FIRING BISCUIT
"The Pope, the Cardinals, and the Princes of the World are
astonished that such excellent and noble works can be made out
of the earth."
--EXIMENUS. Fifteenth century.
When sufficient green shapes have been accumulated and are white dry, the
next stage will be _biscuiting_. This process is the firing of the clay to
a primrose or a white heat according to its fusibility. This permanently
expels the water that is always present, even when dry, and converts the
friable clay into a hard unalterable body. This may vary in colour from
the white of kaolin to ivory, grey, buff, red, or brown, according to the
composition of the clay; it may be vitreous or porous; soft like common
flower pots or so hard that it will spark when struck with steel.
The fire is the ultimate test of the pot and of the potter. It is
indispensable to both. With but a small kiln the craftsman will begin to
appreciate many things that can be learned only at the _fire hole_.
Without a kiln he will not commence to be a potter.
[Illustration: FIG. 45--GAS KILN. SECTION SHOWING CLOSE PACKED BISCUIT.]
The kinds of kilns usually found in schools are the gas and the oil kiln.
The English gas kiln has an arrangement of nine or twelve burners beneath
the muffle. (Fig. 45.) This is a fire-clay box, open at the front, set on
fire bricks and cased round with fire tiles within an iron frame (see
cut). There is an air space all round, except at the open end, leading to
the flues on top which have dampers to regulate the draught. The open end
may be closed by a hinged door or bricked up with fire tiles cut to fit.
The defects of this kind of kiln are too sudden access of flame to the
bottom of the muffle, causing it to split, and the impossibility of
getting the front, where trials are usually placed, fired up equally with
the back. An ideal muffle of this kind would be one with flues all round,
gradual access of flame on all sides, spy holes each end, and the top to
lift off, for placing. American kilns have flues in the door, and the
chimney at the top is placed slightly forward, thus making it easy to fire
the front up hard. They are usually fitted with two large burners, with
air mixers, and a handy mica spy hole.
The oil kiln differs considerably from the gas kiln. The oil is fed
through tubes into fire boxes some distance below the bottom of the
muffle. It burns on asbestos fibre in an iron pan to which the draught can
be admitted. The flames strike the bottom of the muffle and pass up
through fire-clay pipes, which project into the muffle, then pass off
through a twin flue regulated by dampers. With this kiln a long flue is
necessary and any excess of carbon is liable to choke the pipes, and
further it cannot be "sweated" up at the finish so easily as a gas kiln
having a large number of burners.
It is a good plan when a coarse fire-clay muffle is used for glaze and
biscuit to give the sides and top a sagger wash of lead and stone. This
renders the muffle less liable to absorb glaze from the pots in glost
firing. It also lessens the danger of small bits scaling off and sticking
to the finished ware. The bottom should always be kept dusted with finely
powdered flint. When cracks appear or joints open, they should be stopped
with a pugging of fire clay and grog. A mixture of egg silica or water
glass with fine grog and quartz sand will stop small cracks. _Siluma_, a
fire-proof cement, with equal parts of sand, answers admirably for
patching.
In biscuit firing the green shapes may be packed close together, with the
lighter shapes on top of the stronger, but all must be 1/2" to 3/4" away
from the sides of the muffle. Triangular pieces of biscuit, called
saddles, are used to raise the shapes off the bottom, but often a fired
tile, sanded and placed on a spur or saddles, gives the best foundation.
Where two layers are required, small props and fire bats, perforated to
let the heat through, will be necessary. These form shelves as the
exigencies of the packing dictate. (Fig. 47.)
For light shapes, thimbles and fired tiles will serve the purpose. (Fig.
48.) Where a shelf or prop rocks insecurely, a small wad of pugging (grog
and clay) will give a steady bearing. Thrown bowls, if dried together and
well fitting, may be fired together, and large thrown pots may be filled
with little ones. Cast shapes can be placed on top of thrown ones, but no
liberty is to be taken with them. Flint should be used liberally to
prevent sticking, which may happen if the biscuit be over-fired. Tiles can
be fired two together in tile boxes or stacked as dried. Flat ware fired
in a small muffle requires very careful handling. Whenever possible, it
should be placed in the centre, on a flat flint-covered fire tile or bat.
One soon learns to pack a biscuit kiln, using saddles, spurs, stilts,
thimbles, bits of tile or biscuit, and sand or flint as necessary. The
thing that is a little difficult to realize at first is that built or
thrown shapes, and still less tiles or modelled work, should not be
hurried. Twelve hours is none too long to give to a 5/8" tile in the
biscuit kiln. Although to all appearances thoroughly dry, the least hurry
generates steam which will ruthlessly blow our best effects to bits. In
packing, two cones or temperature indicators (Fig. 46) are placed
somewhere near the middle in a position easily seen during the firing from
the spy hole.
These cones are made of different compositions which melt at varying
temperatures. Thus if the firing point of a body is known, a cone of that
degree is used and the firing continued until the cone bends. This it does
soon after it assumes the colour of the surrounding muffle.
To eliminate the uncertainty that is likely to be present at the first few
firings it is as well to use two or even three cones, one just above and
one below the correct temperature. Placed in order there is little chance
then of over- or under-firing unless so much sulphur gets into the kiln
that the cones harden and refuse to turn. Calorites are sometimes used but
are not so reliable. The cones may be sloped to insure bending to right or
left, as a cone bending towards the spy is deceptive.
[Illustration: FIG. 46--CONES.]
