SHAVING THE BASE.Fig. 36
SHAVING THE SIDES.Fig. 37
Care must be taken with the base. If it is not quite true it should be sandpapered to stand flat. A little experience will soon show when it is in the best state for shaving. If the clay is too soft, the tool jumps, forming ridges, or possibly cuts deeply into the form. If it is too dry, the surface crumbles and the pot is liable to break. When turned in the right leathery condition, the shavings curl off like peel from an apple and all tool marks areeasily removed. A beautiful finish may then be imparted with the leather.
To end the process the shape is reversed in the chuck. Those most important parts, the shoulders and lip, are carefully rounded with a piece of felt and polished with the leather. Properly attended to, this will save much work and disappointment later on. Then with all holes filled up, all ridges, bumps, and sharp edges removed, the shape is signed and put to dry.
For very delicate work in transparent glaze or under-glaze painting, the whole surface may be gone over with very fine sandpaper and polished with the hand. This may be done provided the body is a fine one, for with a coarse body this is apt to leave the surface looking gritty. Some little practice will be necessary in throwing stiff chucks and centring the shape securely, but this once mastered, the method here described will be found to be expeditious and satisfactory for turning shapes.
A proper regard for process points to the desirability of leaving built shapes without a high finish.
Yet they also, if built carefully and stoutly, may be turned down in the above manner. The building up and turning down is somewhat tedious, but it is sometimes the only way by which a craftsman can obtain large shapes. A rather more simple process is to centre the shape upon a whirler andturn down with a sharp wire tool, finishing off lightly with a sponge and soft leather.
To accomplish this satisfactorily the shape must be fairly soft, as there can be no quick spinning motion to enable the tool to cut cleanly as in the chuck or the wheel. When the head of the whirleris of plaster, it should be well soaked in water and the centred shape stuck down with soft clay. The hand holding the cutting wire tool must be held steady at a fixed distance from the centred shape and the pot may be sprayed with water blown through a diffuser from time to time, to keep it moist. Some clays will not, however, stand much re-wetting.
POSITION OF TOOLS WHEN SHAVING.Fig. 38
It should not be necessary to caution the craftsmanagainst angular profiles, splayed feet, or sharp mouldings. Such features are foreign to good pottery, however suited to metal or stone.
With taste and judgement the irregular grooving caused by the tool can be made of high decorative value. In no case should the built shape try to masquerade as a thrown shape. When the whirler is used to shave down built shapes, they may be coiled very thick at the base, thus allowing more rapid work.
The attention must then be concentrated on the profile of the inside. In this way many shapes that splay out or curve boldly from the foot may be built, which would otherwise present many difficulties in coiling.
Tile-Making
“VI thousand and fourscore of pavynge tiles delivered at Hampton Court, for to pave the Kinges new hall at XXVJs. VIIJd. the thousand.”—Sixteenth century memorandum.
—Sixteenth century memorandum.
Tiles may be made of various kinds and sizes, but in every case they show an inclination to buckle in drying and firing. The larger the tile the stronger is this tendency to warp. The clay must not be so rich as that which is used for throwing and should be tempered to counteract the tendency to curl.
A tile box as shown in Fig. 39, or a variant of it, will be required for pressing tiles by hand; for plain tiles5⁄8” deep, for others 11⁄4” deep, the size being calculated to allow for shrinking. The sides hinge and the thumb screws keep it steady on the bench during work. For rough tiles, two strips nailed to the table will serve, the clay being rolled out between and cut in lengths as required.
For plain tiles the wedged and tempered clay is batted out into a slab a full3⁄4” thick. The mould is dusted with French chalk, flint, or very finelysifted clay dust, to prevent sticking, and into it is pressed a piece cut to fit easily in the tile box, from the slab. This is firmly pressed into the box, considerable pressure being used. The surplus is scraped off and the top trued with an iron straight-edge. The knife can be run round the sides, then with the frame reversed a smart tap on the back will release the tile.
TILE FRAME—6-1/2” x 13”.Fig. 39
Let the tiles toughen and then pile in stacks with flat pieces of clay or old tile between each corner and a true biscuit tile at bottom and the top. They can be piled one on the other with sand between or stacked like bricks with alternate holes for theair to circulate. This retards the drying, but in any case they must be dried slowly. With plain glazed tiles a little coarse dust sprinkled over the surface before the tile is removed from the box, and well pressed in, will give a slight interest to the surface and take away any mechanical look. If required for painting, the clay must be very finely sieved and the surface will need careful finishing by hand when dry. With coarse clay, a fine surface can be imparted with a flexible broad palette knife.
For raised outline tiles a frame 11⁄4” deep is required. This allows a plaster bat5⁄8” thick to be placed at the bottom. The best way, perhaps, is to oil the frame and cast the bat in it, removing it when set and sandpapering the upper surface flat and true. The design for the tile drawn the exact size on paper is now traced on this surface. It is far better to sketch it directly onto the slab, but this demands some proficiency if the surface is to be preserved. The design is then incised with a firm sharp point, clay squeezes being taken from time to time to show the progress of the work. The plaster should be wetted to insure easy working. At the finish the design should stand out in a fairly strong and deep line—square—not round or angular in section. (Fig. 40.) A chisel-pointed hard pencil will be found best for finishing. This gives a clear-cut line, not too round. The bat,sponged clean and porous, is then placed in the frame and the tile pressed as before.
