Photo from "Craftsmanship in Clay'

Success or failure? It's a suspenseful moment when you open the kiln door.

In the entire field of ceramic art there is no greater thrill for either the amateur or the professional than to open the door of the kiln on a perfectly fired "load" of either bisque or glazed ware.By the same token, there is no greater heart­break than to open the door on a poorly fired "load."

All ware must be absolutely dry before it is fired.The most important item that goes into your kiln is the cone.Pyrometric cones measure heat work due to effect of time, temperature and atmosphere.The degree of maturity in a fired clay body, within limits, is likewise controlled by time, temperature and atmosphere.A cone should not be considered simply as a tem­perature measuring device such as a py­rometer.Important: When cones are heated at different rates of heat rise they have different deformation temperatures.

The cones should be set in place at a slight angle of about 8 degrees to the ver­tical.Ceramic clay may be used to hold cones 022 through 04.Set the cone in a tiny pat of clay so that the number of the cone is not imbedded and the cone is slightly tilted.When clay is used as a cone support, allow it to dry at room tempera­ture before placing the cone in the kiln, or it might explode during the firing.

Using the face of a clock perhaps is the best way to illustrate the degree of tilt and the proper cone position when the firing is completed.When the cone is mounted in the clay, it should tilt at about the one o'clock position.When the cone has bent in the firing to the three o'clock position, your kiln should be turned off.

Your supply house for ceramic materials will be glad to recommend the proper cone number to be used when firing a given material.Always fire ware to the hottest cone first.For example, if you use an 05 body and an 08 glaze—regardless of whether the glaze is suitable for one firing with a body you are not using—you must fire your greenware to an 05 bisque fire to mature the clay before you apply the 08 glaze.

If a clay body is hardened in the kiln before glaze is applied, it is said to have been bisque-fired.When a glazed piece is baked in the kiln it is said to have been glost-fired.There are several distinct dif­ferences between these two types of firing, particularly in the way the kiln is loaded.

Various sizes of shelves, posts and stilts, known as kiln furniture, are necessary for stacking both a bisque and glaze fire.The posts are used to support the shelves.By using taller or shorter posts you will be able to raise, lower or increase the number of shelves in the firing chamber.

All kiln furniture is made of refractory materials which are impervious to the high kiln temperatures.Usually these heat-resistant pieces are cut from the same type of insulating brick used in making your kiln.When exposed to heat, your kiln furniture should not crack, split or melt.

It is a basic law of physics that hot air always rises; cool air always sinks.Thus the lower half of your kiln is always cooler than the top.This makes it necessary, when loading a bisque fire, to place the heavier, thicker pieces on the bottom or cooler area of the kiln.These pieces are more likely to be ruined by sudden changes in tem­perature than are the thinner wares.

Try to arrange the shelves economically in the kiln so that you can fire as much greenware as possible.In a bisque firing, the pieces may touch each other and rest directly on the shelf or floor of the kiln.They should, however, be kept at least one inch away from the heating elements of an electric kiln.It is not a good practice to stack ware on top of or inside other ware, because the weight is liable to shatter the bottom pieces.The ware can be stacked on its side if done with care.If a piece has a cover, such as a cigarette box, it should be fired in place on top of the main body.

All sizes of shelves and supports are used in a kiln, and can be bought to suit your kiln size.

Below is shown how the shelves are supported by posts inside kiln.See photos on following pages.

Square objects in left photo are tile supports; three-pronged metal items are stilts, as are all the other ones, made of ceramic materials.

Generally speaking, sculptured pieces are stacked separately.The chance of such a piece chipping or breaking in the kiln is greater and this may destroy other ware in the kiln.Before it is fired, a piece of sculp­ture should have a minimum drying time of six weeks.Remember it must be hollow or be constructed around an armature which will be oxidized by the kiln heat.Otherwise an uneven distribution of heat through the body of the ware will almost certainly cause it to collapse or, worse, explode.

Sculpture is often considered finished in the bisque state, particularly when ex­ecuted in terra cotta or sculpture clays.Pottery must have a glaze finishing, at least on the inside, to make it usable and easy to clean.Furthermore, the color of many pottery clays is innocuous and unin­teresting as ordinary bisque.The color and texture of glaze are necessary to give it esthetic quality.

Place as much heavy ware on the floor of the kiln as you can, leaving room in the corners for four posts which are about one inch taller than the tallest piece of green-ware.If the kiln is hexagonal, the shelves will be hexagonal or round, and three stilts should be used for support.The posts, of course, should be the same height in order to hold the shelf perfectly level.Gently lower the shelf in place atop the supports.Stack more ware on the shelf.You may stack up as many shelves as you need or space permits.Place the cones on one shelf, where it can be clearly seen through the spy-hole in the front of the kiln.

The ware should be stacked in such a manner that the hot air can circulate freely through the kiln.Passages of a couple of inches should be left open from the top to the bottom of the kiln and from side to side to give the kiln a chance to heat evenly.Otherwise, some of the ware will come out hard and properly matured and some will come out soft and have to be refired.

