Pottery, for Artists Craftsmen & Teachers Part 6

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 m.u.f.fle. (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 s.p.a.ce 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 m.u.f.fle, causing it to split, and the impossibility of getting the front, where trials are usually placed, fired up equally with the back. An ideal m.u.f.fle 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 m.u.f.fle. It burns on asbestos fibre in an iron pan to which the draught can be admitted. The flames strike the bottom of the m.u.f.fle and pa.s.s up through fire-clay pipes, which project into the m.u.f.fle, then pa.s.s 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 coa.r.s.e fire-clay m.u.f.fle is used for glaze and biscuit to give the sides and top a sagger wash of lead and stone. This renders the m.u.f.fle 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 gla.s.s 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 m.u.f.fle. 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 m.u.f.fle 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 a.s.sumes the colour of the surrounding m.u.f.fle.

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.

[Ill.u.s.tration: FIG. 46--CONES.]

A trial piece of biscuit being placed near the spy hole, the next thing is to close the m.u.f.fle. 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 m.u.f.fle, 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 m.u.f.fle 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 c.o.c.k 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.

[Ill.u.s.tration: 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 c.o.c.k.

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 m.u.f.fle 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.

[Ill.u.s.tration: FIG. 48--TILE THIMBLES.]

When the eight burners have been on about one and one half hours the m.u.f.fle will probably show a dull red inside and all danger of blowing will be pa.s.sed. 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 c.o.c.k 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 c.o.c.k 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 m.u.f.fle. This time applies to a m.u.f.fle 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 m.u.f.fle 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."

[Ill.u.s.tration: FIG. 49--SKETCH SEC'^{TN} OF OIL KILN TO SHOW PACKING OF GLAZED POTS.]

CHAPTER XI

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 coa.r.s.er 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 pa.s.sed 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 sc.r.a.ped free of all splashes.

[Ill.u.s.tration: 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 const.i.tuents settling. (Figs. 52 and 53.) This requires considerable dexterity. In a school, glaze is seldom mixed in sufficient quant.i.ties 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 sc.r.a.ped and wiped clean with a sponge and the lip touched up with a brush.

[Ill.u.s.tration: 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 a.s.sume a glossy surface when fired.

[Ill.u.s.tration: 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.