Ten Basics of Firing Electric Kilns

electric-kilns-kiln-sitter

Firing is the most critical part of the ceramics process because it is the one thing that makes clay durable, hence ceramic. Electric kiln firing is one of the most common firing methods because electric pottery kilns are readily available and simple to install.

In this post, we are sharing some of the principles of firing and getting the best results with electric kilns. –Jennifer Poellot Harnetty, editor.


From Mud to Ceramic

Firing converts ceramic work from weak clay into a strong, durable, crystalline glasslike form. Ceramic work is typically fired twice: it is bisque fired and then glaze fired. The goal of bisque firing is to convert greenware to a durable, semi-vitrified porous stage where it can be safely handled during the glazing and decorating process. It also burns out carbonaceous materials (organic materials in the clay, paper, etc.). As the temperature in a kiln rises, many changes take place in the clay.

More Science

Heat in an electric kiln is transferred in three ways:

1.) conduction heat transferred through physical contact
2.) convection heat rising through the air
3.) radiation heat emanating from all the kiln elements

Electricity passing through coiled heating elements (made especially for high temperatures) generates radiant heat, which rises and is absorbed by everything in the kiln.

Use your electric kiln to its highest potential!

The fourth edition of Electric Kiln Ceramics, has been completely rewritten, reorganized, and expanded by Frederick Bartolovic. Loaded with new color images that highlight some of the most beautiful results possible with electric firing, the new edition features step-by-step instruction on forming and finishing pieces for electric firing, schedules for firing both manual and computerized kilns, and even glazing techniques and recipes to try out in your electric kiln!

Learn more and download a free excerpt!


How Hot

pyrometic-conesAll clays and glazes are formulated to mature at certain temperatures. Firing clay too high can cause it to deform or even melt, too low and it will not be durable. Firing glazes too high can cause run-off on the pot, too low and they will be dry and rough. To fire to the right temperature, pyrometric cones are used. Cones are made from various oxide mixtures and bend at known temperatures. In general, the following cones are used in the pottery studio: bisque fire (cone 08-05), low fire (cone 06-04), mid-range (cone 4-7) and high fire (cone 8-10).

Using Pyrometric Cones

Cones are used in every firing. Typically, a three-cone system (either large or self-supporting), consisting of a guide cone that is one cone below the target temperature, the firing cone and a guard cone provides the best information about the firing. Bar cones and small cones are used in a properly adjusted Kiln-Sitter(r), an automatic shut-off device. While the three large cones are not required for kilns equipped with a KilnSitter or an automatic controller, they do provide a second point of reference for how a kiln is operating.

kiln-sitter

Get Ready to Fire your Kiln

Before firing any kiln, vacuum it out if necessary-bottom, sides, element channels and lid. Check the elements for breaks, and chisel off any glaze drips on the shelves. Visually check the electrical cords and connections. Make any repairs required (see owners manual or call your local supplier for service).

Kiln Furniture

An assortment of kiln furniture is needed to hold and support ware during a firing. Furniture consists of shelves, posts, stilts and tile setters made from refractory materials. Kiln furniture is designed to withstand the repeated heating and cooling to high temperatures without deforming.

The Bisque Load

Loading a bisque kiln is a fairly simple task, but there are some basic rules. Fire full loads to take advantage of conduction heating and also save electricity. All work should be bone dry . If the work is cool or cold to the touch, it is not bone dry. Handle all work very carefully because it is extremely fragile at this stage. Place the bottom shelf on 1-inch stilts to aid circulation, and keep ware 1 inch away from elements, walls, thermocouple and KilnSitter (figure 6). Unglazed pieces may touch each other. Place a small cone in the KilnSitter and/or a cone pad on the middle shelf. Fire to cone 08-05, depending on the type of clay and amount of porosity you want for glazing.

