Why does my kiln seem like it overfires the pottery?

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  • See the Sales FAQs for Frequently Asked Sales and Preorder Questions
  • The Knowledgebase is organized into a series of questions and answers having to do mostly with technical troubleshooting and understanding of kilns.
  • Although we write this for our own kilns many of these articles apply to other makes - although L&L takes no responsibility for that.
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Why does my kiln seem like it overfires the pottery?

The work is overfired but there is no error code. Here are some things to look for:

  1. Cones placed wrongly in cone pack- angle is off
  2. Small or wrong cones used
  3. Fired to the wrong cone
  4. Firing with a hold time at the end
  5. Firing a Vary Fire program to too high a temp for the rate of climb
  6. TC offset adjustment needs to be set higher
  7. TCs reading unevenly (from age or loading unevenly or too close to TC) make it take longer to even out(must be firing and hot to see this; press 1,2,3),
  8. Old elements have more and more trouble responding to uneven temps- (from TC age or from uneven loading or loading too close to the TC) this makes the kiln take longer and can make cones look like an overfire
  9. Kiln-sitter (if used) out of adjustment
  10. Kiln-sitter (if used) blobs the cone instead of bends it- replace kiln-sitter tube
  11. Lid/ Hinge out of adjustment- opens a bit in the front when hot.

Additional Actions to Take

Why Calibrate Your Kiln?

  • Most people will be satisfied with the operation of their kiln right out of the box.
  • However for the most precise or accurate work a kiln needs to be calibrated or adjusted to match the load, the temperature that you typically fire to, the unique characteristics of the kiln, and the thermocouples.
  • It is important to establish a relationship between you, your kiln and your work.
  • Although we use the word “calibrate” or “tuning” really what is called for is an understanding of the heating characteristics of your particular kiln, and understanding of how thermocouples and controls work and their limitations, how you load your kiln, the clay and glazes that you use, how pyrometric cones work, and the basic concepts of heat-work. 

The heating characteristics of your kiln

  • Each kiln is different. They vary from brand to brand, kiln series to kiln series, kiln size to kiln size, and even for identical models from kiln to kiln.
  • The age of the elements, the insulation thickness, the distance from the center of the kiln to the elements, the kiln height, how tight the kiln is, if there is a vent, the condition of the thermocouples as they age, whether there is a protection tube on the thermocouple, etc. all make a difference.
  • Even external conditions like voltage and temperature in the room affect the firing.
  • The controls used in most kilns, while perfectly acceptable for ceramics, are not military precision instruments.
  • A kiln is not a standardized “instrument” – it is more like a violin that needs to be tuned and understood.

Controls and Thermocouples

  • The controls in ceramic kilns are typically accurate to about 1% of scale vs. something like .1% of scale in a precision industrial control.
  • The thermocouples can read +/- 10°F from actual temperature even when when they are brand new. (This is a function of the variation in materials in the thermocouple. Even though L&L uses "Special Limit of Error" wire, there are still all kinds of variation in the thermocouples, the thermocouple circuit, and the cold compensation on the control. See this on Wikipedia for more information on thermocouples.
  • In addition to this Type K thermocouples, the most common ones used in ceramic kilns, will drift over time, especially when used over 2000°F.
  • Type S platinum thermocouples do not drift at those temperatures. Although they are available as options on most L&L kilns and most other brands, they cost on the order of $230 each.
  • The thermocouple protection tube that L&L uses introduces a known 18°F offset into the thermocouple reading. Although it is a known constant and is adjusted for, the exact amount can vary slightly based on individual tubes (this does not change with time).
  • The temperature that you see represented on the control will probably not be completely accurate (and even if it were it is only one piece of the information you need to understand what is happening in the kiln).
  • On the other hand it tends to be consistent over time (except for the thermocouple drift). Hence, when you “calibrate” the control to meet the requirements of your work, the experience of many thousands of customers tells us that you will get consistent results firing to firing, all other things being equal.

How you load your kiln

  • How you load your kiln makes a huge difference in how heat reaches and affects your work.
  • For example, fire an empty kiln and you will probably get error codes.
  • Fire it too densely or too heavily in the top or bottom and you might get different error codes.
  • Ideally you will load it with a reasonable amount of work, not too close the elements, and distribute the work as evenly as possible.
  • Don’t put a shelf right next to an element. Most of the heating about 1200°F is radiant heat. Because not all of the work is exposed directly to the elements you need to make space for the convective heat to circulate. Also you need to allow time for the heat to get from the wall elements to the inside of the kiln while not overheating the work towards the outside.
  • There are guidelines for loading a kiln but this is more art than science.

