KILNS
"By many long, laborious, and chargeable experiments he hath found
out."
--Extract from an old potter's patent.
What must be the first representation of a kill, or kiln, is found at Beni
Hassan. It appears to be square in form, and the potter is shown feeding
the fire at the base. In the same illustration he is depicted unpacking or
drawing the ware from the top. The cut from the Greek Hydra gives a very
similar kiln, but a vessel in the museum of Berlin shows one with a
beehive shape.
The kilns left scattered about Europe by the Romans were usually of this
domed kind, circular in plan, with one fire hole. The floor of the kiln
was of pierced slabs, and the flames issuing thence enveloped the ware
piled within and escaped through a vent in the top. The packing and firing
is described in _Ceramic Art in Great Britain_, by L. Jewitt, F.S.A. It
fully explains the trepidation of the old potters, who, before each
firing, were wont to consult the moon and stars and evoke the aid of the
gods. This is happily set forth in Cowper's translation of one of Homer's
epigrams, wherein he expresses the pious hope that if the false potter
"stoops to peep into his furnace, may the fire flash in his face and
scorch it"; a risk often faced by potters, false or true!
The smothering, or reducing, as then practised, was similar to the
_lustring_ methods used in Italy in the sixteenth century, or in the
manufacture of the _blue_ bricks to-day. The Japanese and Chinese built
small kilns in tiers on the side of a hill. Starting with the lowest, the
waste heat was utilized to warm up the kiln above, thus saving time and
fuel. The Chinese used heavy saggars, and specimens of these with portions
of melted pots still adhering to them attest the enormous heats to which
they frequently attained.
Modern kilns subdivide roughly into biscuit, glost, and enamel. The first
is used for firing the green or clay shapes, the second for the hard fire
of the glazed ware, and the last-named for fixing on the added decoration.
Sometimes a kiln is used for the double function of biscuiting and
glazing.
Of modern kilns the one still most widely used approximates to the
bottle-shaped, simple, up-draught kiln. It contains one or two chambers
with hatch for entry, flue or chimney, and anything from three to nine
fire holes. The section of such a kiln is here shown and represents a fair
average up-draught kiln (Fig. 66), variants of which type are working in
most pottery districts to-day. In these kilns the flames rush in at the
fire holes, play on the built-up _bungs_ of saggars, and escape through
the top vent. In a two chamber kiln, as sometimes used for porcelain, the
glaze is put in the lower chamber to receive the hottest fire, the biscuit
in the upper getting a gentle fire. Where the fire enters directly into
the kiln in any large volume, _bags_ or small chimneys are built up inside
the mouths to save the saggars from the worst of the fire. Of late the
single-chamber, down-draught kiln has come into favour, as it is easily
packed and economical of heat. Bags of firebrick protect the saggars from
the roughest fire and direct the flames to the crown of the chamber, from
which point they descend to pass out through flues in the floor of the
kiln. (Fig. 67.) Biscuit ovens are often of this type, either domed or
flat arched.
[Illustration: FIG. 66--UP-DRAUGHT KILN.]
[Illustration: FIG. 67--DOWN-DRAUGHT KILN.]
The most recent innovations are gas and oil kilns. They require
comparatively little manual labour in the firing, are easily regulated,
and the waste heat is frequently used for secondary purposes. Their
freedom from smoke and their economy of labour and money must eventually
render the other types obsolete.
These large kilns are strongly banded with iron supports designed to
prevent too much loosening of the walls when expanded by the great heat. A
very important point is the draining of their site, as large kilns tend
to attract moisture, and the presence of steam in a kiln is productive of
much damage.
[Illustration: FIG. 68--ENAMEL OR MUFFLE KILN.]
Enamel kilns are used for the final processes where the glazed pot is
painted with over-glaze or enamel colours. They are of no great size and
are made of fire-clay slabs or even iron plates when the enamel is soft.
The flames play all round the muffle or fire-clay box during the firing,
but no flame or fume is allowed access to the inside. (Fig. 68.)
In packing a kiln with biscuit or glaze much care, thought, and expedition
have to be exercised. An experienced setter is essential in a factory if
the pots are to have every chance in the fire, for all kilns vary and have
their hot and _cool_ corners. First, the floor is well bedded with quartz
sand or flint that will not vitrify but will provide a good setting for
the bottom saggars. These saggars are fire-clay _boxes_, round or oval,
rarely square, and without lids, in which the pots are placed. Piled one
above another they form the _bungs_, the bottom of one supplying the top
of that beneath. These bungs are built at intervals that permit the flames
to penetrate between them and give a good even fire all over the chamber.