A: POINT FOR FIRST OUTLINE. B: DO. FOR FINAL INCISED LINE. C: SECTION OF INCORRECT CLOISSONS. D: CORRECT.Fig. 40
Raised outline tiles can be even more satisfactorily made by means of outlining slip squeezed from a tube or tracer in the same manner that inscriptions are made on sugared cakes, but this requires much practice to obtain good results. When glazing, the coloured glazes are applied to the different compartments with a brush. As the glaze fuses to about one third of its bulk when dry, it should be applied very liberally.
It will be found that large or elaborate designs are to be avoided, as in this process they tend tobecome mechanical and look thin. The old Spanish and Moorish tiles in this style with jewel-like bits of colour are excellent guides and might be studied with advantage.
SLIP TRACER.Fig. 41
The process of making encaustic tiles is a little more complicated. Within the frame is placed a well-lathered or soaped plaster bat (or tile) high enough to allow only a thin layer of clay1⁄8” thick being run out on top. On this when firm the design is traced or pounced. Then with a thin sharp blade it is cut down vertically to the plaster bat, and the clay removed until the whole design shows in white plaster beneath (see illustration 42). The face of the clay left must be preserved carefully, as it forms the surface of the subsequent press.
The bat with clay is now placed on the bottom of the frame to allow a cast5⁄8” thick to be made. The plaster and the sides of the frame are slightly oiledand the plaster mixed and poured in. A soft hair brush will be useful to dislodge the air bubbles that are certain to hide in some of the many odd corners. When nearly set, the surface of the plaster can be scraped flat, and when set taken out of the frame and detached. The clay is picked out and the whole surface of the design cleaned and trimmed so that it will not hold or bind in pressing. This in turn is placed in the frame and a careful press taken. The result is a sunk design into which a different coloured clay is pressed, the tile being first allowed to toughen. The surface is lightly scraped flat and the tile slowly dried. When hard, the face is scraped again with a steel straight-edge, sandpapered, and dusted, when the design appears in two colours. (Fig. 43.)
CLAY LAYER CUT OUT ON BAT & PLASTER DIE.Fig. 42
The most effective clays are fairly siliceous reds,buffs, browns, and greys. When tempered with flint or quartz sand to a uniform degree, they offer a splendid opportunity for counterchange pattern. If a soft clay that contracts considerably is inlaid in a refractory clay, cracks will appear round the edges of the inlay. Thus it is found best to have the body of the tile made of the clay that contracts most.
STRIKING OFF FACE OF TILE IN FRAME—SECTION ON A-B.Fig. 43
Where only a single tile is required a more direct method is possible. The tile is pressed and allowed to toughen slightly, the design being transferred as before. It is then cut round with a sharp knifeand the waste removed with a wire tool. A certain facility of handling is required, for great care must be taken to preserve the edges and angles. This method is, however, productive of much fresh and vigorous work.
Encaustic tiles must necessarily be simple and bold in character, for anything complicated invites confusion; the best possible guides are the encaustic tiles of mediæval times, especially the simple and spirited English and German Gothic.
The most satisfactory way to make modelled tiles or panels is to run out upon a stout board, cross battened to prevent warping, a flat slab of clay of the required thickness. Sketch in the design with a point and model straight away onto the clay. Care must be taken, if the panel is to be fired, to see that all the added work adheres firmly to the background. Where several presses are to be made, the edges of the modelled slab should be carefully trimmed with a bevel, the board oiled, and the mould made directly after the modelling is finished. The mould may be worked on in moderation. Lettering is much more easily incised in the mould than raised in the clay. For tiles needing much sharpness of detail almost the whole can be carved in the plaster. Unless done with great sympathy, however, this leads to a certain harshness and angularity that should be foreign to clay. Where amoulded frame for a panel is required, astrickle, or profile, is cut in zinc. For short use one made from a thin slab of plaster will serve. The strickle is keyed to a straight-edge and dragged over the clay until the correct moulding emerges. This is then cut into lengths and very carefully dried.
The tendency of all transparent glassy glazes to pool in hollows and run off at high points must be borne in mind. With thick matt glazes any delicacy of detail is apt to be lost labour. These problems should be faced before the design is made, as in this process there is a certain quality of surface required by the glaze. On the other hand, in endeavouring to make a good surface for the glaze to enhance, it is easy to slip into the over-round and slimy treatment that characterizes so much modelled pottery.
Drying: Finishing
“There nis no workeman whosoever he beThat can werke bothe welle and hastile.”—“Merchants Tale.”
“There nis no workeman whosoever he beThat can werke bothe welle and hastile.”
“There nis no workeman whosoever he beThat can werke bothe welle and hastile.”
—“Merchants Tale.”
