain factories of Europe. Their
success or failure has generally followed closely upon the greater or
less interest taken in them by the reigning prince, and few of these
kilns survived the political changes of the end of the eighteenth
century.
No doubt, within the last twenty years or so a certain revival has come
about both in the Far East and in certain European countries, and that
under totally different conditions from those which prevailed in the
eighteenth century. Here and there, at least, the manufacture of
porcelain has come within the sphere of the new impulses that have
brought about such changes in the ‘Arts and Crafts’ at the end of the
nineteenth century.
In its main lines, the history of porcelain is a very simple one. Slowly
developed during the Middle Ages in China, the manufacture became
concentrated at one spot, at King-te-chen, and there reached its highest
development early in the eighteenth century. In Europe, the repeated
attempts to produce a similar ware had about the same time been crowned
with complete success in Saxony; while in England and in France a ware
closely resembling in aspect the Chinese, but softer and more fusible,
had been accepted as an equivalent. Speaking generally, then, we can
make these three statements with regard to the history of porcelain:–
1. That the art had its origin and complete development in China.
2. That it has seldom flourished except under royal or princely
patronage.
3. That porcelain, from the artistic point of view, is essentially a
product of the eighteenth century, and that this statement is true in
the main as regards the country of its origin, though in this latter
case we must make a certain reserve in favour of the earlier wares.
Our subject may seem a simple one compared with some kindred branches of
the industrial arts, such, for example, as the history of glass-making,
or that of cloisonné and other enamels. We come indeed at more than one
time into contact with both these arts, and it is just at these points
that some of our chief difficulties arise. It is in view of such
questions as these, and indeed of many others equally important in the
history of porcelain, that the necessity of a thorough understanding of
the technical and even chemical side of our subject becomes evident. Of
course, if in discussing the different kinds of porcelain we are
concerned only with their merits or demerits as artistic products, we
can put aside these practical questions as ‘beneath the dignity of our
argument.’ But such a treatment of the subject would land us only too
surely in vague generalities and in an arrangement based upon personal
caprice. We require, above all at the start, a firm basis, and this can
only be found in a thorough comprehension not only of the technical
processes that are involved in the manufacture of porcelain, but of the
physical and chemical nature of the substance itself.
But first we need some kind of preliminary definition of what is meant
by the word. Porcelain, then, is distinguished from other fictile wares
by possessing in a pre-eminent degree the following qualities: hardness,
difficult fusibility, translucency, and whiteness of body or paste. Any
specimen of ceramic ware that possesses all these qualities may be
classed as porcelain, and from a practical point of view, the more it
excels under these heads, the better specimen of porcelain it is.
These were the qualities by which the porcelain brought from the East in
the seventeenth century was distinguished from any ware made at that
time in Europe. Our ancestors dwelt especially on the practical
advantages of the hard glaze and the elastic compact paste of the new
ware, which compared favourably with the easily scratched surface and
the crumbly body of the earthenware then in general use.
The greater infusibility that accompanies this hardness was not a point
of much importance to them, but they marvelled at the translucency of
the edges, as of some natural stone, and we find absurdly exaggerated
accounts of the transparency both of the original ware and of the
imitation that they claimed to have made. Finally, they noticed that the
whiteness of the surface was not given by an artificial layer more or
less closely adhering to an earthy base, but was the natural colour of
the paste to which the thin layer of transparent glaze merely gave the
effect of the polish on ivory or on marble. What then was this hard,
white, translucent substance? What wonder if from one end of Europe to
the other, scheming minds–chemists, alchemists, physicians, potters,
and charlatans–were at work trying to make something that should
resemble it? The history of this long search is a very interesting one,
but it would be impossible to explain its failures, its partial failures
(these last resulting in a compromise–soft-paste porcelain), and the
final success of Böttger, without, as it were, going behind the scenes,
and giving some account of porcelain from a modern, scientific point of
view.
And first let us say that, although when treating of porcelain from the
historical and especially from the æsthetic standpoint (and this after
all is our principal business in this book), it is well to take a wide
grasp and include a whole class of china–I mean the soft-paste
ware–which does not come up to our standard of hardness and
infusibility, this is not the case when we are considering the physical,
and especially the chemical, nature of porcelain. By confining
ourselves, for the present, to true hard porcelain, we have the
advantage of dealing with a substance which chemically and physically
may be compared to a definite mineral species. Nay more, we propose here
to confine ourselves to the consideration of the hard pastes used at the
present day in the wares of France and Germany, neglecting for the
present the softer and more irregular porcelain of the Chinese.
