Kilns and firing techniques

The ceramic artist receives his greatest thrill when he opens the door of the kiln and observes the final results of his efforts. The care with which he has wedged his clay, formed the body, and decorated its surface, which may represent hours and days of creative effort, are climaxed when proved in the heat of the kiln. The transmutation of clay and other base earths to "esthetic gold" through the medium of controlled fire implies an almost romantic and spiritual process. The ceramic artist should, by all means, learn the technique of preparing, stacking, and operating the kiln as an essential part of the knowledge of his art.

By definition, ceramics includes the application of heat. Without heat, the clay remains soluble, and with the addition of water, can be returned to a soft state. Also, without heat, glazing is impossible. Heat is an essential factor which changes "mud" to the finest porcelain.

The Kiln: In primitive days, clay objects were fired in an open fire, using wood for fuel. Discoloration and irregular maturing of the clay often resulted, because the heat could not be measured nor regulated, and because of the effects of the direct flame.

However, it was found after a time that, if objects were fired in an enclosed space (a packed clay or brick over), a higher degree of heat could be arrived at, and a slower cooling of the fired objects obtained. Thus was the first kiln born.

By definition, a kiln is an oven, or furnace, for hardening, burning, or drying anything. It is usually lined with fire brick or other highly "refractory" (heat resistant) material, which can support the weight of the objects placed upon it, and which retains the heat within the firing chamber.
Modern flame kilns are of two types: those in which the flame passes through the firing chamber; and the "muffle" type in which the flame passes around and surrounds the walls of the firing chamber but does not enter the chamber itself, since the heat enters the chamber by radiation.

Modern flame kilns are described by the method of heating (the fuel used), the cubic size of the firing chamber, and the maximum attainable temperature. Fuels in common use are natural or artificial gas, oil (used in conjunction with vaporizers and blowers), kerosene, and solid fuels (coal or wood). Economy and availability are prime considerations in determining which fuel to use. The size of the kiln is an important factor limiting the quantity of items which can be fired at one time. If a large class of students desire to use the same kiln, it is obviously more economical and efficient to procure a kiln of a large cubic capacity.

The highest temperature that a kiln can safely reach is important, as this limits the types of clay and glazes which can be fired in the kiln. A low-temperature kiln obviously cannot be used for firing porcelain and stoneware that require a very high temperature to mature.

The maximum temperature to which a kiln can be brought is determined by the material of which it is made. Fire clays permit temperatures up to 2000 degrees Fahrenheit. Above 2000 degrees Fahrenheit the walls must be made of carborundum, or other very highly refractory material.
So far as the student is concerned, an electric kiln is by far the best. The temperature can be regulated and controlled easily, there is no open flame to give undesired effects to the fired ware; it is quiet and safe in operation; and it is easy and convenient to handle. Electric kilns are relatively inexpensive to operate; they require no chimney outlets (as is the case with certain fuel-fed kilns), and can be installed in any room with minimum effort.

There are numerous types of electric kilns for sale on the market, in varying sizes and with other specifications, from which a selection can be made. They are usually so insulated that very little external heat is radiated, and they present very little, if any, fire hazard. Further, electrical heating elements can be easily replaced in a matter of minutes so that little time is lost in kiln operation due to repair.

Note: As a matter of information, "saggers" are boxes of fire clay in which items to be fired are "stacked." These boxes are sealed with fire clay and placed in the chambers of open-flame type kilns. The "saggers" prevent contact of the flame with the clay ware inside the sagger.

Effects of heat. It is important that we understand what takes place in the kiln during the "firing cycle." As clay is heated any "physical" moisture remaining in it is vaporized and driven off. As the temperature rises (about dull red heat), the "chemical water" in combination in the clay is separated and driven off. The clay becomes dehydrated, and will no longer become plastic with the addition of water. With increasing temperature, the organic matter (animal or vegetable matter) which may be contained in the clay is oxidized or burns off. In other words, this organic matter combines with the oxygen of the kiln atmosphere to form carbon dioxide and other gases.