Drying out is quite an important part of pot making. For this a drying cupboard is a necessity. It is easily erected, if the front and sides of wood be backed against a wall. Across the bottom, which should be open, run a row of gas jets protected above by perforated zinc or iron. At the top, which is boarded in, place a small sliding panel to insure a draught. With side brackets and removable shelves it should answer all purposes. A cupboard may be built over a radiator, but here the heat is not so easily regulated.
A thermometer inside the cupboard will be an advantage, for a wet pot straight from the wheel will warp in a warmth that would be quite suited to tough shapes. The green or damp wares should be put on the top shelves and brought nearer the heat as they dry. Large shapes put into the cupboard to dry quickly are very liable to crack acrossthe base. Any flush of heat upon them through any aperture in the shelves will cause them to dry streakily. Turning then becomes difficult or impossible.
Bowls, if not too fragile, may with care be piled one within the other. This helps to retain their shape. Tiles are best stacked in piles dusted with flint or with a piece of clay at each corner between them. Tiles should never be placed in the cupboard until quite dry and straight. Flat platters or dishes require very careful drying to prevent buckling and should be reversed on a piece of sanded glass. When shapes are dried in the open air, they often get hard at the rim before the bottom stiffens. They need reversing to counteract this tendency.
To retard drying, which may often be necessary, a damp-box is needed. A large box, zinc-lined and fitted with plaster slabs, is an excellent device. The plaster must be kept moist with water. A well-tarred box with a close-fitting lid is more easily constructed and will serve most purposes. All work to bestuck upor modelled on should be kept in the damp-box until quite finished and then dried very gradually.
In all kinds of sticking up the body and the addition should be of the same consistency. Re-wetting is dangerous but may be resorted to in moderation with stout thrown shapes. Ornament added when the shape is nearly dry is very likely to leave in thebiscuiting, although apparently quite firm in the green state. Cast shapes dry very rapidly and should be finished before they become white dry. With practice handles, masks, and the like can be affixed in the dry state with slip, but it demands its careful and sparing use. Cracks or holes in dried shapes can with great care be filled, a stiff wedge of clay being firmly pressed in and welded to the slightly moistened sides of the crack. The plaster tools (Fig. 44) will be found very handy for working on the dry clay. With skill and patience much repairing may be done on unfired shapes, but it is a waste of time to attempt it unless the beauty of the piece warrants it. It is far better if the piece be faulty to throw it back into the bin at this stage. Once fired, it is likely to remain an irremediable eyesore.
PLASTER TOOLS.Fig. 44
One or two other points bearing on finish have been mentioned before in previous chapters, but these last touches are so important that they will bear some reiteration.
The finality burned in by the biscuitingshould induce a careful and sound completion of each object; yet paradoxical as it may seem, the less finishing, the better for the piece. It is no part of a craftsman’s work to go finnicking with file and sandpaper; too frequently nothing remains of what might have been a vigorous shape but a meticulous finish. The best Japanese work was superb in this respect and despite the many quaint and surprising shapes into which they fashioned the clay, it very rarely seems to lose its plastic character, it never assumes forms more suited to metal, wood, or stone. Again the character that comes with correct treatment is never smothered. Often the ribs or ridges made by the fingers or the tool in forming the shape are frankly left to contribute their quota to the general effect. The lips are rounded with scrupulous care and angles removed without a suggestion of weakness.
Where large utensils, ewers and the like, are in question, mediæval pottery is rich in suggestion for handles, spouts, and such added forms. It is only when such additions are affixed that one realizes the nicety of adjustment required between the size, shape, and situation of the handle or spout and the vessel to which it is attached. It is here that an appreciation and an intelligent use of historic ornament is necessary.
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.
—Eximenus.Fifteenth century.
When sufficient green shapes have been accumulated and are white dry, the next stage will bebiscuiting. 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 thefire hole. Without a kiln he will not commence to be a potter.
GAS KILN. SECTION SHOWING CLOSE PACKED BISCUIT.Fig. 45
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 equallywith 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 orjoints 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 be1⁄2” to3⁄4” 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 orstacked 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 a5⁄8” 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 somuch 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.
CONES.Fig. 46
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 heldtogether 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 turneddown 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.
THE USE OF BATS & PROPS.Fig. 47
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 hasstopped 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.
TILE THIMBLES.Fig. 48
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 turnedon. 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 tosweatthe 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 handyfor 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 constructionand 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.”
SKETCH SECTION OF OIL KILN TO SHOW PACKING OF GLAZED POTS.Fig. 49
Glost Firing
“When Fortune bringeth thee affliction, console thyself by remembering that one day thou must see prosperity, and another day difficulty.”—FromEl Koran.
—FromEl 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 phosphorbronze 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.
POURING.Fig. 50
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.
SPRAYING.Fig. 51
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.
DIPPING.Fig. 52
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 thana 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.
DIPPING.Fig. 53
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. Softglazes 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.
STILTS, BITS AND SPURS.Fig. 54
As the back is usually hotter than the front the hard glazes should be packed first, and by selecting suitable shapes a goodsetterwill 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. Theslow start is not so important, there being no risk ofblowing. 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 ofthe 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.
Glazes and Lustres