Firing is a potentially hazardous activity and all students must obey safety rules to avoid injury. Instructors must read and understand all the safety information that came with the kiln, and assure that the kiln is properly installed and maintained. If a manual is not available, many companies post them online or you can request a replacement copy from the manufacturer. For operating the kiln, students must:

  • Turn off kiln prior to loading or unloading. Disconnect the kiln for any servicing or when kiln is not in use.
  • Do not touch heating elements with anything since they carry high voltage.
  • Do not place any combustibles within 12 inches of any surface of the kiln.
  • Do not leave kiln unattended while firing.
  • Never look into a hot kiln without properly tinted safety glasses (e.g., welder’s glasses). Sunglasses only block ultraviolet light.
  • Make sure the ventilation system is working properly
  • Never add extra insulation around a kiln to conserve energy. Extra insulation can cause the wiring and the steel case to overheat.
  • Remove all tripping hazards. Keep the power cord out of the way.
  • Do not fire with cracked shelves. They can break during firing, which could damage the ware inside the kiln. Store kiln shelves in a dry area.
  • If you smell burning plastic, turn the kiln off. Examine the wall outlet and power cord for signs of burning.
  • Never wear loose-fitting clothing around a hot kiln.
  • Do not open a kiln until it has cooled to room temperature. Pots may break from thermal shock.
  • Keep the kiln closed when not in use, and never place anything on the kiln lid, even when the kiln is idle-you may forget.
  • Always keep unsupervised children away from the kiln.
  • Do not place any objects under or around the kiln stand.
  • Blocking airflow changes the kiln’s heating characteristics.
  • Remove all flammable materials from the kiln room.

 


This article is included in Electric Kiln Firing Techniques and Tips: Inspiration, Instruction and Glaze Recipes for Electric Ceramic Kilns, which is free to Ceramic Arts Daily subscribers!


**First published in April 2009
Comments
  • What is the general rule of thumb to fire glazed and bisque pots
    I have an electric kiln with kiln sitter.
    1 hour on low with kiln lid 1-2 inch opening?
    2 hours on Med with closed lid then to high until kiln shuts off?

    Is there some generl rules somewhere to follow?
    Thank you, please give me your thoughts.
    Dee

  • I am returning to pottery after 20 years, so your article is an invaluable refresher for me.
    thank you!
    Yvette

  • I too have returned to pottery after a long absence. Just as I get going, my friend with the kiln is moving! So I am considering getting an electric kiln of my own. I live in Denmark, and the kilns here do not work with the “cone” scale, but degrees centigrade. Can anyone tell me why the pottery world is divided – the cone users versus temperature control on an understandable level?! And can anyone tell me if there is guidance somewhere to compare the two – like in a cook book which gives Imperial Measurements alongside Metric Measurements?
    Finally, any advice on buying new against buying second hand? Do kilns last a reallly long time if you look after the elements/replace them? I would so welcome any advice on this subject…
    Melanie

  • Orton cones are not really a seperate measurement from degrees celsius each orton cone melts to a specific temperature in degrees so you use the cones to make sure your work is firing to the temperature it should if you google an orton cone temperature chart you should be able to see which cones equate to what temperatures in degrees celsius

  • Please could you help me get started with an appropriate firing schedule for a bisque and glaze firing for the following kiln.
    I am brand new to Firing a kiln and I have been given a electric kiln that was built thirty years ago but was never used. It has no manual. I am unfortunately still not familiar (after much reading) with all the related terminology so bare with me please. My control panel has the following: a mains light on light; a kiln firing light; a switch with an auto soak label below it and auto off label above it; a round knob with the digits one to six around it; and another round knob with 100 degrees celcius up to 1300 degrees celcius above which is a red needle over another horizontal indicator band in degrees centigrade. The kiln has a little hole with plug in the roof and another small hole in the door at the front. this kiln also has a thermocouple in the back wall. It is a front loading kiln.
    I need to learn how to do a bisque firing to 1000 degrees celcius (cone 6). I have purchased the appropriate cones for this but the whole in the door will not allow for viewing of all three cones!
    I understand this is just the beginning of a big learning cure and many trial and error sessions but I would greatly appreciate your assistance in my first firing by giving me the basics to start from, many thanks Lynda-Anne (South Africa)