First as regards hardness, the surface of the paste of a true porcelain,
when free from glaze, can be scratched by a crystal of quartz, but it is
untouched by the hardest steel. That is to say, it would be classed by
the mineralogist with felspar, and given a hardness of 6 to 6·5 on his
scale.[1]
The freshly broken edge shows a white, perfectly uniform substance, a
glassy or vitreous lustre, a finely granular texture, and a fracture
conchoidal to splintery. When struck, a vessel of porcelain gives a
clear, bell-like note, and in this differs from other kinds of pottery.
When held against the light it allows, where the piece is sufficiently
thin, a certain amount to pass through, but even in the thinnest
splinters porcelain is never transparent.
If a thin section be made of a piece of porcelain, and this be examined
under the microscope by transmitted light, we see, scattered in a clear,
or nearly clear, paste, a vast number of minute, slender rods, and
between them many minute granules (Church’s _English Porcelain_, p. 6).
These belonites and spherulites, as they have been called, doubtless
reflect the light which would otherwise pass through the glassy base in
which they float, and the partial reflection and partial transmission of
the light may not be unconnected with the lustrous fracture so
characteristic of porcelain. Their presence points to the fact that we
are dealing with a more or less definite substance, one which may be
compared to a natural mineral species, and not merely with a semi-fused
clay, something between stoneware and glass. Now when we come to treat
of the chemical constitution of porcelain, we shall find that this view
is confirmed. This structure is developed in the paste by the exposure,
for a considerable period of time, to a temperature of from 1300° to
1500° centigrade, a temperature which is sufficient to reduce all other
kinds of pottery, with the exception of some kinds of stoneware, to a
glassy mass. In the case of porcelain, this great and prolonged heat
allows of a complete rearrangement of the molecules in the softened
mass. The process may be compared to that by which certain minerals and
rocks are formed in the depths of the earth.
We see, then, that not only from the standpoint of history, but on the
basis of the physical properties and intimate constitution of the
material, we are able to draw a sharp line between porcelain and other
fictile wares. This distinction is even more definitely shown by a
chemical analysis.[2]
We are dealing, as in the case of so large a part of the rocks and
minerals of the earth’s surface, with certain silicates of the alkalis
and alkaline earths, with silicates of alumina above all. All natural
clays used for fictile purposes consist essentially of silicates of
various bases, such as alumina, lime, iron, potash, and soda, more or
less intimately combined with water, and with the addition, generally,
of some free silica. If the clay be good in working quality and colour,
the next point the potter has to look to is the question of its
fusibility. It may be said generally that the simpler the constitution
of a silicate, that is the smaller the number of bases that it contains,
the greater will be its resistance to fire. Silicate of alumina is
unaltered at 1500° C., a temperature which may be taken as the maximum
at the command of the potter. The fusing-point is reduced by the
addition of silica, especially if some other bases such as oxide of iron
or lime, or again an alkali, are present even in small quantity. But
beyond a certain point the addition of silica raises the fusing-point,
and it is important to note that it is this excess of silica that
renders certain stonewares and fire-clays so infusible. In the case of
porcelain, on the other hand, the resistance to high temperatures
depends more upon the percentage of alumina present, and the absence or
small amount of other bases. Thus in comparing the composition of
different porcelains, we find that it is those that contain the most
silica that are the most fusible, or rather, to speak more accurately,
that become ‘porcelainised’ at a lower temperature.[3]
The relation of porcelain to stoneware on the one hand, and to ordinary
pottery on the other, will be made clear by the following figures, which
give the composition of stoneware, Meissen porcelain, and of a red
Samian ware:–
Stoneware. Meissen Porcelain. Samian Ware.
Silica, 80 per cent. 58 per cent. 61 per cent.
Alumina, 12 ” 36 ” 21 ”
Potash and Soda, 5 ” 5 ” 5 ”
Lime and Iron, 3 ” 1 ” 13 ”
The refractory stoneware contains a large excess of silica over the
amount required to combine with the alumina and the ‘other bases.’ In
the easily fusible Roman pottery, the ‘other bases’ nearly equal in
amount the alumina, while the Meissen porcelain not only contains less
silica than the pottery, but the ‘other bases’ only amount to a sixth
part of the alumina present.
