THE MATERIALS: MIXING, FASHIONING, AND FIRING
It would be quite foreign to the scope and object of this book to
attempt to describe in any detail the different processes that come into
play in the manufacture of a piece of porcelain. There is the less cause
for any such detailed treatment, inasmuch as the operations involved in
the preparation of the paste and in the subsequent potting and firing do
not essentially differ in the case of porcelain from those employed in
the manufacture of other classes of pottery. The differences are rather
those of degree--greater care is necessary in the selection of the
materials, and these materials must be more finely ground and more
intimately mixed. Again, the great heat required in the kilns
necessitates, in the firing of porcelain, many precautions that are not
called for in the case of earthenware or fayence. Without, however, some
slight acquaintance with the processes of the manufacture, it would be
impossible to avoid an amateurish and somewhat ‘anecdotal’ treatment of
our subject. There are, indeed, many intimate features, many delicate
shades of difference that distinguish the wares of various times and
places, both in Europe and in the East, which can only be rationally
explained by reference to the details of the manufacture.
At the present day there is only one district in Europe where true
porcelain is manufactured on a large scale. This district lies on the
western and south-western border of the central granitic plateau of
France, especially in the Limousin and in Berry. Again at Sèvres, for
the last hundred years and more, a succession of able chemists has
carried on a series of experiments on the composition and preparation of
porcelain. It is no wonder, then, if we find that the literature
concerned with these practical departments is almost entirely French.
One result of this is a greater richness in technical terms than with
us. We find in France names for the various implements and processes of
the potter’s art, that are something better than the workshop terms of
the local potter. Again, the little that has been written in England
upon the technology of pottery has been concerned chiefly with
earthenware of Staffordshire.[6]
As for the English soft-paste porcelain of the eighteenth century, there
is a remarkable dearth of information both as to its composition and as
to its manufacture. We know in fact in much greater detail how the great
potteries at King-te-chen were carried on at the same period, thanks to
the letters of the Père D’Entrecolles, and to the information collected
in Dr. Bushell’s great work, _Oriental Ceramic Art_ (New York, 1899. I
shall always quote from the text edition).
The following technical notes are based chiefly on the processes in use
either at Sèvres or in the great factories of the Limoges district.[7]
To begin with the Kaolin, the ‘premier’ element in the composition of
porcelain. The greatest care is taken to procure a pure white clay which
should approach as near as possible to the more or less theoretical
mineral kaolinite, _i.e._ to a hydrous silicate of alumina. With this
object the rough china-clay brought from the pit is thrown into a large
tank of water and broken up with wooden spades; the milky liquid is now
decanted into a second tank, leaving behind most of the quartz and the
other stony particles. On its way the soup-like liquid passes through
the meshes of a sieve--these may be formed either of brass wire or
sometimes of finely woven silk. On this sieve all but the finest
particles are retained. The greater part of the kaolin is deposited in
this second tank, but a certain portion still remains suspended in the
liquid, which is again decanted; the remaining kaolin then settles down
in the third tank, yielding the finest clay. To dry this slimy mass, it
is first forced by hydraulic pumps into canvas bags, and these bags are
then pressed between fluted wooden trays, strongly clamped together. We
have now got a white chalky mass which may contain as much as 98 per
cent. of the hydrated silicate of alumina.
The other materials, the china-stone[8] and the quartz, have first to be
reduced to the finest powder. To effect this they may, to begin with, be
roasted to effect disintegration, then crushed in a stone-breaking
machine, and finally passed through the grinding-pan in which they are
ground fine between large blocks of chert which rotate upon a pavement
of the same stone. The finely ground materials have now to be mixed in
suitable proportions either by the old process of ‘slop-blending,’ where
the different ‘slops,’ each of known specific gravity, are run in due
proportion into the big ‘blending ark,’ or, as is now usual in the case
of fine wares, by weighing out the materials in a dry state. On the
relative amounts of the three elements, the china-clay, the china-stone,
and the quartz, the nature of the porcelain after firing will depend. M.