  • I too am new to kiln firing. I have purchased a second-hand Duncan kiln, have just had it checked by the electrician, given the green light, and am ready to go. Thank you for this article; it’s the clearest set of basic instructions I’ve found so far. I’ve been very confused with the cone instructions, but I think it’s an experiential thing. Will start with bisque firing several types of stoneware and see how I go. Wish me luck!

  • Thank goodness I found this article! Reading articles about firing and seeing references to cones and guard cones and kiln sitters was very confusing. I just bought a used kiln and after reading the manuals was still very unsure. This clears up a great deal. Thank you!

  • This is really helpful for someone new to firing in an electric kilm. I am just starting to make pottery full time and was given an old Crusader kiln that is made mostly of fire brick (which crumbles), and is likely 30 years old or more. I do have the manual but its poorly written, so having some backup info is reassuring. I have yet to test it out, but hope to this week.

  • I HAVE AN ELECTRIC KILN..COMPUTERIZED..BUT LATELY I HAVE BEEN GETTING PIN POINTS ON THE POTTERY ESPECIALLY ON THE INSIDE..I’M FIRING THE SAME I HAVE FOR FIVE YEARS..SOME TELL ME NOT FIRING HOT ENOUGH ..SOME SAY TOO HOT..SOME SAY GLAZE TOO THICK..WHAT DO YOU SAY?

  • Have you tried (on a hidden part of the pot) to scratch it hard with a pin? If it makes a mark, it is not fired, if it is rock hard and the pin does not leave a groove, you have fired it already.

    However, to answer the question you ask – you won’t do any damage firing it for a second time at the bisque temperature. In theory you will still have the possibility of firing it at a higher temperature with glaze. Hope that helps. Good luck!

  • Do a test….plink with you fingernail a bisqued fire pot, and do the same with non fire. The bisqued fire will have high pitch bling sound and the non bisqued will have a lower thud sound.

  • I had the same problem about the pinholes, I found out that there is a huge difference in some glazes that should be brushed on rather than dibbed into the glaze. I was dipping when I should have been brushing on the glaze.

  • Vern Newbie!

    Can Clay be fired in a glass fusig kiln?

    Where do I find the cones that melt when the kiln is at the proper temp for clay firing?

  • Your site is just the greatest…. so much good information… and very helpful.

    Thanks for doing this. Much appreciated.
    Jay.

  • i am a novice potter, and have run into a problem that i cannot solve. i use a clay body that fires at cone 6. until the last two firings, previous firings have worked out well. now, my electric kiln does not seem to be reaching temperature to turn the sitter off. it will run 48 hours or more if i would allow it to. i finally turned it off, and the clay did not reach maturity. how can i find out at what temperature the kiln is firing, how can i determine how long it should take for the kiln to reach temperature, could outside ambient temperature change the firings so abruptly, or any suggestions what else could have changed it, and lastly, can i refire the clay that did not mature?

  • I am a porcelain painter and now I would like to go into ceramic art. My question is does my electric digital kiln do the ceramic firing as well, but under different heat temperature?

    Thank you and I appreciate your cooperation. Your answers will be the judge to go for ceramics or not.

    Nabila

  • Nabila: my understanding is that porcelain firing is higher than other ceramics, therefore if your electrical digital kiln goes to above circa 900 degrees celcius for earthenware or 1250 for stoneware, the answer is yes.

    Barbara
    its roughly the same answer – if your glass fusing kiln goes upto 900 degrees (earthenware) or 1250 (stoneware) the answer is yes. If not (which I think may be the case) then, no.

    Hope this is helpful.