As the maturing temperature of the particular clay is approached, certain minerals or compounds within the clay begin to fuse, melt, and vitrify. The clay mass softens somewhat, begins to shrink, and becomes more compact. At the maturing temperature, the clay will have the desired density and, upon cooling, will have attained a rigidity of structure. When this maturing point is reached the firing is discontinued. If a temperature higher than the maturing temperature of the particular clay is reached, the clay will continue to melt and vitrify; it may become molten or quite soft, and warp or change its shape and may become extremely dense, hard, and brittle upon cooling. It is of absolute importance that the maturing temperature is not exceeded.

Other aspects of heating to be considered are the rate of speed of rise of the temperature and the length of time of heating. Moist or damp clay will crack or explode if heated too rapidly.

If not enough time is allowed to completely oxidize or burn off the organic matter, the clay may discolour or blacken. Thus, it is essential in firing that the temperature be gradually increased.

If the clay is underfired, it may become too porous and soft upon cooling. Heavier clay objects require a more gradual temperature rise than lighter thin-walled ware.

For your particular kiln, keep a series of records indicating the time and rise to maturing temperature, so that you can determine the best rates for each of the particular clays you use. Generally, the average electric kiln properly regulated will rise with sufficient slowness to avoid the above defects.

At the beginning of firing, leave all vents open (if any) in the kiln including the peephole. The door of the kiln is "cracked" (left slightly open). See that. it is held open with a piece of fire brick, triangle, or piece of shelf wedged in the doorway. This is done to permit the escape from the kiln of water vapor and steam formed from the moisture of the dehydrating clay. Keep the kiln door cracked for at least one-half hour after the start of firing.

The hand is passed above the cracked door from time to time to feel the character of the escaping air. If the air feels moist, keep the door cracked. When the air feels hot and dry, practically all the moisture has escaped. At this point close the vents and the door. The peephole may be kept open for a period of time thereafter to insure the escape of all remaining vapour and fumes. The period during which the door is cracked may be longer than one-half hour, depending on the number of clay pieces, their thickness, and their moisture content.

After the ware has been fired to maturing temperature, turn the heat off and let the ware cool, keeping the door closed. The period of time for cooling bisque is about the same as for firing. The period of time for cooling glazed ware is about twice the time for firing. There is no objection to a longer cooling period. For example, after the heat is stopped the ware can be cooled in the closed kiln overnight.

After this cooling period the door is cracked and gradually cracked wider as the heat diminishes. This can be felt by passing the hand near the opening of the cracked door. The door is finally left wide open. Do not reach in and grasp the ware immediately after the kiln door is opened wide. Although the air inside the kiln may cool rapidly, the ware itself has absorbed a great deal of heat in firing and will continue to give off this heat for a substantial period of time. The ware is removed when it can be handled comfortably with the bare hands.

The thermal shock of too sudden cooling of hot ware may cause it to crack, or may set up strains within its structure that will make the object quite fragile.

Test the temperature of the cooling ware by inserting the hand carefully in the kiln and bringing it close to the ware. The radiation of the ware will indicate if it is cool enough to be handled. This test is similar to testing a flat iron (for pressing clothes) with the hand to see whether it is cool or hot.

When fired, clay becomes bisque. Bisque is porous (sponge-like) due to the evaporation of chemical water, and the formation of vapour and gases within it.

Engobes and glazes are fired to the maturity temperature indicated for each by the manufacturer. Engobes will sink in or crack off if fired too high. Never fire glazes above their maturing temperatures, as they may run, discolour, sink in, craze, blister, or crack off.

Heat measurement and regulation: The heat condition in the kiln chamber can be measured by the use of pyrometric cones; the actual temperature by the. pyrometer.

Pyrometric cones are pyramids of china clay, mixed with feldspar, flint, and other materials; they are designed to soften and bend at various temperatures. Each cone is marked with a standard code number that indicates the heat condition at which the cone will bend over or "deform." See the Cone Temperature Chart below.

