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Pottery Kilns

( Originally Published 1937 )



As A FAMOUS authority has remarked: "...he who finds it impossible to procure or build a kiln had best take to some other craft." I would only add that if one finds it impossible to procure or build a kiln, one should study and follow the Indian methods of firing without one. Let me tell you of a way I saw the Indian potters of Mexico fire their wares.

They heap broken shards loosely together on the ground and level off the top of the heap. On this they pile the pots carefully together and proceed to build a wall of dried cowdung around them, using pieces of a convenient size. But first they ignite a few pieces and distribute them around the first courses of the wall. When they approach the top they gather the wall in and arch it like a bee hive until the pots are quite covered. The fire glows fiercely from within and white smoke pours from the top. They watch carefully and when the pieces are an even red heat, they knock out the walls of dung and let the pots cool. But if they are firing the black ware, they smother them with loose horse dung, and leave them until they have partly cooled inside the smoky atmosphere. The black color is probably caused by two things—the reduction of the red iron oxide (Fe203) in the clay to the black (FeO), and the precipitation of carbon in the pores of the clay from the carbon monoxide (CO) in the smoke.

If you follow the Indian method of firing you must also follow the Indian custom of mixing clays with a large proportion of grit to withstand the rapid changes of temperature in the firing and cooling. The types of clay that we fire so gently in our muffles and saggers over a period of many hours would not at all bear the rapid Indian firing which is up, down and over with before two hours are past.

They obtain grit from local sources found good after generations of use. But you, starting afresh, will have to experiment with materials. "Grog" or crushed burned clay—especially crushed firebrick—is ideal. You can buy it; or pound up old firebricks, or unglazed pottery; or crush unburned clay and fire it in other pots. You will find many natural materials that can be used-very fine sand, crushed sea shells, ground up mica, ground soapstone, and especially the flaky sediment from the decomposition of schists and granites, found in small streams. Put it through a thirty-mesh (30 wires to the inch) sieve. If you lack one of these, shake the material into a pail of water stirring it well and pouring the liquid into another pail before any but the largest particles have settled. After a further settlement of a few minutes in the second pail the cloudy water is poured off. The sediment in this pail should be about the right size.

If the clay mixture you get seems to be too coarse, do not forget that the Indians apply a very fine slip to the surface when the pot is almost dry and burnish it by rubbing with small polished stones. You can make this slip by taking the dry scrapings from your pots, mixing them to a thin cream, allowing most of the grit to settle, and decanting the thin slip into another vessel. Then you may add some Sienna or Indian or light red powder if you wish to make it redder, or umber to make it brown. If you wish to make a lighter colored slip for decorating you may add kaolin, flint or whiting to it.

How TO BUILD A SIMPLE WOOD-BURNING KILN

The kiln illustrated, fig. 13, is used by the Indians of Mexico for burning their low-fired ware.

The fire is kindled on the ground, and the door-way is kept choked up with fuel, leaving plenty of room inside the fire box and just pushing in the burning sticks from time to time with the fresh ones that replace them. If this is not done, cold air rushes in under the top of the door, chills off the pottery on the near side of the kiln, and short-circuits the draft as well. It is an art to fire a kiln with wood, but it can be learned if one watches not only the fire, but the inside of the kiln as well, observing where the flames play and noting whether any parts of the kiln appear darker than others. If they do, it is a sign that cold air or insufficient heat is reaching the spot and the fire must be manipulated accordingly.

A FINER KILN

I designed this kiln for burning wood, but it can also be used for coke or coal. The firing technique has been worked out after a good deal of experience with the kiln and should be followed carefully.

The damper (H) is a plate of cast iron, bolted to a rod to give means of controlling it from below with the handle (K). The fire is started with the damper and draft door (D) nearly closed and fuel is added very little at a time. The fire should be very small and should not be built up to its full size for about five hours. Then the draft door and damper are opened and the fuel introduced more frequently. The wood should be kept piled up in front in (B) and fresh pieces are used to push the burning ones off the pile into the fire over the grate-bars (A). Avoid getting too much fuel on the fire itself. No more should be put on it than it can easily consume at once, and it should be tended every five minutes toward the end of the firing. Small pieces of wood are better than large. Hardwood sawmill slabs (from the outside of the logs) are the best fuel I have found, especially if they are not too heavy.

