Firing with electricity, because it is limited to the application of heat only, is the simplest form of firing and that is where we will start. | say thot it is so limited because there is no practical way to alter the atmosphere from extreme oxidization. A few select laboratery personnel and a few experimental potters moy scream in lovely indignation about that last statement but | make it nevertheless. | hasten ta say that there is a very real ploce for electric kilns in spite of the fact thot gos kilns ore more versatile and economical to operate. Certain ceramic moteriols must have an ox: idizing atmosphere at all times, small electric kilns are quite practical, which cannot be said for small gos kilns, and electric kilns are, relatively speaking, by far the cheapest to buy, They can also be equipped with ail sorts of fairly inexpensive controls, timers, ete., that make automatic tiring possible. With care, and the right elements, an electric kiln can be used to cone 12 but in common use they start to become impractical ond expensive obove cone 6. Hence electric kilns are suitable for small, low-temperature firings requiring on oxidizing otmosphere, and where there is no one fully qualitied to conduct a firing or where the control of a firing hos to be handed over to instruments. No one would ever claim that this is going fo satisfy the needs of on eager potter but it at least makes pottery possible where it otherwise would not be. This of course means most school and many Gmoateur groups. TEMPERATURE in the discussion of “tempercture’ we con take ao great deal for granted; if your kiln can develop a high enough temperature for the moterials thet you ore using and if the rote at which this temperature is developed con be controlled very little more need be said about it, we ore ready to proceed with oa firing. |f your kiln will not do these things it is dragging you down and defeating you. Your kiln might be inadequate from o number of other points of view, size, heat-loss, safety, efc., but that is anather matter, TIME With the availability of temperature assured we can start to look ot the processes of firing, From 250 to about 1400 degrees F, crystal woter, carboan-dioxide, carbon and sulphur are be- ing broken out ond driven from the firing materials; the molecular bonds in which they were in- valved ore being reploced by mare stable molecular bonds. Wore token from the kiln after this stage is soft and porous but much more durable than it was in the raw state. Examination will show that absolutely no fusion has taken place, the strength is all in the molecular bond between par- ticles, and the piece is actually larger thon it wos when it went into the kiln. At this stoge the clay is soid to be “sintered,”’ | find this the ideal bisque firing. Therefore, contrary fo populor concepts, firings storts at 250 degrees. If one is certain thot the cloy is antirely dry when it goes inte the kiln the sintering stage can be taken at 300 degrees an hour, | usually take it more slowly to about 450 degrees. If the rote is too fast in the early stages water will be released from the crystals foster than it can escape to the surface and the piece will blow up from steam pressure. If the rate is too fast in the loter stages carbon and sulphur com- pounds will not be oxidized fast enough ond will be trapped in the interior of the piece causing bloating ond black coring later, The greotar the plasticity of the clay or the thicker the piece the slower this stoge must be token. Many clays and many kinds of products require detailed scheduling throughout this stage for success. From 1400 degrees to the stort of vitrification, which varies with different clays, the temp- eroture moy advance more rapidly, molecular bonds get more numerous and complex but only mic- rascopic points of fusion develop. No really significant changes take place, At this stage there is some danger of perts of the kiln advancing faster than others creating an imbalance that will be hard to correct so | compromise by continuing at 300 degrees an hour or even slowing down on a heavily loaded kiln. Ag soon a5 fusion is generally under woy the reactions become much more ropid, almost spon- foneaus; morrow differences in the degree of heat-work show up as brood differences in firing result, At this stage our concern starts to shift fram clay to glaze. Throwgh the development of fusion temperoture should odvance very slowly, as litthe as 30 rees on hour and you should be relying on cones to fell you where you ore in your schedule. When your last comes are going down you are being told that your firing is nearing completion IN THE VICINITY OF THE CONES. Now you must assure yourself thot all parts of the kiln become equolly developed. The temperature advance must be stepped at this level and held. Heot-work will continue at this temperature for some time, cones will continue to go down, body ond glaze continue ta moture and temperature level throughout the kiln becomes much more uniform, The “holding” period for small kilns (6 cu. ft.) usuolly starts only about holf a cone before maturity and takes 20 minutes to holf an hour; for large kilns (50 cu. ft.) the start is one or two cones before maturity ond may take an hour or two.