GLOST FIRING
"When Fortune bringeth thee affliction, console thyself by
remembering that one day thou must see prosperity, and another
day difficulty."
--From EL KORAN.
The hard porous biscuit shape will now need a coat of glaze and a
subsequent glost fire.
Raw glazes for green shapes are now seldom used except for the coarser
wares or peasant pottery. In commerce it is, however, largely used on
tiles, mouldings, and big sanitary appliances. For common cheap crockery a
soft lead glaze, often galena, is generally used. It is applied with a
brush, or the pot still leathery and tough is dipped in the glaze. Raw
glazes have a strong inclination to leave in the firing. They are very
difficult to manipulate unless they are of the simplest formula and fired
very slowly. The addition of a little clay in the form of slip to the
glaze will often counteract the tendency to leave the pot.
The ordinary glazes in dry powder form are mixed with water to the
consistency of cream and passed with the aid of a stout brush through a
phosphor bronze sieve into a large basin or tub. The sieves may range
from eighty to two hundred mesh, according to the delicacy of the work.
For ordinary work No. 100 suffices. The biscuit to receive the glaze
should be quite clean and free from dust or dirt with the insides
carefully dusted or blown out. Grease will stop absorption, but with
opaque glazes discoloration is not of great importance. If the biscuit is
hard and inclined to be non-porous, the glaze will need to be mixed fairly
thick before it will cling; with soft and porous shapes a comparatively
thin mixture will take readily.
No immutable law can be laid down for the exact thickness of the coat of
glaze. All glazes vary. One sixteenth inch may be enough for one and far
too thin for another. From one twentieth inch for thin transparent glaze
to one eighth inch for matts is a fair average. It is well to err on the
thick side to avoid an impoverished look. Trials on odd pieces of biscuit
fired in horizontal and vertical positions will best settle the point.
In all cases the inside is first half filled with glaze which is rolled
quickly round and out. The deposit is then tested with a knife. The glaze
for the inside should be slightly thinner in composition than for the
outside, as it is inclined to pool in the bottom if too thick. The outside
and the neck are then scraped free of all splashes.
[Illustration: FIG. 50--POURING.]
To glaze the outside of the pot the methods employed are painting,
spraying, dipping, and pouring. In painting, two or three coats applied
with a flat soft brush may be necessary. In spraying, the glaze is thinned
considerably and blown through a vaporizer by means of a foot pump. (Fig.
51.) The shape, its mouth closed with a small bowl or saucer, is placed on
a whirler in a draught box and slowly revolved. The draught draws the
waste spray away from the operator, who should wear a respirator. This
method is excellent where any gradation is required. It is easily learned.
Unfortunately, it requires expensive apparatus to render it safe with lead
glazes. Unless it is done on a big scale and the waste glaze retained, it
is also wasteful.
In dipping, the pot is plunged right under in a tub of glaze which is kept
well stirred to prevent the heavy constituents settling. (Figs. 52 and
53.) This requires considerable dexterity. In a school, glaze is seldom
mixed in sufficient quantities to permit of this being acquired.
Perhaps the most satisfactory method is pouring. (Fig. 50.) The shape is
reversed and stood upon a big stilt or two strips of wood over a bowl or
tub. Then the glaze is poured from a jug round the edge of the base, until
every part is covered. With a little practice a quite even coat can be
thus applied. Owing to the inversion of the shape it is slightly thicker
at the shoulder and neck, an excellent point in glazing. When dry, the
base is scraped and wiped clean with a sponge and the lip touched up with
a brush.
[Illustration: FIG. 51--SPRAYING.]
With matt glazes, as has been noted, a thick coat is necessary. If very
porous, the shape may be soaked first in clean water to take some of the
suction out of the biscuit. When glazed, it is essential that all dips and
inequalities be removed. If thinly coated, the glaze will assume a glossy
surface when fired.
[Illustration: FIG. 52--DIPPING.]