Vogt (_La Porcelaine_, Paris, 1893) gives a useful table showing the
limits within which the materials may be varied. We may note that in the
case of a normal china-stone or petuntse being used instead of felspar,
very little additional quartz is required. These limits are: kaolin, 35
to 65 per cent.; felspar, 20 to 40 per cent.; and quartz, 15 to 25 per
cent. The larger the percentage of the first material, the harder and
more refractory will be the resultant porcelain.
This question of the composition of the paste has been the subject of
many experiments lately at Sèvres. A somewhat animated discussion has
raged around it. M. Vogt, who is the director of the technical
department in the National Porcelain Works, is well qualified to speak
on the subject. We shall not hesitate then to avail ourselves of the
conclusions which he arrives at, the more so as they put tersely some
important points of which we shall see the importance later on. I refer
especially to the relations of the glazes and the coloured decorations
to the subjacent paste.
These are, then, the results that M. Vogt arrives at:--
The two extreme types of porcelain, one with 65 per cent. of kaolin and
the other with only 35 per cent., when taken from the kiln do not differ
in appearance, though one has been subject to a temperature of 1500° C.
to ensure vitrification and the other to only 1350° C. Their physical
properties, however, are very different. The first, rich in alumina
derived from the excess of kaolin, stands without injury variations of
temperature, it suits well with a glaze made from felspar, a glaze hard
enough to resist the point of a knife. These are excellent qualities for
domestic use, but such porcelain does not lend itself well to artistic
decoration. At the high temperature required in this case in the
firing, the colours of the paste and of the glazes assume dull and tame
hues, so as to offer little resource to the artist. In a word, in that
part of the decoration that has to be subjected to the full heat of the
kiln, the artist has command only of a restricted and relatively dull
palette. Again, in the decoration of the muffle-stove the vitrifiable
enamels do not become incorporated with the glaze on which they rest. If
a decoration in opaque or translucent enamels is attempted, these
enamels are apt to split off, carrying with them a part of the glaze. To
sum up: the porcelain of which the hard paste of Sèvres, introduced by
Brogniart, may be regarded as a type, though excellent for domestic use,
is incapable of receiving a brilliant decoration.
Porcelain of the second type, more silicious and less aluminous, is
fired at a lower temperature. In order to get a glaze sufficiently
fusible to melt at such a temperature to a fine uniform surface, it is
necessary to introduce a certain amount of lime into its composition; by
this the glaze is rendered at the same time a little softer. But now the
lower temperature of the fire will allow of a greater variety and
greater brilliancy in the colours either combined with or used under the
glaze. When we come to the muffle-fire we can employ enamels of the
widest range of colour, yielding a brilliant decoration. On the other
hand, this type of porcelain offers less resistance than the other to
the action of hard bodies and to rapid changes of temperature--enough
resistance, however, so M. Vogt thinks, for all ordinary usages. It is
to this type that the porcelain of China, and Japan, as well as the ‘new
porcelain’ of Sèvres belongs. The latter comes nearer to the porcelain
of the East than any other European ware. Finally, M. Vogt points out
that most of the other European porcelains, those made in the Limoges
district, in Germany and in Denmark, are of an intermediate type, and
that they allow the use of either a felspathic or of a calcareous glaze
(Vogt, _La Porcelaine_, pp. 144 _seq._).[9]
To return to our raw materials, which we may now suppose to be weighed
out in a dry state in the required proportions. These are once more
thoroughly mixed with water to form the slip or _barbotine_, which is
again passed through a fine sieve. To remove any particles of iron which
may have come from the machinery or elsewhere, and which if allowed to
remain would form unsightly stains on the finished ware, it is usual to
pass the slip at this stage through a vessel in which a number of
horse-shoe magnets are suspended. In some of the large French factories
a more complicated machine is used for this purpose. The superfluous
water has now to be removed either by evaporation or by pressure between
canvas bags in the manner described above. The paste may then be passed
through a pug-mill to render it uniform in consistency.