The cone measures the combined effect of temperature and time. It measures the maturity of the clay object under exposure to a given heat over a given period of time. For example, cone 014 will bend to right angles at 1463 degrees Fahrenheit when fired slowly (20 degrees centigrade per hour), and will bend to right angles at 1526 degrees Fahrenheit when fired rapidly (150 degrees centigrade per hour). Clay with a maturity equivalent of cone 014 will mature at these temperatures under the same conditions also.

The cones are usually mounted in fire clay "pats" or plaques at a slight angle. The base of the cone should be set firmly in the fire clay pat about one-eighth of its length, and the clay pressed in around the cone (to avoid the cone toppling over after the heating begins). The clay for the pat should be raw fire clay and not a compounded clay, and must have a maturing temperature higher than that of the highest cone in the pat.

Fig 39

In placing the cones in the pat they should be slanted sideways about 8 degrees, and forward about 8 degrees. The cones are set in the pat about 1/4 inch apart. If the cones slant to the right, the cone with the lowest bending temperature should be to the farthest right. Thus, it will bend first, and not touch the other cones in bending. The pat should be "bone-dry" before placing in the kiln (Fig. 39A).

As the maturing temperature of the cone is approached the cone becomes incandescent (in shades of red or white heat). The cone bends and begins to arch over; it bends to right angles when the maturing temperature of the particular cone is reached (Fig. 39B). After the switch is turned off, the cone will continue to bend over a little more. Do not be alarmed-unless you forgot to turn off the switch when the maturing cone was at right angles.

Standard cone chart

Temperature equivalents when fired rapidly (150° C. per hour)

Cone Degrees Fahrenheit Cone Degrees Fahrenheit
022 1121 09 1706
021 1139 08 1742
020 1202 07 1814
019 1220 06 1859
018 1328 05 1904
017 1418 04 1940
016 1463 03 2039
015 1481 02 2057
014 1526 01 2093
013 1580 1 2120
012 1607 2 2129
011 1661 3 2138
010 1643 4 2174
    5 2201

When you are first beginning to fire, you should mount cones in threes. For example, if your clay matures at Cone 04, use cones 05, 04, 03, mounted in a unit (Fig. 39A) . The three cones are employed to determine when the maturing temperature is being approached, when maturity is reached, and when it is being exceeded. After practice, only one cone (maturing temperature) need be used (Fig. 39C).

If practicable, place the pat of cones about two-thirds of the distance from the front toward the back of the firing chamber, and about two-thirds up from the bottom of the chamber. When stacking ware in the kiln, make provision for seeing the cones from the peephole. It may be necessary to raise the cones on a stilt or support in the kiln so that they can be seen from the peephole (Fig. 39D).

Fig 40

The reason for placing the cones toward the rear of the firing chamber is because there may be an escape of heat from the open or imperfectly closed peephole, which will cool the cones if placed immediately behind the peephole. If this occurs they will not indicate the true condition of the chamber (Fig. 40A).

After stacking and placing the cones, before firing, check the position of the cones by closing the kiln door and, with the aid of a flashlight, seeing if there is an unobstructed view of the cones through the peephole.

Cones are quite inexpensive and are available at all ceramic supply houses. They come in standard and junior sizes. Junior sizes are especially desirable for small kilns. Both work equally well.

In most ceramic supply house catalogue, the maturing temperatures for clays and glazes arc indicated by the equivalent cones. You can obtain the temperature chart for these cones from the firm from which you bought them.

The pyrometer is an electrical device (a thermocouple) which is inserted into the kiln firing chamber. It is connected to an electrical meter which gives accurate temperature readings. Pyrometers are relatively costly as compared to pyrometric cones.

Remember that cones not only indicate their maturing temperatures, but respond to the kiln atmosphere. Their reactions (cones being also of ceramic material) are coordinated with approximate accuracy with that of the ware in the kiln.

Records: When using a pyrometer, keep a record showing the temperature every 15 minutes, so that the rate of rise can be determined for that kiln. Some kilns have several heat switches. Keep records of the respective rates of temperature climb with each switch, and also the results on certain clays and glazes with each switch as a reference for the best firing "speed" to use in each case. Regardless of whether cones or pyrometers are used, keep records of the total time it takes from the start of firing, to arrive at each maturing temperature. Based on these records, each time a "burn" (a kiln load for firing) is started, an alarm clock can be set to ring, at the future time (which the record indicates) when the switch must be changed or just before the ware can be expected to mature. At this time the cone or pyrometer can be inspected for maturing temperature and the heat turned off if necessary.