  • Just unloaded a whole kiln full of earthenware glaze fire and almost every piece has blisteres or pin holes in the glaze. Several different glazes were used, but all on the same clay, all bisque fired in the same load. Could it be my bisque fire didn’t get hot enough. I used a a ^04 in the kiln setter, but my witness cone, 04, barely bent. Any thoughts on what to try? Can I try firing it all again at a higher cone?

  • I HAVE OLD BOOKBINDERS GOLD SPOOLS..14K I AM TRYING TO USE AND EXPERIMENT WITH GLAZE. I HAVE A POUND AND A HALF. ANY HELP WOULD BE WELCOME. KEN VALOIS

  • I had to turn my kiln off before the cone was fully bent. It was a cone 6 glaze firing. Should I ref ire or will this affect the glazes?

  • I have a nude sculpture I made 20 years ago never fired. Still in green state. Can I still fire the piece?

  • If I have fired a pot and can’t stand the glaze, my fault, terrible application. Can I apply more glaze and refire the pot?

  • I have a brand new Olympic Kiln that fires to 2350 degrees. It allows for 6 different settings to be manually programmed depending on what you are doing (i.e. enameling, metal clay, etc.) I have Buncombe White Clay, which fires at Cone 6 and is a mid-range clay from what I have been reading. I would like to make clay beads and pendants, but I have not the slightest idea as to go about firing these small items. I know my learning curve here is going to be significant, as I am just starting out, but any advice on how to go about getting started would be wonderful. I have already been enameling and that has gone well so far, with a few misshaps here and there. Like I said, my learning curve is going to be significant! Thank you and Cheers!

  • I, too, am having problems with pitting. Where do I find answers to the previous posts??

  • I have over fired my greenware, (cone feel off of sitter) is there something I can put on my wares to make the glaze stick?

  • Wow. This nice posting sure attracted a lot of questions. I’m not spotting many answers. This is probably because the people who know the answers don’t click into articles on “ten basics”. I just moved to firing my own work this year and so have had many of the same questions. Firing number 19 is running as I type this. They are coming out well. Here are answers for the questions that I have figured out so far.

    Welder glasses – I called the pottery supply place, asked, and bought what they recommended. Sorry – don’t recall the result. They are helpful.

    Beads & Pendants – The pottery supply shops sell stands and wire which is able to withstand kiln temperatures. You string your hanging items on the wire and fire it suspended.

    There should be no problem with firing old greenware.

    Underfired glazes are often disappointing. You can refire the pieces to the right temperature to get better results. They won’t necessarily look the same as if you had fired them correctly in one session because of how overlapping glazes behave. (There can be more blending in the two fire case.)

    I don’t think there is any general problem with firing pieces multiple times. Some people do this intentionally to build up complicated layers of glazing. It costs more than firing once and that’s a reason people avoid it.

    If your kiln is running 48 hours then some elements are not heating. The new post talks about how to visually check whether an element is burned out.
    http://ceramicartsdaily.org/ceramic-studio-equipment/ceramic-kilns/how-to-replace-electric-kiln-elements/
    I have also found that the switches on my kiln don’t always turn on. The kiln sat for a while before I bought it and a small layer of rust can form on the switch contacts. I have found that just switching on/off several times can cause enough friction rub it off and get a good contact again. When the kiln is cold I can hear the elements turn on (they hum with the power.) Before I start my firing I check that each switch creates a hum by itself. This is harder to hear later when the kiln is up and running, fans are on, etc. If I clear them before the firing they usually come on easily during the firing.

    Re: Pinholes – I leant my copy of “mastering cone 6 glazes” to a friend, so can look up their advice. I do recall them saying that the holes are caused by material in the glaze burning out. Glazes with a lot of material that burns are more subject to the problem. The “Try a thinner application” comment below sounds good. I think another option is to cool the kiln more slowly. If you let the kiln turn itself off at the top of the temperature range then the temperature will drop quickly. If you “fire it down” then the gasses in the glaze may have time to bubble out and smooth over again.