A curious question arises with regard to the prepared clay. There was
formerly a widespread idea, which may contain an element of truth, that
instead of handing the clay at once to the potter, it should be kept,
under certain conditions, for a long space of time that it may undergo a
process of ‘aging’ and fermentation. By the ‘aging,’ the working
qualities, especially of a ‘short’ or non-plastic paste (such as that in
use at Sèvres in the eighteenth century, in making the _pâte tendre_),
were doubtless increased, the more so when the clay was at intervals
subjected to fresh kneading and watering. With regard to the long
periods for which the clay was kept by the Chinese, the most exaggerated
statements were formerly made. Mr. William Burton is of opinion that
there may be in some cases an evolution of carbonic acid and
sulphuretted hydrogen when natural plastic clays are used, for these may
contain both vegetable remains and small quantities of iron pyrites. But
the change, he thinks, is chiefly a physical one, due to the settling
down of the mass. Might there not also, I would suggest, be a change of
a more intimate nature, due to the formation of gelatinous silica and
perhaps also of fresh alkaline or other silicates, among these minutely
comminuted particles of various materials now freshly brought together?
We know very little of the conditions that give to natural clays their
peculiar unctuous quality and their plasticity.
We come now to what has been called the ‘shaping’ of the clay, using
that word as an equivalent to the French _façonnage_ to include all the
processes, throwing on the wheel, turning of the lathe, ‘pressing’ and
‘casting,’ by which the desired form is given to the vessel.
The POTTER’S WHEEL, perhaps the most ancient of all mechanical
contrivances, is still largely used in the shaping of porcelain, and
that, too, in a simple form which differs little from that employed
three or four thousand years ago in Egypt,[10] and perhaps for nearly as
long a period in China. From an æsthetic standpoint, the wheel holds the
same relation to the art of the potter as the brush does to that of the
painter. It is perhaps a just cause of reproach against that branch of
the ceramic art with which we are now concerned, that so comparatively
little use is made of the potter’s wheel. Not only in Europe, but for
long ages in China also, the use of the wheel, for many classes of
vessels, has been replaced by various processes of moulding. With us,
but not in the East, a third process, that of ‘casting’ with liquid
slip, is largely used. But when made either by casting or moulding, the
hand of the potter is not seen in the shape of the finished vessel. By
means of the wheel alone do we get the full expression of the peculiar
qualities of a plastic material. This was recognised by the Greeks, when
the potter who made the vase signed his name by the side of the painter
who decorated it. This it is that gives a certain charm to the roughest
earthenware which we may look for in vain in the most elaborately
decorated specimen of either Chinese or European porcelain.
The clay as it comes from the filter-presses or from the drying-beds is
subjected to a series of kneading processes to ensure uniformity of
texture. The last of these is the ‘slapping,’ when the clay is made up
into hollow balls, and thrown vigorously on to a board until all bubbles
and irregularities of texture are removed.
The thrower’s wheel is essentially a revolving vertical spindle, with a
small round table at the top, beside which the thrower sits. The clay is
handed to him in balls, and he throws it upon the whirling table between
his knees. The table is put into motion either directly by the pressure
of the workman’s foot on a lower table, or by some arrangement of straps
and pedals. If the movement is given by the potter himself, as is still
the case at Sèvres, and to some extent in China, there is the advantage
that a more delicate and intimate control of the speed is possible. The
movement of the clay under the potter’s hand is instinctively regulated
by him. Every one has seen and marvelled at the wonderful process. The
clay is first drawn up into a pillar, and then depressed into a flat
cake, so that the circular arrangement of the particles may spread
through the whole mass. The thrower then opens the hollow of the vessel
with his thumbs, and proceeds to give it the desired shape, moistening
his hands at intervals by dipping them into the slip. Small pieces are
shaped between the thumb and first finger, either of one or of both
hands. For larger pieces the whole hand and wrist is called into play,
with the assistance, it may be, of a sponge. Still larger vessels are
built up by piling on to the circular edge as it revolves strips of the
clay. Delicacy of hand is of the greatest importance--the pressure
applied and the movements of the fingers must be regulated by the nature
of the clay, and especially by its greater or lesser plasticity. It is
essential that the workman should not only press evenly and steadily on
the clay as it rises, but that the speed of the rotation should have a
definite relation to the rate at which he raises his hands. With a ‘fat’
or unctuous clay especially any irregularity of pressure will betray
itself, and the marks will be more prominent after firing. This is the
origin of the spiral ridges that we often see on the surface not only of
common earthenware, but sometimes of high-class porcelain. To this cause
are due the rings so characteristic of Plymouth porcelain; this
‘wreathing’ or ‘_vissage_’ is sometimes seen on Chinese porcelain also.
