Porcelain is a kind of pottery ware whose paste is fine grained,
compact, very hard, and faintly translucid; and whose biscuit softens
slightly in the kiln. Its ordinary whiteness cannot form a definite
character, since there are porcelain pastes variously coloured. There
are two species of porcelain, very different in their nature, the
essential properties of which it is of consequence to establish; the one
is called _hard_, and the other _tender_; important distinctions, the
neglect of which has introduced great confusion into many treatises on
this elegant manufacture.
_Hard_ porcelain is essentially composed, first, of a natural clay
containing some silica, infusible, and preserving its whiteness in a
strong heat; this is almost always a true kaolin; secondly, of a flux,
consisting of silica and lime, composing a quartzose felspar rock,
called _pe-tun-tse_. The glaze of this porcelain, likewise earthy,
admits of no metallic substance or alkali.
_Tender_ porcelain, styled also vitreous porcelain, has no relation with
the preceding in its composition; it always consists of a vitreous frit,
rendered opaque and less fusible by the addition of a calcareous or
marly clay. Its glaze is an artificial glass or crystal, into which
silica, alkalis, and lead enter.
This porcelain has a more vitreous biscuit, more transparent, a little
less hard, and less fragile, but much more fusible than that of the hard
porcelain. Its glaze is more glossy, more transparent, a little less
white, much tenderer, and more fusible.
The biscuit of the hard porcelain made at the French national
manufactory of Sèvres is generally composed of a kaolin clay, and of a
decomposed felspar rock; analogous to the china clay of Cornwall, and
Cornish stone. Both of the above French materials come from Saint
Yriex-la-perche, near Limoges.
After many experiments, the following composition has been adopted for
the _service paste_ of the royal manufactory of Sèvres; that is, for all
the ware which is to be glazed: silica, 59; alumina, 35·2; potash, 2·2;
lime, 3·3. The conditions of such a compound are pretty nearly fulfilled
by taking from 63 to 70 of the washed kaolin or china clay, 22 to 15 of
the felspar; nearly 10 of flint powder, and about 5 of chalk. The glaze
is composed solely of solid felspar, calcined, crushed, and then ground
fine at the mill. This rock pretty uniformly consists of silica 73,
alumina 16·2, potash 8·4, and water 0·6.
The kaolin is washed at the pit, and sent in this state to Sèvres, under
the name of _decanted earth_. At the manufactory it is washed and
elutriated with care; and its slip is passed through fine sieves. This
forms the plastic, infusible, and opaque ingredient to which the
substance must be added which gives it a certain degree of fusibility
and semi-transparency. The felspar rock used for this purpose, should
contain neither dark mica nor iron, either as an oxide or sulphuret. It
is calcined to make it crushable, under stamp-pestles driven by
machinery, then ground fine in hornstone mills, as represented in
_figs._ 897, 898, 899, and 900. This pulverulent matter being diffused
through water, is mixed in certain proportions, regulated by its
quality, with the argillaceous _slip_. The mixture is deprived of the
chief part of its water in shallow plaster pans without heat; and the
resulting paste is set aside to ripen, in damp cellars, for many months.
When wanted for use, it is placed in hemispherical pans of plaster,
which absorb the redundant moisture; after which it is divided into
small lumps, and completely dried. It is next pulverized, moistened a
little, and laid on a floor, and trodden upon by a workman marching over
it with bare feet in every direction; the parings and fragments of soft
moulded articles being intermixed, which improve the plasticity of the
whole. When sufficiently tramped, it is made up into masses of the size
of a man’s head, and kept damp till required.
The dough is now in a state fit for the potter’s lathe; but it is much
less plastic than stoneware paste, and is more difficult to fashion into
the various articles; and hence one cause of the higher price of
porcelain.
