1. _Clay._--The best clay from which the Staffordshire ware is made,
comes from Dorsetshire; and a second quality from Devonshire: but both
are well adapted for working, being refractory in the fire, and becoming
very white when burnt. The clay is cleaned as much as possible by hand,
and freed from loosely adhering stones at the pits where it is dug. In
the factory mounted by Mr. Wedgewood, which may be regarded as a type of
excellence, the clay is cut to pieces, and then kneaded into a pulp with
water, by engines; instead of being broken down with pickaxes, and
worked with water by hand-paddles, in a square pit or water-tank, an old
process, called _blunging_. The clay is now thrown into a cast-iron
cylinder, 20 inches wide, and 4 feet high, or into a cone 2 feet wide at
top, and 6 feet deep, in whose axis an upright shaft revolves, bearing
knives as radii to the shaft. The knives are so arranged, that their
flat sides lie in the plane of a spiral line; so that by the revolution
of the shaft, they not only cut through every thing in their way, but
constantly press the soft contents of the cylinder or cone obliquely
downwards, on the principle of a screw. Another set of knives stands out
motionless at right angles from the inner surface of the cylinder, and
projects nearly to the central shaft, having their edges looking
opposite to the line of motion of the revolving blades. Thus the two
sets of slicing implements, the one active, and the other passive,
operate like shears in cutting the clay into small pieces, while the
active blades, by their spiral form, force the clay in its comminuted
state out at an aperture at the bottom of the cylinder or cone, whence
it is conveyed into a cylindrical vat, to be worked into a pap with
water. This cylinder is tub-shaped, being about 4 times wider than it is
deep. A perpendicular shaft turns also in the axis of this vat, bearing
cross spokes one below another, of which the vertical set on each side
is connected by upright staves, giving the movable arms the appearance
of two or four opposite square paddle-boards revolving with the shaft.
This wooden framework, or large blunger, as it is called, turns round
amidst the water and clay lumps, so as to beat them into a fine pap,
from which the stony and coarse sandy particles separate, and subside to
the bottom. Whenever the pap has acquired a cream-consistenced
uniformity, it is run off through a series of wire, lawn, and silk
sieves, of different degrees of fineness, which are kept in continual
agitation backwards and forward by a crank mechanism; and thus all the
grosser parts are completely separated, and hindered from entering into
the composition of the ware. This clay liquor is set aside in proper
cisterns, and diluted with water to a standard density.
2. But clay alone cannot form a proper material for stoneware, on
account of its great contractility by heat, and the consequent cracking
and splitting in the kiln of the vessels made of it; for which reason, a
siliceous substance incapable of contraction must enter into the body of
pottery. For this purpose, ground flints, called flint-powder by the
potters, is universally preferred. The nodules of flint extracted from
the chalk formation, are washed, heated redhot in a kiln, like that for
burning lime, and thrown in this state into water, by which treatment
they lose their translucency, and become exceeding brittle. They are
then reduced to a coarse powder in a stamping-mill, similar to that for
stamping ores; see METALLURGY. The pieces of flint are laid on a strong
grating, and pass through its meshes whenever they are reduced by the
stamps to a certain state of comminution. This granular matter is now
transferred to the proper flint-mill, which consists of a strong
cylindrical wooden tub, bottomed with flat pieces of massive _chert_, or
hornstone, over which are laid large flat blocks of similar chert, that
are moved round over the others by strong iron or wooden arms projecting
from an upright shaft made to revolve in the axis of the mill-tub.
Sometimes the active blocks are fixed to these cross arms, and thus
carried round over the passive blocks at the bottom. See _infrà_, under
PORCELAIN, figures of the flint and felspar mill. Into this cylindrical
vessel a small stream of water constantly trickles, which facilitates
the grinding motion and action of the stones, and works the flint powder
and water into a species of pap. Near the surface of the water there is
a plughole in the side of the tub, by which the creamy-looking flint
liquor is run off from time to time, to be passed through lawn or silk
sieves, similar to those used for the clay liquor; while the particles
that remain on the sieves are returned into the mill. This pap is also
reduced to a standard density by dilution with water; whence the weight
of dry siliceous earth present, may be deduced from the measure of the
liquor.
The standard clay and flint liquors are now mixed together, in such
proportion by measure, that the flint powder may bear to the dry clay
the ratio of one to five, or occasionally one to six, according to the
richness or plasticity of the clay; and the liquors are intimately
incorporated in a revolving churn, similar to that employed for making
the clay-pap. This mixture is next freed from its excess of water, by
evaporation in oblong stone troughs, called _slip-kilns_, bottomed with
fire-tiles, under which a furnace flue runs. The breadth of this
evaporating trough varies from 2 to 6 feet; its length from 20 to 50;
and its depth from 8 to 12 inches, or more.