When the thrower has finished his vessel, it is cut off from the table
by a piece of thread or by a brass wire, and taken to the stoveroom to
dry and harden. When sufficiently dry the vessel is placed on a lathe,
and the turner shaves off all superfluous clay. The finer mouldings
(using the word here in its architectural sense) may also be given at
this stage, and sometimes the surface is shaped by a ‘profile’ of steel
(it may be a piece from the blade of an old saw), which cuts the
surface down to the desired shape. The shavings are carefully preserved
and returned to the slip-house, to be blended with the new clay, the
working qualities of which are thereby improved.
There are certain parts, especially handles, spouts, and projecting
ornaments, which must in all cases be separately moulded. The foot also,
in the case of large vases, is separately prepared and subsequently
attached. These parts are made in plaster moulds by the ‘handler,’ whose
duty it now is to fix them to the vase. Carefully marking the exact
place, he spreads on it a thin layer of slip with a spatula, and then
presses home the handle or other appendage. Should, however, the two
surfaces be dry and absorbent, it may be necessary to add some gum to
the slip thus employed. A similar process, but one requiring greater
care and skill, is that of fixing together the separate pieces of large
vases and figures. This is done in the way we have already described in
the case of the handles and spouts--that is by applying a coating of
slip to the parts to be joined.
It is at this stage that any decorations in relief that may be required
are applied to the surface. These are often made in flat moulds, and to
fix them it is enough to run a little water from a camel’s hair pencil
behind the ornament after adjusting it to its proper place. These
processes of fitting on of appendages and ornaments are included by the
French under the term _garniture_.
MOULDING AND PRESSING.--It is evident that only vessels of a cylindrical
or conical form, or, more exactly, such as have a circular section when
divided horizontally, can be formed on the wheel. To produce any other
form, the vessel must be either shaped directly by the hand or made in
some kind of mould. The use of moulds for pottery is as old, if not
older than that of the wheel. It was in this way that the _Ushabti_
figures of the old Egyptians were made, and many of these date back to
the Early Empire. So in China, the further back we go, the more the use
of moulds seems to have prevailed. I take from the excellent article on
the manufacture of pottery in the _Penny Cyclopædia_ the following
account of the process in use in England at the beginning of the last
century:--
‘The mould is made in two parts, and each is separately filled by laying
in a cake of clay which has been beaten out to the proper thickness on a
wet plaster-block; it is pressed into the mould by repeated blows from a
ball of wet sponge, then squeezed into all the angular parts and
smoothed with sponge, wet leather, and horn. When both sides of the
moulds are thus lined with clay, they are joined together, and the man
lays a roll of clay along the inside of the joining, which he works down
until the whole is smooth and solid.’ The mould is then carried into a
stoveroom, and the plaster here absorbs the moisture so as to release
the clay. The contents are carefully taken out, and the empty mould
returned to the stove previous to being filled again. The seam that
remains on the outside of vessels after fitting the two parts
together[11] is removed by scraping and burnishing with wet horn; the
handles and other appendages are then attached.