Checking the kiln: After being installed, the kiln should initially be dried out by heating at a low temperature for an hour or two with the door open. After this initial drying out, close the kiln door when the kiln is not in use. The kiln may now be filled with items to fire; the first group of pieces should not be of great value, because we are experimenting with, or testing, the kiln. The kiln should be filled fairly full of green ware. If a pyrometer is used, after the switch is thrown on, record the temperature readings every 15 minutes until the maturing temperature is reached. From this record you can judge how long it will take to fire a "batch" of this type of clay item. If pyrometric cones are used, they should be observed through the peephole every 15 minutes until the lowest cone bends. Then observe every 5 minutes until the centre cone arches over. Record the time from starting to reaching the maturing temperature for future reference. After the kiln cools, examine the bisque ware. If it is cracked, or discoloured, perhaps the kiln was heated too rapidly -in which case (if the kiln has several switches), run a slower heating test with new green ware. Generally speaking, most up-to-date electric kilns heat up at a satisfactory rate and good results should be expected in practically all cases.

In testing a large kiln (3 cubic feet or larger), place the pyrometric cone triplets at the bottom, on each shelf, and near the top as well as behind the peephole. After the triplet in rear of the peephole indicates maturity and the kiln is cooled, examine the other cones. They will indicate the respective temperatures at the various levels. Some kilns run hotter at the bottom than at the top. This knowledge is of advantage, because the bisque ware requiring a higher maturing temperature can be placed below, and glazed ware requiring a slightly lower temperature placed at the top, and both can be fired at the same time (assuming that their maturing temperatures are fairly close).

Stacking the kiln: The basic principle of kiln stacking is to utilize as much space in the kiln chamber with objects to be fired as is possible. To assist in stacking ware, kiln "furniture" is employed-this "furniture" comprises shelves, shelf supports, stilts, and triangles (Fig. 39E). It is made of highly refractory fire clay or other materials. Before each stacking, the ware to be fired is accumulated, and a study made as to how best the most ware can be put into the chamber. Shelves supported by shelf supports can be erected just above smaller ware, and additional ware stacked on these shelves. The shelf material is quite strong and will support comparatively heavy loads. Tall objects can be placed on one side of the chamber, while shelves are erected in the remaining space to take smaller objects (Fig. 40A).

Green ware can be set very close together, or in most cases even piled on one another, without damage; the heavier pieces on the bottom, the lighter on the top. Bisque pieces will not stick together unless they are heated above maturing temperature. However, glazed pieces must not touch. If they do, when the glaze melts and later cools, the pieces will fuse together and cannot be separated without damage.

Triangles are used to support smaller clay objects such as statuettes, whereas stilts are used to support plates, vases, and bowls (Fig. 39E). No glazed objects should rest directly on the floor of the kiln. Use the triangles and stilts to keep them off the floor. If there are heating elements in the floor of the kiln, all ware must be elevated above the floor with triangles or stilts. Stilts have three points so that there is only one "point" contact with the object. When firing glazes on objects, the objects never rest directly on a shelf or on the bottom of the kiln; stilts or triangles are always used. Stilts leave little pimples of glaze at the points of contact, whereas triangles leave a line marking. The marks left by the stilts or triangles can be later removed with a grindstone. Only the bottoms of the objects should rest on stilts or triangles. Markings elsewhere are thus avoided on the surface of the objects.

Fire green tiles horizontally and support them on all four corners. If laid on a flat shelf, the corners will warp up when firing. Some ceramic supply houses have special refractory supports on which to fire tiles.

Kiln furniture is quite inexpensive, is easily available, and can be used over and over again. If shelves crack or break, the pieces may be used again as smaller shelves.