But it is not enough for the manufacturer to discover a clay of which
the chemical composition corresponds to that of the type of porcelain
which he proposes to make. The question, as an experiment of Brongniart
long ago proved, is more complicated. Brongniart weighed out the
separate constituents for his porcelain–the silica, the alumina, and
the alkalis–and from them he formed his paste. He found, however, that
the paste readily melted at the heat of the porcelain furnace. The
analysis then of any ceramic product can give us but an imperfect clue
to the nature and properties of the ware. We want to know how the
elements are arranged, and this can only be inferred from a knowledge of
the materials employed in the manufacture. I will illustrate this point
by comparing the composition of Meissen porcelain with that of our
Dorsetshire pipe-clay, the most famous of our English clays, but a
material not sufficiently refractory for use in the manufacture of
porcelain. Both substances contain the same amount of alumina–36 per
cent.; in the Poole clay (after removing the water) there is 55 per
cent. of silica and 9 per cent. of ‘other bases,’ against 58 per cent.
and 6 per cent. respectively in the porcelain. The composition,
therefore, of the two bodies is nearly the same: the clay, while it
contains more iron-oxide and lime than the porcelain, is poorer in
silica.
True porcelain has indeed never been made from any other materials than
those so long employed by the Chinese and first described by the
missionary, Père D’Entrecolles, nearly two hundred years ago.
The two essential elements in the composition of porcelain are–(_a_)
The hydrated silicate of alumina, which is provided by the white earthy
clay known as kaolin or china-clay, a substance infusible at the highest
temperature attainable by our furnaces (about 1500° C.); (_b_) The
silicate of alumina and potash (or more rarely soda), that is to say
felspar. But the felspar is generally associated with some amount of
both quartz and mica, and is itself in a more or less disintegrated
condition. This is the substance known as petuntse or china-stone. It is
fusible at the higher temperatures of the porcelain kiln.
Of those substances the first is an immediate product of the weathering
of the felspar contained in granitic rocks; while the second, the
petuntse, is nothing else than the granite (or allied rock) itself in a
more or less weathered condition.
We see, then, that speaking generally, granite is the source of both the
materials whose intimate mixture in the state of the finest comminution
constitutes the paste of porcelain. It thus happens that it is only in
regions of primitive rocks, far away as a rule from centres of industry
and indeed from the usual sources of the clay used for fictile ware,
that the materials essential for making porcelain are found. By the term
granite we mean here a crystalline rock consisting of felspar, quartz,
and mica, and we include in the term gneiss, which differs only in the
arrangement of its constituents. The many varieties of rock that are
named as sources of kaolin and petuntse, such as pegmatite, graphic
granite, or growan-stone, are as a rule varieties of granite[4]
distinguished by containing little or no mica, and above all by the
absence of iron in appreciable quantity. As felspar is also the sole or
at least the principal element in the glaze with which porcelain is
covered, it will be seen that it is the mineral with which we are above
all concerned.
Now, of the three minerals that enter into the constitution of these
granitic rocks (the others are quartz and mica), felspar is the one
most easily acted on by air and water. The carbonic acid which is always
present in the surface-water gradually removes the alkaline constituents
in the form of soluble carbonates, the silicate of alumina which remains
takes up and combines with a certain quantity of water, and in this form
it is washed down into hollows to form the beds of white crumbly clay
known as kaolin. This is, of course, a somewhat general and theoretical
statement of what happens. If we were to examine the actual position and
geological relation to the surrounding rocks of the beds of kaolin in
Cornwall and in the south-west of France, there might be some exceptions
to be made and difficulties to explain. Where, indeed, as in many places
in Cornwall, the kaolinisation has extended to great depth, the
decomposition may have been caused by deep-seated agencies; in such
cases the kaolin is often associated with minerals containing fluorine
and boron.[5]
As for the other constituent of porcelain, the petuntse or china-stone,
we have called it a disintegrated granite, and this is the condition in
which it is usually excavated. It corresponds to the French _cailloux_,
the stony or gravelly material as opposed to the clay. In French works
it is not generally distinguished from felspar, and indeed some
varieties of petuntse may contain little else. However, if pure felspar
is used, the second constituent in granite or in petuntse, I mean
quartz, will have to be added to our porcelain paste in the form of sand
or powdered flint. The third constituent of the china-stone, the mica,
is usually neglected: in many cases the mother rock contains but little,
and what there is is eliminated in the washing. Mica is more fusible
than felspar; the white variety, muscovite, is practically free from
iron, and only from granite rocks containing this variety can petuntse
suitable for the manufacture of porcelain be obtained. The importance of
mica as an element of the Chinese petuntse has only recently been
recognised (Vogt, _Comptes Rendus_, 1890, p. 43). As much as 40 per
cent. of muscovite has been found in samples brought from China. The
pegmatite of the Limoges district, on the other hand, contains only 30
per cent. of this white mica, and of this only a small portion passes
into the paste. We have here, perhaps, the principal cause of the
greater hardness and the higher softening-point of European compared
with Oriental porcelain.