A kiln which loads through a front door affords a better view of shelf clearance, but not as much assurance with respect to the space between pieces of ware being fired.The opposite is true of the top-loader.

What chemical changes take place in the kiln when you have begun the bisque fir­ing process? First you should know that no matter how dry the greenware appears when you put it into the kiln, there is al­ways a minute quantity of chemical mois­ture present.This moisture is known as water smoke.And it must be eliminated, after which the clay loses forever its plas­ticity.

You should start bisque firing at a rela­tively low heat.Keep the temperature at about 500 to 600 degrees Fahrenheit for an hour or two.If possible, keep the kiln door slightly ajar during this period to permit free escape of the chemical mois­ture in the clayware.This period of firing is called water-smoking or "water out." If the heat were increased too rapidly at the start, the moisture would turn to steam and would blow up the stacked ware in the kiln.Even after you close the door, leave the spy-hole open until the inside of the firing chamber begins to glow red from the heat.The open spy-hole will act as a vent for water smoke.

By holding a mirror to the spy-hole, you can check to see if water smoke is still being baked from the ware.

Photos above and adjacent at left show the ware about to be stilted and, second, on top of stilts.Naturally, stilting process is done in the kiln.

As long as there is moisture escaping, the mirror will be­come steam-misted and there may be mois­ture around the spy-hole.When the mirror does not become steamed, you will know that all the moisture has been driven out.

When this happens, seal the kiln tightly.Gradually increase the temperature until the ware reaches its maturing point.The heat should then be turned off and the ware slowly cooled within the kiln.If you are using a periodic electric kiln, the tem­perature will automatically rise as long as the power is on.In other words, the longer the power is on the hotter will become the kiln.Periodically remove the plug from the spy-hole and check the cone.When it is properly bent, you know the maturing point has been reached.

Do not open the kiln immediately after turning off the power.A safe rule-of-thumb is not to open the kiln until twice the firing time has elapsed.Then you may unplug the spy-hole and open the door no more than an inch or so.When another hour has passed, the door may be nudged open about one-quarter of the way.Wait another hour before removing the fired bisque from the kiln.

Be cautious that you don't burn your fingers.If you want to remove the ware now, rather than letting it cool in the kiln chamber, use heavy gloves or tongs.Place the still-hot pieces on some fireproof ma­terial, such as Transite or asbestos.Never set hot ware on paper or wood lest a fire be started.Freshly fired bisque should be kept out of drafts, since a quick tempera­ture change may cause it to shatter.For this reason you shouldn't use unheated tongs to remove your ware from the kiln.

The hardness of bisque ware which comes out of the kiln can be tested by scratching the base with a knife.This can be done after the ware has cooled com­pletely.If the knife cuts into the baked clay, it is called soft paste.If it is difficult to cut into the bisque and the knife leaves a dark mark, it is called hard paste.The hardness can also be judged by hitting the bisque lightly with a nail.If it has a flat sound, it is soft.If it has a bell-like ring, it is hard.

Hard-paste bisque has usually been fired at a higher temperature than soft-paste.It must be kept in mind that the higher the fire, the more a piece shrinks, and thereby becomes more dense and less porous.This will make the glaze application somewhat more difficult.

The stacking of a kiln for glaze, or glost, firing is more complicated than for bisque firing.Much more care is needed when ar­ranging a glaze kiln.Most important is that none of the glaze pieces touch one another, nor touch the shelf or any part of the kiln.Glaze will flow when heated and will stick to anything with which it comes in contact.

Paint the floor of the kiln and the top side of the shelves with kiln wash.(See preceding chapter.) With many kilns it is also necessary to brush kiln wash on the walls.Prepared washes can be purchased from supply houses, although this is hardly necessary since it is so easy to prepare yourself.The wash tends to seal these sur­faces so particles of dirt, sand, clay, etc., will not fall onto the ware and become imbedded in the molten glaze.It is advis­able before stacking the glazed ware in the firing chamber to brush down the ceiling and the walls of the kiln with an ordinary whisk broom to remove all such foreign particles.Many ceramists used a vacuum cleaner to clean the interior of their kilns.

In stacking ware for a glost fire, place the thinner ware on the bottom of the kiln, just the reverse of the bisque fire.The hottest area will provide the best soaking position, since heavy ware will require more heat soaking than the thin ware.Use the kiln furniture as you did for bisque firing.Load your kiln as economically as possible, but do not expect to get as many pieces of glazed ware in the chamber as you did greenware.Each glazed piece must be separated from the next piece and from the shelf-supports by at least three-eighths of an inch.

If the piece is glazed on the bottom, it must be supported on stilts.Otherwise it will stick to the shelf or the kiln floor.Stilts are flat pieces of refractory brick or tile which have three or four wire points sticking upward, upon which the ware is placed.

Cross-sectional view of the kiln will give you idea of how wares can be arranged.