OIL AND GAS FIRED KILNS

These are sold as complete ready-made units and directions are supplied by the makers. They are easier to operate than kilns burning solid fuel, and are always built as muffle kilns, the ware being loaded into the kiln on shelves set on props. Set the kiln up well away from any wood work and as near the chimney as possible. Avoid turns in the pipe connecting with the chimney. Every turn cuts down the effective draft about 25%. A good high chimney is necessary, especially with oil burning kilns that operate by gravity. Take care not to give these kilns too much oil at the beginning of the firing. Apart from the strain on the kiln, of too sudden a heating, you waste the fuel and form deposits of coke in the tubes. Bulletin 133 by Prof. Paul F. Cox of Iowa State College, along with much useful information on clays and glazes, contains plans for a small gas fired kiln that needs no chimney.

A ceramist can build his own oil fired kiln, firing it with a Hauk kerosene torch (operates like a gasoline torch). The torch is mounted so as to throw its flame against baffle plates that support the floor of the kiln. The floor and the baffles are made of 1" "splits" (firebrick 1" thick). The products of combustion pass right through spaces in the floor or up the sides, and there is neither muffle nor sagger. This type of kiln is quite efficient and needs little draft, but the terrific noise of the torches takes away from the pleasure of working with it.

Estimating the Temperature

Pottery is fired to alter the chemical composition of the clay by driving off the combined water, which ordinary drying does not remove. (Note formulae of kaolin and calcined kaolin on page 111). Once this is driven off, the clay is never softened by water again. The firing also fuses various fluxes and impurities (feldspar, lime, iron oxide, etc.) cementing together the fine particles of clay substance and flint. According to the composition of the body, different degrees of heat are necessary to bring this fusion to the correct point. If carried too far "vitrification" (literally "glassification") results. The firing techniques for porcelain and stoneware are based on bringing the clay body just to the point of complete vitrification and stopping before it starts to sag and melt right down. Pottery is distinguished from porcelain and' stoneware by the fact that the firing is stopped at a point short of complete vitrification leaving the body still somewhat porous.

The point at which to stop the firing may be determined empirically by standing up within sight of the spy hole, several pieces of clay with holes in them presented toward the spy hole. At suitable intervals a piece is speared with a sharp poker and brought out. Being allowed to cool quickly, it is examined for hardness by scratching with a knife, tested for porosity by being applied to the tongue (if it sucks strongly to the tongue it is porous and underfired). When the trial piece indicates that the correct point has been reached, allowing for the fact that clay nearer the fire will get a greater heat, the firing is stopped, doors closed and the kiln allowed to cool.

The glaze firing may be gauged by putting a glazed pot near the spy-hole and watching until the glaze melts. In an open fire kiln, a hole is left in one of the saggers where this method is used.

Most workers use pyrometric cones. These are pointed strips of clay substance, so combined with fluxes, or refractory materials, that they soften and bend over at a series of temperatures approximately 30° C. apart.

They are set in pats of clay, three in a row. A softer (or more fusible) cone (for warning), is placed to the right, and a more refractory one (to guard against over-firing), to the left. They are all given an inclination to the right of about 70°. These pats of cones are placed in sight of spy-holes in different parts of the kiln, and the fire manipulated to make them all curve to the same degree at once.

In the small kiln (fig. 16) the pat of cones is placed on top of the sagger cover right in the center. Only one set is used, and the fireman's judgment is exercised in guiding the firing to make the flames play evenly on all sides of the saggers. The spy-hole is placed in the door opposite to this point, covered with a piece of glass or mica, supported by little angles of sheet metal that are set into the concrete or the joints of the bricks.

Setting the Kiln

In setting the kiln consider that the temperature varies with the distance from the fire. The thick pieces need more heat than thin ones, and should be placed accordingly. Modeled heads or figures need a very gentle firing. For most work, fire the biscuit rather soft and then make the glaze firing high enough to mature the glaze and clay at the same time. Plates, however, are better given a high biscuit firing. During their softer glaze firing, they may be supported on stilts, or thimbles, the latter being the more economical of space. Saggers, shelves and kiln furniture of all kinds should be washed with a slip of equal parts kaolin and flint, to prevent glaze from sticking to them. If the glaze is scraped from the bottoms of pots, they may be fired right on the shelves or bottoms of saggers without the use of stilts. Biscuit pieces may be piled together in the kiln, but glazed pieces must always be separated by about 3/8" to be sure that they do not touch and fuse together.

Saggers or shelves can be made of a mixture of sagger clay or ball clay, kaolin, and crushed fire-brick. Saggers and shelves of silicon carbide are best, but have to be made to order by the manufacturer. The kiln in fig. 16 uses two round saggers, 15" in diameter and 7" and 9" high (outside measurements). The bottom sagger is bedded on "wad" made of clay and sand, at six points about its circumference, leaving room between the wads for the up-ward play of the flames. The upper sagger is covered by a disc made of the sagger-mix. Smaller discs are used inside the saggers for shelves, supported by props of a similar material.



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