[Illustration: 891]
A, _fig._ 891., is the profile of the English potter’s lathe, for
blocking out round ware; C is the table or tray; _a_ is the head of the
lathe, with its horizontal disc; _a_, _b_, is the upright shaft of the
head; _d_, pulleys with several grooves of different diameters, fixed
upon the shaft, for receiving the driving-cord or band; _k_ is a bench
upon which the workman sits astride; _e_, the treadle foot-board; _l_,
is a ledge-board, for catching the shavings of clay which fly off from
the lathe; _h_ is an instrument, with a slide-nut _i_, for measuring the
objects in the blocking out; _c_ is the fly-wheel with its winch-handle
_r_, turned by an assistant; the sole-frame is secured in its place by
the heavy stone _p_; _f_ is the oblong guide-pulley, having also several
grooves for converting the vertical movement of the fly-wheel into the
horizontal movement of the head of the lathe.
D is one of the intermediate forms given by the potter to the ball of
clay, as it revolves upon the head of the lathe.
In large potteries, the whole of the lathes, both for throwing and
turning, are put in motion by a steam-engine. The vertical spindle of
the lathe has a bevel wheel on it, which works in another bevel toothed
wheel fixed to a horizontal shaft. This shaft is provided with a long
conical wooden drum, from which a strap ascends to a similar conical
drum on the main lying shaft. The apex of the one cone corresponds to
the base of the other, which allows the strap to retain the same degree
of tension (see the conical drum apparatus of the _Stearine-press_),
while it is made to traverse horizontally, in order to vary the speed of
the lathe at pleasure. When the belt is at the base of the driving-cone,
it works near the vortex of the driven one, so as to give a maximum
velocity to the lathe, and _vice versâ_.
During the throwing of any article, a separate mechanism is conducted by
a boy, which makes the strap move parallel to itself along these conical
drums, and nicely regulates the speed of the lathe. When the strap runs
at the middle of the cones, the velocity of each shaft is equal. By this
elegant contrivance of parallel cones reversed, the velocity rises
gradually to its maximum, and returns to its minimum or slower motion
when the workman is about finishing the article thrown. The strap is
then transferred to a pair of loose pulleys, and the lathe stops. The
vessel is now cut off at the base with a small wire; is dried, turned on
a power lathe, and polished as above described.
The same degree of dryness which admits of the clay being turned on the
lathe, also suits for fixing on the handles and other appendages to the
vessels. The parts to be attached being previously prepared, are joined
to the circular work by means of a thin paste which the workmen call
_slip_, and the seams are then smoothed off with a wet sponge. They are
now taken to a stove-room heated to 80° or 90° F., and fitted up with a
great many shelves. When they are fully dried, they are smoothed over
with a small bundle of hemp, if the articles be fine, and are then ready
for the kiln, which is to convert the tender clay into the hard
_biscuit_.
A great variety of pottery wares, however, cannot be fashioned on the
lathe, as they are not of a circular form. These are made by two
different methods, the one called _press-work_, and the other _casting_.
The press-work is done in moulds made of Paris plaster, the one half of
the pattern being formed in the one side of the mould, and the other
half on the other side: these moulding-pieces fit accurately together.
All vessels of an oval form, and such as have flat sides, are made in
this way. Handles of tea-pots, and fluted solid rods of various shapes,
are formed by pressure also; viz., by squeezing the dough contained in a
pump-barrel through different shaped orifices at its bottom, by working
a screw applied to the piston-rod. The worm-shaped dough, as it issues,
is cut to proper lengths, and bent into the desired form. Tubes may be
also made on the same pressure principle, only a tubular opening must be
provided in the bottom plate of the clay-forcing pump. The other method
of fashioning earthenware articles is called _casting_, and is, perhaps,
the most elegant for such as have an irregular shape. This operation
consists in pouring the clay, in the state of pap or slip, into plaster
moulds, which are kept in a desiccated state. These moulds, as well as
the pressure ones, are made in halves, which nicely correspond together.
The slip is poured in till the cavity is quite full, and is left in the
mould for a certain time, more or less, according to the intended
thickness of the vessel. The absorbent power of the plaster soon
abstracts the water, and makes the coat of clay in contact with it quite
doughy and stiff, so that the part still liquid being poured out, a
hollow shape remains, which when removed from the mould constitutes the
half of the vessel, bearing externally the exact impress of the mould.
The thickness of the clay varies with the time that the paste has stood
upon the plaster. These _cast_ articles are dried to the green state,
like the preceding, and then joined accurately with _slip_. Imitations
of flowers and foliage are elegantly executed in this way. This
operation, which is called _furnishing_, requires very delicate and
dexterous manipulation.
The saggers for the unglazed coloured stoneware should be covered inside
with a glaze composed of 12 parts of common salt and 30 of potash, or 6
parts of potash and 14 of salt; which may be mixed with a little of the
common enamel for the glazed pottery saggers. The bottom of each sagger
has some bits of flints sprinkled upon it, which become so adherent
after the first firing as to form a multitude of little prominences for
setting the ware upon, when this does not consist of plates. It is the
duty of the workmen belonging to the glaze kiln to make the saggers
during the intervals of their work; or if there be a relay of hands, the
man who is not firing makes the saggers.
The English kilns differ from those of France and Germany, in their
construction, in the nature of their fuel, and in the high temperature
required to produce a surface sufficiently hard for a perfectly fine
glaze.
When the ware is sufficiently dry, and in sufficient quantity to fill a
kiln, the next process is placing the various articles in the baked
fire-clay vessels, which may be either of a cylindrical or oval shape;
called _gazettes_, Fr.; _kapseln_, Germ. These are from 6 to 8 inches
deep, and from 12 to 18 inches in diameter. When packed full of the dry
ware, they are piled over each other in the kiln. The bottom of the
upper sagger forms the lid of its fellow below; and the junction of the
two is luted with a ring of soft clay applied between them. These dishes
protect the ware from being suddenly and unequally heated, and from
being soiled by the smoke and vapours of the fuel. Each pile of saggers
is called a _bung_.