Kiln wash: In glazing, some of the glaze upon fusing may run and drip on to the shelves, or floor of the kiln. It is therefore necessary to apply a "kiln wash" to the shelves and floor. Kiln wash is made of dry pulverized kaolin and flint, mixed with water to a thick slip. The wash is sponged or painted on the floor of the kiln chamber before stacking the kiln. Each shelf is washed on the upper surface before glazed ware and kiln furniture are placed on it. Let the wash air-dry before stacking the kiln. When glaze drips on to the wash, after cooling, the glaze droppings and wash may be peeled off. The wash thus preserves the life of the kiln and the shelving. Kiln wash need not be applied if you are bisque-firing only.

Hints: Take care in loading and unloading the kiln. Avoid tumbling the shelves and supports. After a "burn" carefully remove and keep the pieces of furniture in a secure place for future use. Clean the kiln after each firing, remove the glaze drippings and, if necessary, apply new wash if more glazed ware is to be fired.

Every time a kiln is fired, the life of the heating elements is shortened. It is therefore most economical to wait until a full kiln load is accumulated, instead of firing a few pieces at a time. Moreover, the cone or pyrometer reactions will not give true indications of the condition of the ware in partially empty kilns.

Most electrical kilns are constructed so that heating elements are easily replaceable. Instructions usually accompany the kiln indicating the type of heating element used, and how it is installed when replacing a defective or worn-out element.

Making your own kiln: It is possible to construct an electrical kiln but it takes a great deal of specialized skill and considerable effort. The plans, bills of materials, and details of construction can be obtained by writing to the General Extension Service, University of New Hampshire, Durham, New Hampshire, and enclosing a nominal amount for the cost of the pertinent pamphlet. The construction involves cutting grooves in fire brick; the coiling, winding, and insertion of heating coils in the grooves of the fire brick; the construction and welding of a steel frame and door supports; the assembly of fire brick, insulation brick, and asbestos board components; the electrical connection of heating elements to switches and power sources; the mounting of the door, cutting a peephole, and the painting of the exterior. A well-equipped workshop is essential for such construction.

Purchasing an electric kiln: If you intend to use the kiln in your studio for your own use, a chamber of about 1 cubic foot should be ample. However, if the kiln is to be used by a class of students, the manufacturer should be consulted. He should be informed of the number of students, and his recommendations for the appropriate size kiln should be considered.

Firing procedure: A typical "firing cycle" for maturing bisque (low firing clay) is given here. It is assumed that the electric kiln has low, medium, and high temperature switches.

Heating
Time Switch Remarks
1/2 hour Low Kiln door cracked, peephole open, test for moist air with hand
1 hour Low Kiln door closed, peephole open
1 1/2 hours Medium Kiln door closed, peephole open
1/2 hour High Kiln door closed, peephole open
1 hour High Kiln door closed, peephole closed.Observe cones every 15 minutes until low cone bends, afterward watch cones every 5 minutes until middle (maturing) cone bends to right angle.
  Off Turn to OFF immediately when middle cone bends to right angle.
4 1/4 hour Total + to maturing heating period

 

Cooling
4 1/4 hours Off Kiln door closed (same period as total heating period)
1/2 hour Off Kiln door cracked
1/2 hour Off Open door wider every 10 minutes to full open.
  Off Remove ware when cooled sufficiently to handle by hand. Test air in vicinity of ware by hand.

For firing glaze on bisque ware of the type clay assumed above the procedure is the same except that the period during which the medium switch is on is reduced to 1 hour-making a total heating period of 4 hours +. The cooling period with the kiln door closed for this glazed ware is 8 hours (or double the heating period).

Comments: With experience it will not be necessary to check cones every 15 minutes. Set an alarm clock to indicate the times when switches should be changed and when the low cone is expected to bend.

The purpose of keeping the peephole open so long is to provide escape for remaining steam and fumes from the glaze media. As a point of information, electrical heating units for temperatures up to 2000 degrees Fahrenheit are usually made from Nichrome wire. For higher temperatures, "Glo-bars" of silicon-carbide are employed.

Low firing clays and glazes have maturing temperatures below 2000 degrees Fahrenheit.


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