In actual practice some parts of a kiln are hotter than others, and it is
here that a good setter shows his capabilities, setting the thin wares in
the softest places and putting the heavier biscuit or hard glaze in the
hottest corners. With biscuit the setting is not difficult except where
delicate or friable ware may need very careful bedding and propping. With
clean saggars the biscuit may touch the sides, and a competent man will
pile an enormous amount of biscuit into a kiln without risk. In the glost
kiln the ware requires gentle handling and must touch nothing but the spur
or support. The saggars are usually given a saggar wash of lead and stone
to prevent them absorbing glaze from the wares, for a glazed pot placed
too near a raw saggar is very likely to come out with a thin or dry patch.
When the pot is firmly placed on its stilt, a roll of _pugging_ is placed
round the rim of the saggar; this provides a firm bed for the next above
it and also stops the entry of dust and flame. This pugging is made from
clay or marl mixed with ground sherds, sieved gravel, or some non-vitreous
dust to prevent it sticking to the saggars. The plugging is made malleable
with a little water and rolled out by hand or pressed through a die. It
will readily be seen that a carelessly built bung of any height may slip
in the stress of firing, and its fall would most likely involve others,
whilst any slight movement may be sufficient to cause a vase to topple off
its stilt. In a down-draught kiln the bungs over any vent must be raised
on fire tiles to permit the escape of the flames. When all the bungs are
filled up and piled in position, the trials and cones placed, the hatch is
bricked up. Spy holes are left where necessary, and the whole well clammed
to prevent the loss of heat during firing. When fired, the hatch may be
very gradually loosened to accelerate the cooling.
[Illustration: FIG. 69--DOWN-DRAUGHT KILN.]
The fireman's job is one of the most arduous and important of all the
prolonged processes of potting. Coming as it does at the end, it may make
or mar kilns of great value, and it requires more than ordinary judgement
and nerve. The chief troubles arise from bad or irregular draught or
impure fuel. They express themselves in the form of clinkered holes,
excessive smoke, and irregularly fired or sulphured ware. The term
_sulphured_ is applied to pots that come out gloomy or dulled and is said
to be equally due to a reducing fire or an oxidizing one. In the former an
insufficiency of air causes excess of carbon in the kiln and the
absorption of oxygen from the glaze. In the latter an excess of air
(oxygen) allows sulphur vapour, if present, to attach itself to the glaze.
The fire bars, fire holes, and flues must be kept clear and the fuel
carefully selected if these things are to be avoided. The aim, as
previously stated, is a gradually increasing fire, sharper towards the
finish. For this purpose some potters finish off with wood which gives a
long flame free from sulphur and clears the glaze. Even when sulphured, a
clear flame at the finish will remove many ill effects. The baiting should
be fairly heavy at the start, lighter and more frequent towards the
finish, when smoke in any quantity should be avoided.
For temperature gauges, the cones previously described are used and should
be placed in an average place, screened as much as possible from draught
or flame. Small sight pots and glaze rings placed near the spies can be
hooked out and examined and are valuable aids, but their exposed position
and the corrosive action of the flames must be taken into account.
Pyrometers are also used, but with all these aids the experienced fireman
pays due regard to sight and colour.
For a craftsman, the kilns that have been described are unsuitable on
account of their size, if they are not altogether beyond his means. But a
kiln of some sort is indispensable to a potter. The craftsman is nothing
if not inventive. Small trials can be fired in crucibles in an open fire
or on a gas ring. I have heard of some preliminary success with a gas
cooker, discounted later by a frontal attack from the cook. Considerable
knowledge can be gained from small, easily constructed, experimental
kilns.
The small trial kiln shown (Fig. 70) was constructed with a stout old
_crock_, over which was built a core of bricks plastered with marl. The
fire was started at each corner at the bottom, and when well alight, fed
from the top with coke. A small spy at one corner closed with a piece of
biscuit gave some idea of the progress of the firing. This, of course, had
to be practically rebuilt at each firing, but as a makeshift was quite
satisfactory.
[Illustration: FIG. 70--CROCK WITH IMPROVISED SPY HOLE. CONE INSIDE.]
[Illustration: FIG. 70A--CROCK BRICKED OVER & PLASTERED WITH MARL.]
The gas and oil kilns described in Chapter IX are excellent as far as they
go, and indeed the only practicable kind for indoor schools. But their
restricted size soon becomes irksome to a craftsman, whilst the expense of
firing makes only the finest work remunerative. For over-glaze work they
are excellent, but for some reason glazes fired in them seem to lack some
of the richness and maturity the same glazes exhibit when fired in the
slower and more soaking fire of a brick kiln. The dug-out kiln here
depicted (Fig. 71) would be quite suitable for summer schools or for a
craftsman making soft peasant pottery. The section and sketch will
indicate its construction. The materials are hard bricks and stout old
boiler plates, or sheet iron. To pack or unpack, the middle section of the
roof would have to be removed each time, and all glazed pots would need
protection from scalings and gravel from above. The roof will sag at any
big heat, and if of thin iron, will need propping. The firing would be
done with soft coal or wood; a very slow start, with a brisk draught and a
long flame at the finish.