    There were a couple questions on firing schedules. This describes what I’m doing. I have a small Gare kilnsitter with kiln switches and 5 manual switches (just on/off not high/low/off). I was able to find a manual online. My firing schedule is roughly the following for a cone 6 glaze firing:
    – One switch on with lid cracked for a couple hours to dry any damp glaze (probably not necessary.)
    – Eventually, close the lid & start turning on another switch each hour
    – The manual says to leave a peep hole open during the hole process. This is so that vapors cooked out of the clay and glaze don’t come out through the kiln-sitter mechanism and eat away the metal parts. I built a down-draft fan that I installed and so I run this with the peep holes closed.
    – I’ll hit cone 6 five hours after I turn on the second switch.
    – The Kiln Sitter with a cone 6 bar turns off too soon. I turn lift the shut-off mechanism gently and turn it back on and watch the “real” cones through my peep holes (not easy.) I’ve considered getting a cone 7 bar to see if that would give me “true cone 6” results.
    – When the cones look good, I turn off all switches for 15 minutes. This gives a quick drop in temp.
    – After 15 minutes I turn on the top and bottom switches. This gives a slow cool of the glaze through a critical temperature zone (see the Mastering Cone 6 Glazes or online lectures by the authors.) I chose top & bottom because they are buy the massive lid and floor which suck heat. This slow cooling runs for about 4 hours. After that all switches go to off.

    I bought an inexpensive thermometer, drilled a hole and installed it. Search eBay for “Digital Pyrometer”. Mine was $66 including postage. This has been a great help in understanding what my kiln is doing. I think the temperature reads a little low (based on how the cones behave) but I can see the general trend for how temperature is climbing. This helped me decide how many switches to leave on to bring the kiln down slowly. Too many and it stays level. Too few and it would drop too quickly. Well worth the price given the cost of clay, glaze and time.

    Lynda Anne wrote about the difficulty in seeing all of the cones through the peep hole. Mine is small too. I set them almost in a line pointing away from the peep hole. The cone 5 is closest & cone 7 farthest away. If they were in a straight line away from the hole I would see just cone 5. I move the back slightly to the right so I can see a piece of all three. During firing cone 5 will tip towards me and out of sight. I’ll be able to see just a piece of two then. When close to done I’ll see cone 6 tipping towards me and 7 standing behind. These can be hard to see because the kiln glows so brightly at cone 6. The glasses help create some contrast. You can also blow in through the peep hole to push glowing air out of the way and catch a glimpse of your cones.

    Melanie asked why some people refer to cones and some refer to temperature. Cones are more reliable because they behave just like your glazes are going to behave. Cones (and glazes) don’t melt at a specific temperature. They bend based on time and temperature. If you stopped short of the target temperature but held it there longer the cone would react and slowly bend over. This is OK because your glaze is reacting in the same way. The Orton cone temperature charts show the temperature associated with each cone. They typically have several columns based on whether your kiln is firing fast or slow. (Fast meaning the temperature is climbing quickly.) If the temp climbs quickly it will climb higher before the cone is done. If it climbs slowly the heat is applied longer and the cone (and glazes) will finish at a lower temperature. When a kiln is new the temperature can climb faster than when the elements are old. If you fire with cones you’ll get good results through this change. If you fire based on temperature you could overcook the later slower loads.

    Another reason to use cones is that you can put a set on each shelf of the kiln. My thermometer measures from one spot. The different shelves can behave differently (because of spacing, kiln elements per area, pottery mass, top vs. bottom.) I like getting evidence back from all 3 shelves. (I have peep holes to see two of these. The center shelf is a mystery until I unload.)

    One additional thing to note is that the cones and glazes continue to “cook” after you turn the power off. All of the mass of the pottery and kiln keep cooking each other. You want to turn the kiln off before the cone looks perfect because of this extra cooking that will take place in the minutes after.

