Choose your kiln to suit the kind of ceramics you'll do, and learn how to use it for maximum performance.

Kilns have been used by man for many thousands of years.Excavations of ancient sites of early civilizations indi­cate that the most primitive kilns were of the updraft type.In most cases, they con­sisted only of a firing chamber, a perforated floor and a wall.The construction of a permanent kiln was too much of a struc­tural problem for early potters.Recourse was made to a temporary roof of green poles and raw clay which would hold to­gether long enough for the very low firing temperatures used at that time.

The modern kiln has vast improvements over the primitive types.However, the four main essentials which must be present in all kilns have not changed.There must be a means for producing heat, a support for the ware to be fired, a wall to contain the heat, and a means for transferring the heat from its source to the ware.

Firing a piece of ceramic ware is gener­ally the final step.It is also the most neces­sary.If a piece is not fired properly, all the work that has gone before is lost.Many types of kilns—or heat-treating ovens—are in use today.Most common in home work­shops is the periodic kiln, which has a com­plete cycle from cold to warm to hot to cold.By contrast, in large commercial potteries, the kilns maintain a constant heat and the ware is passed through them on conveyer belts or flatcars at a predetermined speed.When you place a piece of dough with yeast in your kitchen oven, the heat causes certain chemical changes which result in a loaf of bread.A periodic kiln can be likened to your kitchen oven, the only important difference being its ability to attain, much hotter temperatures which likewise cause certain chemical changes in clay and glazes.Most kilns are constructed of refractory bricks or tiles for inner walls.These walls are insulated in order to keep the heat evenly distributed throughout firing cham­ber.This is of utmost importance and by no means easy to achieve.In fact, few large periodic kilns show a difference of less than 50 degrees Fahrenheit from top to bottom.

Another advantage of insulation is that it reduces the amount of fuel necessary to reach the desired temperature.Some kilns, particularly those in commercial potteries, are heated with gas or oil fuels.Electricity, however, is much more practical for the hobbyist, since it is much easier to control, extremely flexible and available at reason­able cost in most parts of the United States and Canada.The studio with a gas- or oil-burning kiln is rare nowadays.

The main shortcoming of an electric kiln is that the atmosphere in the firing chamber cannot be controlled.In some rare instances the professional ceramist may want a re­ducing atmosphere rather than the ordi­nary oxidizing condition.A reduction flame firing will have to be made in a gas or oil kiln, if one can be found nearby.

Before choosing a kiln for your studio, you must take into consideration the amount of electricity you have available.If only normal electricity for lighting is available, then your kiln size or firing area is limited to just that power.

Also important to consider is heat loss.Obviously it requires more power to heat a large kiln than a small one to the same temperature.The main principle involved in firing a kiln is that the outlet is smaller than the inlet, so that there will always be a pressure.Without this pressure, it would be impossible to build up high tempera­tures in the kiln.

Hence, a kiln chamber must be confined, and heavily insulated as well, in order to prevent too great a heat loss.It follows, too, that the more power available, the larger the kiln interior can be, and many more pieces can be fired at one time.

Finally, you must consider how you want to load the kiln for firing.You can choose between a front-door opening, or one which is loaded from the top.This is pretty much a matter of personal preference.A little experience loading both types will de­cide the question for you.

Many people, interested in trying their hand at making pottery, become discour­aged when the cost of firing equipment is learned.Yet, to leave a piece unfired or to depend upon someone else's skill is shirking, for the firing of a piece is the real test of its structural and decorative merit.

Though the initial outlay for a kiln can be fairly expensive, the actual cost is low.Properly used and maintained, the life span of a well-made kiln can be measured in generations.Considering the importance of a kiln to any studio and the many years of service it is capable of giving, a kiln is a prudent investment if it is purchased wisely.Remember, however, that even the best kiln improperly used can be reduced to a machine of ugly disappointments within a short time.

The beginner is advised against firing his own pieces until he has had some instruc­tion in the use of a kiln.An exception may be made, though, in the firing of jewelry, small tiles and small pieces cast in a mold.This is because small electric kilns for this type of firing can be purchased for as little as $35 to $50 today.Thus the beginner is not forced to pay out large sums of money until he is certain he can and wants to op­erate a larger kiln.Because of the rapid rise in temperature in a test kiln, as these small models are called, only thin objects no more than one-quarter inch in thickness should be fired.Any object thicker than that may explode in the kiln.

Below left, the author's son has removed plug from the spy-hole to see if proper firing has been reached, as determined by position of cone.Plugs are put in only after moisture has escaped, when pyrometer shows about 500 °F.Right, switches on face of top-loading kiln are for three heat levels.

Shelves having glaze drippings on them (torn previous firings must be cleaned to prevent glaze from sticking to next batch fired.Chisel off glaze, then cover what remains with kiln wash, applying it in a milky-thin mixture made by adding water to kiln-wash powder.Use paint brush; brush hairs that adhere to shelves will fire out.Too heavy wash may peel or adhere to piece resting on it during firing.Apply kiln wash only to side of shelf that is to support wares; do not reverse shelves in the kiln—always keep same side up, or kiln wash may fall on ware on lower shelf during the firing.