We shall see later on that this softer Chinese paste has many
advantages, especially in its relation to the glaze and the enamels, but
for the present we will continue to take the more ‘severe’ European
porcelain as our type.
Let us consider what takes place during the firing of a paste of this
latter description. After all the water, including that in combination
in the kaolin, has been driven off, we have, as the temperature rises,
an intimate mixture of two silicates, one of which, if heated alone,
would be unaltered by any temperature at our command–this is the
silicate of alumina derived from the kaolin; while the other is a
fusible silicate of alumina and potash. There is also present a certain
amount of free silica. There is reason to believe that at a certain
point a chemical reaction takes place between these constituents,
accompanied by a local rapid rise of temperature in the materials, the
rise being due to this reaction. As a result there is a rearrangement of
the molecules of the mass, although no complete fusion takes place. It
is now, says M. Vernadsky (_Comptes Rendus_, 1890, p. 1377)–we are now
following the account of his experiments–that the sub-crystalline
rods–the baculites of which we have already spoken–are formed. M.
Vernadsky claims to have separated these rods from the glassy base by
means of hydrofluoric acid, in which the former were insoluble. He found
them to consist of a very basic silicate of alumina, containing as much
as 70 per cent. of that earth, while the glassy base was chiefly
composed of silica in combination with the potash and with a small
quantity of alumina. In their optical properties the crystals or
baculites resemble the mineral known as sillimanite, a natural silicate
of alumina.
This is all that scientific research has so far been able to tell us of
the intimate constitution of porcelain; but as far as it goes, it is
evidence in favour of our claim that we are dealing with a definite
substance, _sui generis_, and not merely with a casual mixture of
certain superior kinds of clay, something, as we have said, between
glass and stoneware.
There are certain other elements that enter at times into the
composition of porcelain–magnesia, which may have been added to the
paste in the form either of steatite or magnesite; and lime, derived
either from gypsum or chalk. These additions generally tend to increase
the fusibility of the paste, especially when accompanied by an
additional dose of silica; but as their presence is not essential we are
not concerned with these substances here.
The glazes used for porcelain are as a rule distinguished by their
comparative infusibility and by their containing no lead. The
composition of these glazes follows more or less that of the paste that
they cover, with such modifications, however, as to allow of a somewhat
lower fusing-point: as in the case of the paste, there is a harder and
more refractory, and a softer and more fusible, type. The harder glazes
are composed essentially of felspar, with the addition in most cases of
silica, kaolin, and powdered fragments of porcelain. At Sèvres, a
natural rock, pegmatite, consisting chiefly of felspar, has been melted
to form a glaze without further addition. Of late years, however, the
introduction of a milder type of porcelain has necessitated the use of a
more fusible glaze, containing a considerable quantity of lime, and it
is a glaze of this latter type that has with few exceptions found favour
in other districts where porcelain is made.
We have attempted in this chapter to give some idea of the nature of
porcelain from a physical and chemical point of view, and in doing so
have taken as our type the hard, refractory paste of Europe. When we
come to describe the porcelain of the Chinese, we shall notice some
important divergences from this type. We say nothing here of the
soft-paste porcelains, seeing that so long as we confine ourselves to
the question of chemical composition and physical properties, they lie
entirely outside our definitions. It is only from the point of view of
its history and of its artistic qualities that this group has any claim
to the name of porcelain.