    Three cheers for the Ceramics Arts magazines and website.

  • Carrie,
    Didn’t see an answer to what to do when you over fire your greenware but this is what I was told and seems to do the trick…but some old fashion, cheap hairspray and spray a lite coating on it, let dry then glaze…seems to do the trick! Hope that helps!

  • Hello!!!!

    I am working in my first bjd in porcelain… but, I do not know how I must put the painted doll parts in the kiln to prevent them from sticking in the shelf … that I must use under them? stilt? Blanket? Sand? (I will use engobe and glazes)

    I am so disoriented!!

    Thank you for your help

  • Hello!!!!

    Can I ask you for a little help?

    I am working in my first bjd in porcelain and other miniatures… but, I do not know how I must put the little painted doll parts in the kiln to prevent them from sticking in the shelf … that I must use under them? stilt? Blanket? Sand? (I will use engobe and glazes)

    I am so disoriented!!

    Thank you for your help

  • I am new to the process of running a kiln. I have a Paragon Kiln sitter. This is my question.

    I fire the second firing and first glaze firing last week end at cone 6, which did not complete it cycle because after 13 hours I shut the kiln down. My pieces had not all glazed up. After getting some feedback, I put the pieces back in at a cone 8. My kiln has been running since 6 pm last night. It is now 11:30 am the next day. My sitter has not shut down yet. I’m concerned because it has been 17and 1/2 hours. How long does this process take and should I shut it down? thanks for any help. Diana

  • I am new to the process of running a kiln. I have a Paragon Kiln sitter. This is my question. I fire the second firing and first glaze firing last week end at cone 6, which did not complete it cycle because after 13 hours I shut the kiln down. My pieces had not all glazed up. After getting some feedback, I put the pieces back in at a cone 8. My kiln has been running since 6 pm last night. It is now 11:30 am the next day. My sitter has not shut down yet. I’m concerned because it has been 17and 1/2 hours. How long does this process take and should I shut it down? thanks for any help. Diana

  • I had this problem and found that one of my switches had burned out. We replaced it and it works fine now. I refired some of my pieces and had mixed results. Some improved some didn’t.

  • Go to the Orton cone website, you can download a chart which shows Farenheit, Centigrade and cone equivalents…

  • I have got pinholes inside my ware for 3 reasons.
    1. The lid was closed the whole time and gasses could not escape.
    2. Flecks of kiln wash fell on the ware. I now wipe off the bottoms of shelves when loading the kiln.
    3. Clay dust from touching up dry ware settled in the bottom. I rinse bisqueware out if I know it might have shavings in it.

  • Wondering if anyone knows if a stoneware glaze firing that kiln misfired to 2175 degrees instead of 2232 degrees would be food safe or if another firing is required. Thanks bunches 🙂

  • Cones measure ‘heat work’ not just temperature. Heat work is a function of temperature x time. For example, if you fire the kiln super fast to a particular temperature it will achieve less heat work than if it was fired slowly to that same temperature. It is like the volume of water you get out of a tap which is a function of the rate of flow x the time you have the tap on. Filling a particular bucket will be a function of the rate of flow and how long you leave the tap on. Think of the maturing of your glazes as filling the bucket. Only cones measure this heat work accurately, and so are far better than just relying on a temperature reading.

  • it could be any of or a combination of the possibilities you mention as well as; dusty surface, bisque temp not high enough, change in chemical composition of your glaze or clay materials, not soaked long enough . Trial by elimination to see what the problem is. When I have this problem it is usually that it was in a cooler part of the kiln and/or needs more of a soak at top temp.

  • a greenware pot is much softer than a bisqued pot. It is usually heavier as well (due to chemically bonded water) and much less porous. It should be obvious.

  • If you flick a pot with your finger, a greenware pot will only give a sound like a “thunk” whereas a bisqued pot will ring. Be careful where you flick it as greenware is fragile.

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