The round plates and dishes are shaped on plaster moulds; but sometimes
the paste is laid on as a crust, and at others it is turned into shape
on the lathe. When a crust is to be made, a moistened sheep-skin is
spread on a marble table; and over this the dough is extended with a
rolling pin, supported on two guide-rules. The crust is then transferred
over the plaster mould, by lifting it upon the skin; for it wants
tenacity to bear raising by itself. When the piece is to be fashioned on
the lathe, a lump of the dough is thrown on the centre of the horizontal
wooden disc, and turned into form as directed in treating of stoneware,
only it must be left much thicker than in its finished state. After it
dries to a certain degree on the plaster mould, the workman replaces it
on the lathe, by moistening it on its base with a wet sponge, and
finishes its form with an iron tool. A good workman at Sèvres makes no
more than from 15 to 20 porcelain plates in a day; whereas an English
potter, with two boys, makes from 1000 to 1200 plates of stoneware in
the same time. The pieces which are not round, are shaped in plaster
moulds, and finished by hand. When the articles are very large, as
wash-hand basins, salads, &c., a flat cake is spread above a skin on the
marble slab, which is then applied to the mould with the sponge, as for
plates; and they are finished by hand.
The projecting pieces, such as handles, beaks, spouts, and ornaments,
are moulded and adjusted separately; and are cemented to the bodies of
china-ware with slip, or porcelain dough thinned with water. In fact,
the mechanical processes with porcelain and the finer stoneware are
substantially the same; only they require more time and greater nicety.
The least defect in the fabrication, the smallest bit added, an unequal
pressure, the cracks of the moulds, although well repaired, and
seemingly effaced in the clay shape, re-appear after it is baked. The
articles should be allowed to dry very slowly; if hurried but a little,
they are liable to be spoiled. When quite dry, they are taken to the
kiln.
The kiln for hard porcelain at Sèvres, is a kind of tower in two flats,
constructed of fire-bricks; and resembles, in other respects, the
stoneware kiln already figured and described. The fuel is young aspin
wood, very dry, and cleft very small; it is put into the apertures of
the four outside furnaces or fire-mouths, which discharge their flame
into the inside of the kiln; each floor being closed in above, by a dome
pierced with holes. The whole is covered in by a roof with an open
passage, placed at a proper distance from the uppermost dome. There is,
therefore, no chimney proper so called. See STONE, ARTIFICIAL.
The raw pieces are put into the upper floor of the kiln; where they
receive a heat of about the 60th degree of Wedgewood’s pyrometer, and a
commencement of baking, which, without altering their shape, or causing
a perceptible shrinking of their bulk, makes them completely dry, and
gives them sufficient solidity to bear handling. By this preliminary
baking, the clay loses its property of forming a paste with water; and
the pieces become fit for receiving the glazing coat, as they may be
dipped in water without risk of breakage.
The glaze of hard porcelain is a felspar rock: this being ground to a
very fine powder, is worked into a paste with water mingled with a
little vinegar. All the articles are dipped into this milky liquid for
an instant; and as they are very porous, they absorb the water greedily,
whereby a layer of the felspar glaze is deposited on their surface, in a
nearly dry state, as soon as they are lifted out. Glaze-pap is
afterwards applied with a hair brush to the projecting edges, or any
points where it had not taken; and the powder is then removed from the
part on which the article is to stand, lest it should get fixed to its
support in the fire. After these operations it is replaced in the kiln,
to be completely baked.
The articles are put into saggers, like those of fine stoneware; and
this operation is one of the most delicate and expensive in the
manufacture of porcelain. The saggers are made of the plastic or
potter’s clay of Abondant, to which about a third part of cement of
broken saggers has been added.
As the porcelain pieces soften somewhat in the fire, they cannot be set
above each other, even were they free from glaze; for the same reason,
they cannot be baked on tripods, several of them being in one case, as
is done with stoneware. Every piece of porcelain requires a sagger for
itself. They must, moreover, be placed on a perfectly flat surface,
because in softening they would be apt to conform to the irregularities
of a rough one. When therefore any piece, a soup plate for example, is
to be _saggered_, there is laid on the bottom of the case a perfectly
true disc or round cake of stoneware, made of the sagger material, and
it is secured in its place on three small props of a clay-lute,
consisting of potter’s clay mixed with a great deal of sand. When the
cake is carefully levelled, it is moistened, and dusted over with sand,
or coated with a film of fire-clay slip, and the porcelain is carefully
set on it. The sand or fire-clay hinders it from sticking to the cake.
Several small articles may be set on the same cake, provided they do not
touch one another.