Many ambitious ceramists have con­structed their own kilns.Unless you are an accomplished builder, however, it is advisable to buy a ready-made kiln.No matter how fine a set of instructions for building a kiln one uses, some problems almost always arise.Often one has to hunt for a long time before he determines the cause of the difficulty and eliminates them; sometimes the advice of an expert is neces­sary.Such delays and worries are dis­couraging to the beginner.The majority of hobbyists who have built their own kilns will readily admit the finished product was not worth the effort.Almost without ex­ception, the homemade kiln is less de­pendable than the factory-produced kiln.

One word of caution, however—always check the reputation of a kiln manufac­turer before buying.There have been some shoddy models put on the market in the past and some unwary buyers learned to their regret that they had been bilked.When you buy a kiln, make certain it is the best available at the price you wish to pay.Compare competing lines.Do not hesitate to ask for the advice of another ceramist who is qualified to give counsel.Larger kilns are heavy and costly to ship, so if pos­sible try to select one that is manufactured in your section of the country.

The cost of a kiln is also determined largely by the temperature it is capable of attaining.A small test kiln, such as that recommended for firing tile and jewelry, can reach 1800 degrees Fahrenheit in one or two hours.Larger kilns will take longer to get this hot.However, if a higher temper­ature is desired, only a larger kiln can give it to you.

A good all-around kiln will be able to deliver up to approximately 2300 degrees Fahrenheit in a reasonable length of time.There are kilns which go higher, of course, but the price begins to rise in a geometric proportion to the amount of heat which can be had in these high-fire kilns.For most purposes, 2300 degrees Fahrenheit is sufficient.Fine, hard-paste porcelain, for example, can be fired at this temperature.

Before you fire any ware, naturally you must know the maturing points of both the clay and the glaze.You must know at what temperature the ware will collapse, vitrify or melt.Otherwise, how would you know when to shut off the kiln? Time or dura­tion of fire depends a great deal on weather conditions, weight of the ware in the kiln, or mass.Thus, while the time factor is important, it is not sufficiently accurate.

A thermometer is useless at the high temperatures attained in a kiln.Most thermometer liquids would boil if exposed to such intense heat, and the container for such liquid would surely melt.

This situation resulted in the develop­ment of the pyrometer.It is an old prin­ciple of physics that certain dissimilar metals, twisted together and welded at the end, will develop small amounts of electricity when subjected to heat.

Cracks in bottom of kiln will allow heat escape from this area and affect your ware; see below.

Vacuum out bottom, mix bonding cement in plastic bowl and fill in cracks or depressions in kiln.

Smooth, workable mixture of bonding cement was applied and sanded.Cover lightly with kiln wash.

The ratio of electricity increases with the amount of heat.The amount of electricity can be re­corded on a microvoltameter.This is cali­brated and charted in terms of degrees, permitting you to read the temperature in­side the firing chamber.

Another method of determining temper­ature is by the use of cones.These are small pyramid-shaped pieces of clay, which will melt at a predetermined tem­perature.Most electric kilns have spy­holes cut in the front of the kiln with a removable plug.The cone is placed inside the kiln so that it can be seen through the spy-hole.

Each cone has a number on it.When you observe the cone beginning to bend inside the kiln, the temperature denoted by the number on the cone has been reached and the kiln is shut off.The use of cones is quite an accurate means for gauging temperature.Cones are often employed as a check against the pyrometer.

This method of measuring kiln heat has become so universal that maturing tem­peratures are more often than not given in terms of cone numbers.(See chart of temperature equivalents of pyrometric cones in this chapter.) For example, if you used a clay which matures at Cone 8, you would know that the equivalent in Fahren­heit degrees is roughly 2250.Likewise, a glaze which fuses properly at Cone 08 would require a temperature of about 1733 degrees Fahrenheit.

In the first instance, you would place a cone bearing the number 8 in the kiln and wait until it bends before shutting off the kiln.In the second instance, you would use a cone numbered 08 and wait for it to bend before shutting off the kiln.

There is, as you have seen, a large differ­ence between Cone 8 and Cone 08.The numbering of cones begins at 022, equiva­lent to 1085 degrees Fahrenheit, and con­tinues up to 20, equivalent to 2770 degrees Fahrenheit.When there is a zero in front of the number, the equivalent temperature rises as the number decreases.When there is no zero before the number, the tempera­ture continues to rise as the number in­creases.(See chart of equivalents.)

Many ceramists employ another check on the accuracy of cones.Instead of using a single cone, they line up three in full sight through the spy-hole.Suppose they are firing a ware which matures at Cone 06.Then they would place one 05 cone, one 06 cone and one 07 cone in the kiln.When the 05 and 06 cones have bent, but before the 07 cone has started to bend, they will shut off the kiln.

Left are shown cones used to determine firing temperature.Usually you place three cones in kiln, unless you have a pyrometer.To fire at Cone 6.you place Cones 5.6 and 7 in the kiln at eye level.When Cone 5 bends, that's the signal that you're approaching desired temperature; if you wait too long and Cone 7 bends, you've overtired.Above, the straight cone is bent as shown when you shut off kiln.