This is the process that is called ‘hollow-ware pressing’ or
‘squeezing.’ In ‘flat-ware pressing’ the mould is used to give the shape
to the inside of the vessel only. The mould is placed on the extremity
of the ‘whirler,’ a vertical revolving spindle provided with a circular
table, similar to that of the thrower’s wheel. The plate-maker takes a
cake of clay, which he has previously flattened out with his ‘batter,’
places it on the mould, and presses down with his hand. The upper
surface of the cake of clay (what will ultimately be the bottom of the
plate) is now shaped by an earthenware ‘profile.’ The mould is now taken
off the whirler and at once replaced by another. Flat-ware, especially
when greater finish is required, is also made in a double mould, and the
clay may then be first thrown on the wheel so as to approximate to the
shape required before being placed in the mould.
Processes very similar to the hollow and flat-ware pressing are largely
used by the Chinese. Dr. Bushell has unearthed a passage from a
technical work, written in the time of the Chou dynasty, more than two
thousand years ago, in which a distinction is made between the ordinary
potters who worked with the wheel, and the moulders who made oblong
bowls and sacrificial dishes. In a somewhat later work (19-90 A.D.) the
writer notes both the advantage resulting from regularity of size, and
the obstacles arising from the shrinkage of the parts in firing, when
vessels are made in moulds.[12]
CASTING.--There is yet another process which is largely resorted to in
European works, but which appears to be unknown to the Chinese. It
depends upon the rapidity with which dry plaster of Paris will absorb
the water from a slip of creamy consistency, without allowing any of the
solid particles to pass along with the water absorbed. The slip-mixture
is poured into the plaster mould, which at once absorbs the water,
leaving a uniform deposit upon the surface of the mould. After pouring
or otherwise drawing off the water, a second and thicker slip may be
added so as to form a second layer. The paste of the porcelain so
prepared is likely to be of a lighter and more porous consistency than
when made by throwing or pressing. This process was used in the
eighteenth century at Derby, and doubtless elsewhere, and it was
preferred to moulding for making statuettes. Some account of it is given
by Haslem, a good practical authority, in his _Old Derby China_. For
small objects, ‘casting’ has long been employed in France, and more
lately Ebelmen and Regnault have so improved the process, that vessels
of all shapes and dimensions are made by it. This has been rendered
possible by the introduction of compressed air into the interior of the
vessel, by which means the paste is kept in position until it is
sufficiently dry to support itself. A still better way of doing this is
to exhaust the air _on the outside_, by placing the mould in an
air-pump; the upper part can then be left open, and the whole operation
is under the eye of the workman. M. Vogt (_La Porcelaine_, pp. 157
_seq._) laments that in France the increased use of these mechanical
processes had so reduced the demand for skilful potters, that the race
is nearly extinct.
FIRING AND FURNACES.--So far in our treatment of the operations involved
in the manufacture of porcelain, the same general description has been
applicable, with trifling exceptions, to the processes in use both in
Europe and in the far East, and to soft as well as to hard paste. But
now that we have to describe the firing of the ware, a division into
three classes is necessary:--
1st. The Chinese system. This is the simplest plan. The glaze is applied
at once to the air-dried ware, which is then subjected to but one
firing--that of the ‘_grand feu_.’
2nd. The French system for hard paste. The unglazed vessel is exposed to
a heat varying from dull to full red, generally in the dome over the
main body of the furnace. It is then glazed, and again fired to the full
point required by the paste. This is essentially a French process, and
the preliminary fire is known as the _feu dégourdi_.
3rd. The English system used for bone pastes. In this case it is the
first firing that is the most severe. The ‘biscuit oven,’ therefore, in
which this is effected, must not be confused with the _feu dégourdi_
just mentioned. After dipping, the ware is heated again in the ‘glozing’
or glazing oven, but only to a temperature sufficient to melt the glaze.
In the case of ware decorated with enamel colours over the glaze, there
will be required in all these cases one or more additional firings at
comparatively low temperatures in the muffle-stove.