The saggers containing the pieces thus arranged, are piled up in the
kiln over each other, in the columnar form, till the whole space be
occupied; leaving very moderate intervals between the columns to favour
the draught of the fires. The whole being arranged with these
precautions, and several others, too minute to be specified here, the
door of the kiln is built up with 3 rows of bricks, leaving merely an
opening 8 inches square, through which there is access to a sagger with
the nearest side cut off. In this sagger are put fragments of porcelain
intended to be withdrawn from time to time, in order to judge of the
progress of the baking. These are called time-pieces or watches
(_montres_). This opening into the watches is closed by a stopper of
stoneware.
The firing begins by throwing into the furnace-mouths some pretty large
pieces of white wood, and the heat is maintained for about 15 hours,
gradually raising it by the addition of a larger quantity of the wood,
till at the end of that period the kiln has a cherry-red colour within.
The heat is now greatly increased by the operation termed _covering the
fire_. Instead of throwing billets vertically into the four furnaces,
there is placed horizontally on the openings of these furnaces, aspin
wood of a sound texture, cleft small, laid in a sloping position. The
brisk and long flame which it yields dips into the tunnels, penetrates
the kiln, and circulates round the sagger-piles. The heat augments
rapidly, and, at the end of 13 or 15 hours of this firing, the interior
of the kiln is so white, that the watches can hardly be distinguished.
The draught, indeed, is so rapid at this time, that one may place his
hand on the slope of the wood without feeling incommoded by the heat.
Every thing is consumed, no small charcoal remains, smoke is no longer
produced, and even the wood-ash is dissipated. It is obvious that the
kiln and the saggers must be composed of a very refractory clay, in
order to resist such a fire. The heat in the Sèvres kilns mounts so high
as the 134th degree of Wedgewood.
At the end of 15 or 20 hours of the great fire; that is, after from 30
to 36 hours’ firing, the porcelain is baked; as is ascertained by taking
out and examining the watches. The kiln is suffered to cool during 3 or
4 days, and is then opened and discharged. The sand strewed on the cakes
to prevent the adhesion of the articles to them, gets attached to their
sole, and is removed by friction with a hard sandstone; an operation
which one woman can perform for a whole kiln in less than 10 days; and
is the last applied to hard porcelain, unless it needs to be returned
into the hot kiln to have some defects repaired.
The materials of fine porcelain are very rare; and there would be no
advantage in making a gray-white porcelain with coarser and somewhat
cheaper materials, for the other sources of expense above detailed, and
which are of most consequence, would still exist; while the porcelain,
losing much of its brightness, would lose the main part of its value.
Its pap or dough, which requires tedious grinding and manipulation, is
also more difficult to work into shapes, in the ratio of 80 to 1,
compared to fine stoneware. Each porcelain plate requires a separate
sagger; so that 12 occupy in the kiln a space sufficient for at least 38
stoneware plates. The temperature of a hard porcelain kiln being very
high, involves a proportionate consumption of fuel and waste of saggers.
With 40 _steres_ (cubic metres) of wood, 12,000 stoneware plates may be
completely fired, both in the biscuit and glaze kilns; while the same
quantity of wood would bake at most only 1000 plates of porcelain.
To these causes of high price, which are constant and essential, we
ought to add the numerous accidents to which porcelain is exposed at
every step of its preparation, and particularly in the kiln; these
accidents damage upwards of one-third of the pieces, and frequently
more, when articles of singular form and large dimensions are
adventured.
The best English porcelain is made from a mixture of the Cornish kaolin
(called china clay), ground flints, ground Cornish stone, and calcined
bones in powder, or bone-ash, besides some other materials, according to
the fancy of the manufacturers. A liquid pap is made with these
materials, compounded in certain proportions, and diluted with water.
The fluid part is then withdrawn by the absorbent action of dry stucco
basins or pans. The dough, brought to a proper stiffness, and perfectly
worked and kneaded on the principles detailed above, is fashioned on the
lathe, by the hands of modellers, or by pressure in moulds. The pieces
are then baked to the state of biscuit in a kiln, being enclosed, of
course, in saggers.
This biscuit has the aspect of white sugar, and being very porous, must
receive a vitreous coating. The glaze consists of ground felspar or
Cornish stone. Into this, diffused in water, along with a little
flint-powder and potash, the biscuit ware is dipped, as already
described, under stoneware. The pieces are then fired in the glaze-kiln,
care being taken, before putting them into their saggers, to remove the
glaze powder from their bottom parts, to prevent their adhesion to the
fire-clay vessel.