TEMPERATURE EQUIVALENTS ORTON STANDARD PYROMETRIC CONES

Temperature equivalents shown for these cones were not determined in tests.Values given have been estimated for your convenience.

COLOR SCALE FOR TEMPERATURES

COLOR                                                           APPX.CONE         DEGREES F.
Lowest visible red to dark red........................022 to 019                 885 to 1200
Dark red to cherry red ..:..............................018 to 016               1200 to 1380
Cherry red to bright cherry red..........             015 to 014               1380 to 1500
Bright Cherry red to orange............................013 to 010               1500 to 1650
Orange to Yellow..............................................09 to 03                 1650 to 2000
Yellow to light yellow................„ „ „                 02 to 10                 2000 to 2400

Pyrometer is used to determine approximate tem­perature in kiln; for accuracy, cone must be used.

If only the 05 cone has bent, the kiln has not reached the maturing point.If all three cones have bent, the ware has been overfired.

The use of cones and pyrometers re­quires the attendance of the kiln operator, who must be on hand to shut the kiln off when the proper temperature has been reached.This has been overcome recently with the introduction of thermostatic con­trols which automatically snap off the kiln at the desired temperature.

One such device is called the Kiln-gard.It is patented and sold by dealers through­out North America.It consists of two pro­jecting fingers, spring-loaded and held apart by a pyrometric cone.These fingers are inserted through the spy-hole in the kiln.When the cone melts in the firing chamber, the fingers close, shutting off a switch which in turn snaps off the kiln's source of electricity.

There are simple rules to follow in the care and maintenance of kilns to obtain properly fired ware, uninterrupted pro­duction and a long kiln life with a mini­mum of repair.

1. Do not overfire.If you do not know the normal safe temperature ceiling for your kiln, write to the company that manu­factured it.The firm will be glad to furnish this and any other information about the kiln.

2.Do not overload.Get circulation throughout.Set all ware so that heat may pass freely through the firing chamber.It is a temptation to load a kiln too tightly, especially when the ware consists of small pieces.

3.Replace damaged parts.Wear in­ vites wear.Faulty parts almost always cause poor results in the firing.

Small enameling kiln by the L&L Manufacturing Co.reflects the current craze for copper jew­elry with ceramic glaze.Below is large-capacity Paragon kiln.

Model H-145 hobbyist kiln made by D.H.C.Kilns.Inc..measures 13 inches high, has maximum tem­perature of 2000'F; around $65.

D.H.C.porcelain kiln, below, stands 29 inches high and costs about $124.Its maximum heat is 2400 °F; case is of welded steel.

Model TL-6 top-loading kiln made by Harrop Ceramic Service Co.sells for about SI00 crated (prices given on all kilns is with crating).Height is 20 inches.Switch has three heat speeds.Harrop bench-model kiln is shown at the beginning of chapter, has a pyrometer.

Model SF-2 front-loading kiln is 56 inches high, comes with pyrometer, pilot lights and other features (bottom stand not shown in photo) for $390; it's made by Paragon Industries, Inc.

L&L Model 424 front-loading kiln above heats to 2300°F, is fine for studio; cost, over $1000.Manufacturer makes various kiln types, sizes.

4. Do not rush the firing.Building up the temperature gradually during early stages avoids excessive strain on both the ware and the kiln.
   
   
5. The kiln should be kept free of dust at all times.Should any foreign matter reach a heating element in an electric kiln, it should be carefully removed before firing.
   
   
6. If small cracks appear on the inside walls, leave them alone.These cracks are inherent with insulating firebrick.They are not harmful in any way, since they close when the kiln is heated, due to the expansion of the material.It is the same principle as the expansion joints in side­ walks, highways and bridges.
   
   
7. Kiln wash should be used on the tops of shelves and on the kiln floor before firing ceramic glazes.It is painted on with a brush and usually consists of one-half kaolin and one-half water, mixed with water to the consistency of light cream.Kiln wash is used to prevent glaze drip­ pings from sticking to the shelf or the kiln floor.It is not generally recommended to use kiln wash on the kiln walls.
   
   
8. Be certain that your electrical service is capable of delivering sufficient power for the kiln.Do not overload the circuit.It is wiser to install a new service which delivers more power.Your local power and light company, or your electrician, will be able to advise you on the service your kiln requires.

9.Electrical connections in the kiln may in time become loose due to the effect of expansion and contraction.At any sign of excessive heat at the plug or at the terminal of the kiln, the connection should be examined.If severe blacking of the copper wire is noted, this should be sanded bright, and a new connection made.
10.Should at any time a heating element begin to come out of the grooves, place a heating element staple at the center of the bulge and very carefully push it into the brick wall of the kiln.Caution must be used when handling the elements after they have been heated and cooled.After the staple is in place, fire the kiln to at least Cone 06 to soften the element and allow it to take a new set.

In the event that the heating element has moved entirely out of the groove before maintenance is attempted, do not try to put all the element back at once.With a staple, move it part of the way toward the groove, fire the kiln, then push the staple to a better position and fire the kiln again.Repeat as many times as necessary until the element is back in the proper place.•

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