A trial piece of biscuit being placed near the spy hole, the next thing is
to close the muffle. In a kiln with hinged doors the spy hole is fixed and
this fact has to be taken into account. But with a bricked-up door the spy
and vent may be left where it is most convenient. These orifices have
plugs that fit them loosely so that if necessary they can be pulled
without disturbing the clamming. This clamming or stopping is a mixture of
sand, sieved dust, ground pitchers, or other infusible siftings held
together with a very little waste glaze and water. Where much is required,
moist sand will suffice. This is plastered into the cracks that would
otherwise let heat out of, or air into, the muffle, and so seals up the
door. With hinged doors little stopping is required, except round the spy
or vent. The crack above the door should not be clammed until the muffle
begins to get warm. With this done and the two plugs out, all is now ready
to light up.
With an English kiln, a good middle course is as follows: For cone .01,
taking 12-14 hours. See that all the burner taps are off with the main
cock on one third to one half. Take the reading of the metre. Pull the air
regulators right back and the dampers out nearly half. Then take out the
plug of the lighting hole and insert a taper. Turn on tap number 1. When
lit, withdraw the taper and turn on tap number 2. Turn down to about one
half and continue until every burner is lit, making sure that each one is
burning freely with a yellow flame. With a kiln having twelve burners turn
out all but numbers 3, 6, and 9. With these on one third, very gently push
forward the air regulators until a roaring noise tells that air is being
admitted to the bunsen burners.
The flame at this time should be blue, and the stopper should be replaced.
If the flame appears at all fierce, turn the taps down a little. If
turned down too much, the gas lights back and will have to be turned out
and relighted. The same thing happens when too much air is admitted. The
burners require watching until the right pressure is known.
[Illustration: FIG. 47--THE USE OF BATS & PROPS.]
For biscuit of any thickness three hours on the three burners is not too
much, the taps being gradually turned on to increase the length of the
flame until at the end of the three hours the taps are at three fourths.
At the end of the first half hour the bottom spy hole is plugged and when
all steam has stopped issuing from the top vent, that is stopped also and
the whole clammed, leaving just a small crack as vent. When three hours
are up, the regulators are pulled back and all burners lit at half cock.
Then all but 2, 5, 7, 9, and 11 are turned out. Starting at the half, they
are gradually increased to three fourths in two hours. Then numbers 1,
2-4, 5-7, 8-10, 11 are lighted in the same way. Colour will begin to show
inside the muffle about the fifth or sixth hour, and the top dampers can
come out a little, the front one more than the back, to draw the flame
towards the door.
[Illustration: FIG. 48--TILE THIMBLES.]
When the eight burners have been on about one and one half hours the
muffle will probably show a dull red inside and all danger of blowing will
be passed. Two more burners at three fourths may now be put on and at the
expiration of another one and one half hours the remaining two may be
turned on. If the pressure weakens and the flame shortens, the main cock
should be turned on. Just above the base of each chimney is a small hole
and the flame should show through these towards the finish. If the flame
flaps out of the chimney tops, it is so much waste and it should be
checked at the taps. The dampers can come right out towards the end, being
used to _sweat_ the flame to the front if the back appears to come up too
fast.
As white heat approaches, the cones should be observed occasionally. They
turn the colour of their surroundings when about to topple over and as the
first begins to curl the trial near the spy can be hooked out quickly and
tested. When the right cone is well down, the main cock and then the
burner taps are turned off. The air regulators are now pulled back and
after a few minutes the dampers are closed. The metre is then checked and
entered in the firing book.
The kiln should be allowed to cool slowly for at least 12 hours, but the
clamming at the top may come away and this will expedite the cooling
without risk to the muffle. This time applies to a muffle of about 20" ×
15" × 30" dimensions. A smaller kiln will fire up and cool in a shorter
time.
Gas kilns as made in America vary in the arrangement of their burners, but
the principle is the same. They are simple to manipulate and especially
handy for firing "on" decoration, as they are fitted with shelves and
uprights. The burners once alight, the flame is gradually increased, but
where the pressure is uncertain, it is well to keep something in reserve.
Firing with oil is somewhat different. The asbestos fibre in the pans is
well saturated with kerosene, the tanks filled, and the taps turned off. A
light is applied to the burners and when both are burning freely the taps
are turned on to allow a thin stream of oil to flow into the pans. From
the merest trickle at first, the flow should be gradually increased as the
heat develops. This is observed through the mica spy hole in the door and
the one above tells when the flame is reaching its maximum. Should it
flare over irregularly before the finish it means that the combustion is
not perfect and there is danger of clogging. The supply of oil should be
reduced and the draught regulated until the flame in the combustion
chamber burns clear.
All soot or carbon forming in the fire box should be raked out and the oil
supply checked, as it indicates a too liberal supply. As the oil in the
tank subsides it should be refilled and the taps checked, as the increase
in pressure is apt to vary the flow.
The later patterns of oil kilns have several advantages over the kiln
described. The muffle construction and the burner arrangements are
ingenious and practical, and need little manipulation to insure even
distribution of heat.
The oil tanks and taps will need attention at each firing, otherwise
sediment will collect and choke the even flow of paraffin oil or kerosene.
With both gas or oil kilns the amount of fuel consumed should be recorded,
together with the time, weather conditions, cones, and results of firing,
in the "Kiln Log."
[Illustration: FIG. 49--SKETCH SEC'^{TN} OF OIL KILN TO SHOW PACKING OF
GLAZED POTS.]