Kiln photos from "Craftsmanship in Clay"

Use the largest stilt available for the piece being stacked so that it is per­fectly balanced.This is important, for if a single piece of glazed ware topples over during the firing, it may start others falling like the proverbial house of cards.Porce­lain pieces are set directly on the kiln-washed floor or shelf and are never placed on stilts.

Stilts are not necessary to support glazed ware, if it has been dry-footed.To dry-foot a piece of ware, remove all the glaze from the part of the ware that will rest on the shelf or the floor of the kiln.The glaze may be removed with a wet sponge, or a wet scrap of wool.Since no glaze will thus come into contact with shelf or kiln, no stilt is required.However, if the glaze is over-fired it is liable to flow to the bot­tom of the ware and stick to the shelf or kiln floor.

Keeping each level of your kiln stacked with pieces of similar height will enable you to get the most out of your firing area.When placing shelves in the kiln over glazed ware, be certain that the underside of the shelf is clean.Dust or foreign matter on the underside of a shelf will drop on the ware below, stick to the glaze and cause defects in the finished ware.

Glost firing need not be as gradual as bisque firing, since the clay has usually been already fired and water-smoked.If, however, you are firing glazed greenware, follow the same procedure to remove chemical moisture as you did for bisque firing.

To fire glazed bisque ware, use a low heat for about 20 minutes and let the door stand open for this time.Without closing the door, increase the heat to high.Wait another 20 minutes before closing the door.

Be careful never to jar or shake the kiln during a glost firing.Otherwise, the pieces may fall from their delicate perches on the stilts and be spoiled.If a glazed piece is stacked so it overhangs a shelf, molten glaze is likely to drip off and ruin any ware below.

See that the heat is increased steadily and that it is not allowed to drop back at any time.If the firing does drop back at the time the glaze begins to melt, blisters and defects will form which will mar the finished ware.

Shut off the kiln at the proper maturing temperature, when the cone has bent to the three o'clock position.Do not open the kiln until it has completely cooled.A sud­den draft of cold air will cause crazing on most glazeware.If a glaze has been un-dersired, it will look dull and lusterless.Too much heat will cause the glaze to run, leaving large areas of the surface bare and unglazed.An improperly fired piece can be glazed and fired again, but you will probably find it difficult to make the second coat of glaze stick to the once-glazed sur­face.As many as five coats are often re­quired to get a heavy enough coverage for retiring.

This presents another problem.If glaze is applied too generously it may begin dripping from the ware when it is heated to the molten state.Upon cooling, this ex­cess glaze usually is left hanging from the bottom of the ware, like miniature stalac­tites.These hangings must be removed with a file or a carborundum wheel.Take care always to grind in, toward the ware, lest the glaze be pulled from the surface in large chips.

Some colored glazes will "travel" in the firing if placed in the kiln with other un-matured glazes.This will result in a gray cast to an otherwise beautiful glaze.This phenomenon is caused by the gaseous chemicals forced from some glazes by the intense heat.These gases often affect the atmosphere in the firing chamber and bring about undesired chemical changes in other glazes they contact.

Improperly mended greenware led to whole section of repair separating from body of ware in firing.

Another cross-sectional view of cutaway kiln, here you see spacing between bowl and shelf.

Crack occurred in finished ware in kiln due to ■ small neglected crack that broadened in firing.

Should you happen to fire such a com­bination of glazes to produce this effect, note the colors fired together so that in the future you will fire these colors separately —unless, of course, you desire the gray tinge.

Overglaze decorations generally fire at a much lower temperature than glazes, since they are fired only to the softening point of the glaze over which they are applied.For this reason, the ware on which the overglaze decoration is applied affects the maturing point of the overglaze decoration.

All overglaze paints must be dry before being placed in the kiln.If this simple rule is not followed, you are likely to have trouble with the colors on the finished piece.Overglazed ware should be stacked the same as glazed ware.When you are doing a gold, china paint or luster firing it is necessary to leave the door of the kiln slightly open to allow the escape of the strong fumes which are generated by the oils evaporating and burning off as the heat rises.Although these fumes are annoying, they are quite harmless and proper ventila­tion will eliminate them.

Certain overglaze paints take less firing than others.All overglaze colors are not fired to the same cone even when placed on the same piece of ware.The colors to be fired to the hotter cone must be fired before the colors to be fired to the cooler cone are placed on the ware.Overglaze on a hard china piece must be fired to a hotter cone than the same overglaze ap­plied to softer clay body.

It always pays to test your kiln by placing several cones at different spots in the firing chamber and thereby determining which are the cool and hot spots in your particular kiln.Always place the more sensitive overglaze paints in the coolest spots.

Not until you have mastered the tech­nique of firing can you truly call yourself an accomplished ceramist.You are almost certain to have a number of failures and disappointments the first few times you operate your kiln.However, if you use the finest materials and equipment available, and if extreme care and cleanliness are used in firing your ware, you should not suffer too many disappointments.To get the most pleasure from your ceramic hobby, by all means learn how to use a kiln and do your own firing.•

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