[Illustration: FIG. 71--"DUG-OUT" KILN OF COMMON RED BRICK.]
Of course the front will be hotter than the back, but if saggars can be
obtained, the glaze may be placed in them to the front with the biscuit
protected behind. Clay shapes fired up at the front may bend towards the
fire, and any broken crocks should be used to screen them.
The making of rough saggars is not difficult if a supply of fire clay is
to hand, or clay and grog will serve at a pinch. The clay is rolled out
and the saggar stuck up, much as described on page 32. Every joint must
be carefully welded and the whole thoroughly dried. Then they are fired up
in the kiln, _very_ gently at first, and carried up to a temperature
considerably above that which they will be subjected to when in use.
[Illustration: FIG. 71A--SECTION OF "DUG-OUT" KILN.]
For small trial crucibles ordinary clay mixed with pitchers and powdered
coke proves satisfactory; the coke when fired out renders the body porous
and the heat penetrates more swiftly.
To construct the muffle kiln shown at Fig. 72 the aid of a competent
bricklayer would probably be required. In this kiln glaze and biscuit
would fire up without saggars, but will take rather longer. The plan and
elevation of this particular kiln are given with all reserve. The design
would probably require considerable adjustment and modification before
complete satisfaction was obtained.
[Illustration: FIG. 72--SECTIONS OF MUFFLE KILN TO SHEW SUPPORTS. FLUES.
ETC.]
A down-draught kiln, although more difficult to construct than an
up-draught, is more economical in the end and does its work more evenly.
For the benefit of those that may like to build a small and moderately
cheap kiln drawings are here given. This kiln was built by me when I had
very little practical experience of kilns other than gas, but it served
its purpose well. It was not banded, but this is essential if the kiln is
to stand hard and frequent fire. Firebricks were used for the fireholes,
flues, floor, core, and dome; strong stock bricks for the rest. The glazed
ware, which was fired harder than the biscuit, was saggared in bungs as
usual up to about five feet; the biscuit piled on top and protected by old
saggars and cracked pots from the roughest fire. There were no bags in
this kiln, but the saggars used were very strong and had stood a much
greater heat than that to which they were subjected in this kiln, so that
they showed very little wear or tear after twenty firings. The stack of
sixteen feet gave a good sharp draught, increased if necessary by the
addition of an iron chimney and regulated by an iron damper. A wind screen
or _hovel_ is advisable for rough nights, and some sort of roof is
necessary to protect the crown from the weather. It is important to have
the arch of the hatch very strongly built, as it has to stand a lot of
strain, and an iron support too near the fire soon corrodes and needs
replacing. Another essential is that the site be as dry as possible and
the foundation made solid with concrete; otherwise even a small kiln is
liable to settle and crack. With a little extra expense a kiln of this
kind could be banded round the impost and fireholes, thus considerably
prolonging its life.
[Illustration: FIG. 72A--PLAN SHEWING ARRANGEMENT OF FIRE TILES FORMING
MUFFLE.]
[Illustration: FIG. 72B--MUFFLE KILN BUILT OF FIRE TILES, FIRE BRICKS,
AND COMMON REDS.]
[Illustration: FIG. 73--POKER. SLICE. TONGS.]
In firing this kiln about half a ton of coke and three quarters of a ton
of good hard coal, giving a long flame, was used. Coke for the slow fire
was first started on the bottom and maintained for sixteen or eighteen
hours, lifted up on to the bars for another six or eight hours, when the
saggars would begin to show signs of colour. The coal fire was then
started about the 24th or 25th hour and continued another 18 hours, more
or less, according to the varying conditions, making in all some 40 or 45
hours. This gave a very evenly graduated heat from cone 1 at the base to
cone .03 at the top. Bags were tried experimentally, but whilst giving a
more uniform heat, took much longer to fire up. At the finish of the
firing the fireholes were bricked up, the damper closed when the fires
died down. In about 24 hours the vent and the hatch were eased a little
at the top, and in 48 hours it was pulled down and the drawing commenced.
Packed with the hard glazes at the bottom and the soft at the top this
kiln answered excellently for the purposes for which it was required.
[Illustration: FIG. 74--SAGGAR WITH A POT INSIDE.]