By the dissipation of the water, and careful agitation of the pap, an
uniform doughy mass is obtained; which, being taken out of the trough,
is cut into cubical lumps. These are piled in heaps, and left in a damp
cellar for a considerable time; that is, several months, in large
manufactories. Here the dough suffers disintegration, promoted by a kind
of fermentative action, due probably to some vegetable matter in the
water and the clay; for it becomes black, and exhales a fetid odour.
The argillaceous and siliceous particles get disintegrated also by the
action of the water, in such a way that the ware made with old paste is
found to be more homogeneous, finer grained, and not so apt to crack or
to get disfigured in the baking, as the ware made with newer paste.
But this chemical comminution must be aided by mechanical operations;
the first of which is called the potter’s _sloping_ or _wedging_. It
consists in seizing a mass of clay in the hands, and, with a twist of
both at once, tearing it into two pieces, or cutting it with a wire.
These are again slapped together with force, but in a different
direction from that in which they adhered before, and then dashed down
on a board. The mass is once more torn or cut asunder at right angles,
again slapped together, and so worked repeatedly for 20 or 30 times,
which ensures so complete an incorporation of the different parts, that
if the mass had been at first half black and half white clay, it would
now be of a uniform gray colour. A similar effect is produced in some
large establishments by a slicing machine, like that used for cutting
down the clay lumps as they come from the pit.
In the axis of a cast iron cylinder or cone, an upright shaft is made to
revolve, from which the spiral-shaped blades extend, with their edges
placed in the direction of rotation. The pieces of clay subjected to the
action of these knives (with the reaction of fixed ones) are minced to
small morsels, which are forced pell-mell by the screw-like pressure
into an opening of the bottom of the cylinder or cone, from which a
horizontal pipe about 6 inches square proceeds. The dough is made to
issue through this outlet, and is then cut into lengths of about 12
inches. These clay pillars or prisms are thrown back into the cylinder,
and subjected to the same operation again and again, till the lumps have
their particles perfectly blended together. This process may
advantageously precede their being set aside to ripen in a damp cellar.
In France the stoneware dough is not worked in such a machine; but after
being beat with wooden mallets, a practice common also in England, it is
laid down on a clean floor, and a workman is set to tread upon it with
naked feet for a considerable time, walking in a spiral direction from
the centre to the circumference, and from the circumference to the
centre. In Sweden, and also in China (to judge from the Chinese
paintings which represent their manner of making porcelain), the clay is
trodden to a uniform mass by oxen. It is afterwards, in all cases,
kneaded like baker’s dough, by folding back the cake upon itself, and
kneading it out, alternately.
The process of _slapping_ consists in cutting through a large mass with
a wire, lifting up either half in both hands, and casting it down with
great violence on the other; and this violent treatment of the clay is
repeated till every appearance of air-bubbles is removed, for the
smallest remaining vesicle expanding in the kiln, would be apt to cause
blisters or warts upon the ware.
Having thus detailed the preparation of the stoneware paste, we have
next to describe the methods of forming it into articles of various
forms.
_Throwing_ is performed upon a tool called the potter’s lathe. (See
fig., _infrà._) This consists of an upright iron shaft, about the height
of a common table, on the top of which is fixed, by its centre, a
horizontal disc or circular piece of wood, of an area sufficiently great
for the largest stoneware vessel to stand upon. The lower end of the
shaft is pointed, and runs in a conical step, and its collar, a little
below the top-board, being truly turned, is embraced in a socket
attached to the wooden frame of the lathe. The shaft has a pulley fixed
upon it, with grooves for 3 speeds, over which an endless band passes
from a fly-wheel, by whose revolution any desired rapidity of rotation
may be given to the shaft and its top-board. This wheel, when small, may
be placed alongside, as in the turner’s lathe, and then it is driven by
a treadle and crank; or when of larger dimensions, it is turned by the
arms of a labourer. Sometimes, indeed, the wooden plate is replaced by a
large thick disc of Paris plaster, which is whirled round by the hand of
the potter, without the intervention of a pulley and fly-wheel, and
affords sufficient centrifugal power for fashioning small vessels. The
mass of dough to be thrown, is weighed out or gauged by an experienced
hand. The thrower dashes down the lump on the centre of the revolving
board, and dipping his hands frequently in an adjoining tub of water, he
works up the clay into a tall irregular cylinder, and then down into a
cake, alternately, till he has secured the final extrication of
air-bubbles, and then gives the proper form to the vessel under a less
speed of rotation, regulating its dimensions by wooden pegs and gauges.
He now cuts it off at the base with a piece of fine brass wire, fastened
to a handle at either end. The vessel thus rudely fashioned is placed in
a situation where it may dry gradually to a proper point. At a certain
stage of the drying, called the _green state_, it possesses a greater
tenacity than at any other, till it is baked. It is then taken to
another lathe, called the turning lathe, where it is attached by a
little moisture to the vertical face of a wooden chuck, and turned
nicely into its proper shape with a very sharp tool, which also smooths
it. After this it is slightly burnished with a smooth steel surface.