Whatever process is used, the lip and base of a pot should receive careful
attention, the rims especially should be thickly and evenly coated with
glaze. The thickness can be tested with the point of a knife and any
scratches or bare spots should be filled in and rubbed flat as they are
apt to show at the finish if the glaze is at all refractory. Never starve
the glaze, for a thin coat imparts a poor cheap look to any pot that is
seldom remedied with entire success.
When only one kind of glaze is used, the application is quickly learned.
With hard and soft, lead and leadless glazes, both opaque and transparent,
and possibly a combination of spraying, painting, and pouring, the
difficulties are greatly increased. Practical experience will be the only
safe guide. But speaking generally, a sprayed coat can be thicker than a
poured coat. Hard glazes give less trouble than soft if too liberally
applied. Tin glazes will stand a lot of over-firing even when thin and the
reverse holds good of matt glazes. Raw borax glazes require much more
careful firing than raw lead glazes but often give better colour results
and are less liable to sulphuring.
[Illustration: FIG. 53--DIPPING.]
When glazing with a transparent glaze over under-glaze painting a thin
coat is advisable. If it be thick and run, the painting is spoiled; but
if, after firing, it appears thin, another light spray may be tried. But
whole chapters of writing will never settle these points. Repeated trials
on odd pieces of biscuit will elucidate more than many books. In packing a
glost or glaze kiln stilts and spurs instead of saddles must be used to
prevent the pots from sticking to the floor or shelves. (Figs. 49 and 54.)
For the same reason the pots must not touch each other. With glazes
requiring a high temperature the space between pots of different colour
should be considerable. An appreciable amount of glaze is liberated in the
intense heat and a blue pot will often leave a distinct blue patch on any
light pot placed too near. Soft glazes liable to run or drip should be
placed on separate bats well flinted beneath. Any crack in the muffle
should be well stopped and lime may be used to neutralize the effects of
any sulphur that may enter.
[Illustration: FIG. 54--STILTS, BITS AND SPURS.]
As the back is usually hotter than the front the hard glazes should be
packed first, and by selecting suitable shapes a good _setter_ will pack a
surprising amount into even a small kiln. The clamming should be done with
care, as bits are liable to flake off and stick to the glaze. The top plug
only need be left out and the lighting up proceeded with as before.
The gradual increase of heat is very essential in firing glaze, for any
sudden jump or reduction will play all manner of games in a glost oven.
The slow start is not so important, there being no risk of _blowing_.
Indeed the glaze is best in a sharp quick fire.
Although cones should always be used and are excellent indicators,
experience will soon show when a kiln is fired up. It may be advisable to
go on a little after the cone is over until all looks fluxed so as to get
rid of bubbles. These appear in some glazes until the last. They do not
always go down on cooling, but those glazes that bubble through
over-firing should be avoided. Possibly the trouble will arise from
sulphur in the body which must be corrected. Trials are always useful but
they must be small and easily hooked out, as constant poking about at the
spy hole lowers the temperature of the muffle at the front, just where it
is most difficult to get it up.
When fired, a glost kiln should cool very slowly, for any sudden access of
cold air is liable to stunt or crack the pots. When firing over glaze
decoration where the glaze does not run, the packing is much simplified.
With low-firing lustres the pots may even touch each other.
All this has been set out at great length but we shall find over and above
this that each kiln has its own little ways which must be studied and
humoured before the best results can be obtained.
The chief thing to avoid is a sudden flush or jump in the heat. The main
thing to aim at is a slow start steadily increasing to a sharp finish. The
state of the muffle, a dry or a damp day, will modify each firing a
little, but the above generalization will have a fairly wide application
to the working of a small kiln.
Unpacking is a compound of despair and delight, and is best done slowly.
As the colour dies the clamming, if any, may be knocked away, and later on
the spy plugs removed. When comparatively cold, the door may be opened
slightly, and left so for one or two hours. Then the front pots can be
extracted but those at the back should be allowed another half hour. Be
very careful of the razor edges of broken stilts or glaze dips. A steel
chisel should be used for chopping rough edges or removing refractory
stilts. If necessary the bases may be ground on an emery wheel until the
pot stands true.