The furnaces, ovens, or kilns in which porcelain is fired are always of
the reverberatory type; that is to say, the fuel is burned in a separate
chamber or fireplace, and the products of combustion pass over or among
the ware that is being fired. Such furnaces differ on the one hand from
the arrangement in a blast furnace, or that often used in the burning of
bricks, where the fuel is mixed with the material to be heated, and on
the other hand from the muffle-stove, where the object exposed to the
heat is protected from the direct flame by the box of fireclay or iron
in which it is placed.
Kilns of many shapes and sizes have been used for firing porcelain, but
they may most of them be included in one or the other of the following
broad classes.
1st. The old bee-hive ovens of China, the use of which appears to have
been abandoned in that country by the end of the seventeenth century.
These ovens were generally small, in some cases only holding one vase. A
row of them may be heated from one fireplace, and they are then built on
a rising slope. This type has survived to the present day in Japan.
2nd. The oblong horizontal furnaces, often of considerable dimensions,
used during the present dynasty in China. They resemble in section the
ordinary type of reverberatory furnace found in metallurgical works. A
very similar form was long employed at Meissen.
3rd. The large conical furnaces, now in general use in the porcelain
factories of Europe. They may be heated by either direct or by reversed
flame.[13]
In China the fuel is generally pinewood, in billets of uniform size. In
many European kilns wood is still used: birchwood, cut in lengths of
fifteen to twenty inches, is the only fuel used at the present day at
Sèvres. In England, however, the difficulties attendant on the use of
coal appear to have been overcome.
The reader will find in the third volume of Brongniart’s great work
(_Traité des Arts Céramiques_, Paris, 1877) several plates giving plans
and sections of all these types of furnaces. From a careful examination
of these engravings more is to be learned than from any amount of verbal
description. A thorough grasp of the process of firing is of the
greatest assistance in understanding the problems and difficulties that
arise in the manufacture of porcelain, and we shall have to return to
the subject when we come to treat of the several wares.
Whatever differences there may be in the shape of the furnaces, when it
comes to filling the interior with the ware to be baked, there is one
precaution which has been adopted in nearly every country.[14] The ware
must be protected from the direct heat of the flame by means of a case
of fireclay in which it is placed. These are the seggars (French
_cassettes_; the process of filling and arranging them is called
_encastage_), to the preparation of which so important a department has
to be set apart in all porcelain works, and whose manufacture adds so
much to the working expenses.
The seggar proper is a cylindrical pan of fireclay, in shape and size
like a hatbox. They are piled, in the furnace, one over the other, and
these piles or ‘bungs’ are arranged in the furnace so as to allow a
free circulation of the hot gases between them, but otherwise they are
packed as closely together as possible. These seggars may be used
several times over. When broken, the fragments are ground up and mixed
with fresh fireclay or _argile-plastique_ to form new cases--without
this addition the clay would be too plastic or ‘fat’ for the purpose.
The greatest precautions are taken in the packing of the seggars in the
furnace. The giving way of one pile from any inaccuracy in the
arrangement may destroy the contents of the whole oven. So again
infinite care must be taken in the arrangement and support of the
objects in each seggar. The bottom is covered with ground flint or other
infusible material, and the vessel is supported, when necessary, by
various forms of struts, props, or crow-claws, which sometimes leave
their mark on the base or side of the finished object. In spite of these
precautions, a large quantity of defective pieces or ‘wasters’ are
produced in all works, and these are usually cast aside. The finding of
such fragments in after days is sometimes the only proof we have that
porcelain or pottery has formerly been made at the spot. But the proof
is final, for defective pieces and ‘crow-claws’ are not objects likely
to have been imported from a distance. Again, the indelible marks left
on the porcelain, either on the edge which rested directly on the seggar
or at the points where the object was supported by the crow-claws, often
give valuable hints as to the _provenance_ of the piece in question.[15]
In the case of valuable wares these rough edges and marks are removed as
far as possible by grinding on a small wheel, and then polishing the
surface with pumice or with putty.