Clay and Glazes

  • Talk to your clay and glaze suppliers to find out their firing recommendations.
  • Some glazes can be very fussy about heat-work, others not so much.
  • The main point, however, is to “tune” or “calibrate” the kiln to the appropriate level of precision that you need. Don’t overdo it.
  • Also be aware that clay and glaze vary with different batches.


  • Ceramics and glazes develop from heat-work, which is a function of time and temperature.
  • So, for any given temperature, the amount of time spent at that temperature would determine how much heat-work is done.
  • Looking at one part of this equation can be misleading.
  • Assuming you had a perfectly known temperature you would then have to determine how much time at that temperature will give you the right result.


  • Pyrometric Cones are probably the best way to “see” ceramic heat-work. They represent and reflect the actual ceramic process taking place. They are far more representative of what you want to see than temperatures as shown on the control.
  • Even pyrometric cones are telling a story that is only representative of what your work will experience.
  • Where they are placed in the kiln can make a difference, for instance.
  • They will always need to be interpreted based on your experience with your own work.
  • Calibrating the kiln with cones is your best chance of coming close to what you want.
  • Remember: The kiln calibration only has to be as good as what you need to produce the work you want – not what the cones say.

Repeatable Results

  • Given all the variables involved the only way to get completely consistent results is to repeat the same process, with the same time-temperature cycle, with a kiln that changes as little as possible, and with the same materials as possible.
  • This is how industrial ceramics is done but with far higher precision and with process engineers using statistical process control.
  • Most art ceramics has a much higher degree of variability.
  • There are exceptions to this rule and those who pursue those exceptions need to and often do put a tremendous amount of time and effort into this achievement. Some of the crystalline ceramic artists out there are good examples of this.

How to Calibrate Your Kiln

Tools for Calibrating your Kiln

  • There are two basic tools to calibrating your kiln: thermocouple offset and cone offset.
  • Pyrometric cones are used as the main tool to "see" what is happening in the kiln.



  • Sometimes a new kiln does not get to temperature during the test firing.
  • This is generally because of an empty kiln.
  • It is generally a good idea to fire with your kiln furniture to put some mass in the kiln even during the test firing.


  • One difference between an empty and full kiln is that an empty kiln cools a lot quicker which will freeze the cone very quickly. In a full kiln there is a lot of mass in the kiln that is just as hot as the kiln around it. It is this mass (the load in the kiln), which is radiating it’s heat as well, that will continue to melt the cone for a little longer after the kiln has been shut down.
  • Once the kiln is fine-tuned, it is the variable of how you have loaded the kiln that will account for many of the variations you will see from firing to firing. 
  • Loading will affect the speed of firing - an empty kiln will fire differently than a full one. Although the control does compensate for this that compensation is not totally perfect.


  • The kiln cannot be calibrated until it has reached temperature and affected a witness cone. The reaction of the witness cone to the firing is how you begin the calibration process. (Kilns are not fired before they ship).
  • You can fine-tune how the kiln reads temperature by adjusting the Thermocouple Offset. (See below for how to do this).
  • Fire the kiln with witness cones. Use the most typical or critical cone you fire to. Typically the most important cone you will fire to is your glaze temperature. Generally, bisque temperatures are less critical. 
  • Thermocouple offset will change the reading of the thermocouples the same amount from room temperature to the maximum temperature of the kiln. (Cone Offset, which will be covered later on, will adjust how the kiln reacts to a particular cone/temperature).


  • Adding thermocouple offset lowers the temperature in the kiln (relative to the temperature reading on the control. For instance lets say the control reads 2000°F and you add 10°F of thermocouple offset. Now the control thinks there is 10°F more in the kiln and, for a setpoint of 2000°F, it will actually control to 1990°F in the kiln. 
  • Subtracting thermocouple offset raises the temperature in the kiln. For instance lets say the control reads 2000°F and you subtract 10°F of thermocouple offset. Now the control thinks there is 10°F less in the kiln and, for a setpoint of 2000°F, it will actually control to 2010°F in the kiln. 
  • Thermocouple Offset affects both Easy-Fire programs and Vary-Fire programs on a Dyna-Trol or Bisque & Glaze Programs and Custom Programs on a One-Touch (Cone Offset, on the other hand, only affects Easy-Fire programs and only on a Dyna-Trol).
  • All temperature readings that you see in your control are interpreted by the control from the thermocouple signal. Thermocouple readings can be inaccurate from thermocouple drift or signal change through many factors like inductive current. The only absolute that you can have good confidence in is a witness cone. When we change the thermocouple offset we are not changing the setpoint of the control only the temperature interpretation with thermocouple offset.
  • You can think of the control setpoint as the other absolute and the thermocouple signal reading as the one variable that gets changed.
  • When we add temperature to the thermocouple signal we make the control believe the kiln is hotter than it would otherwise think and it will shut off sooner and fire cooler. 
  • When we subtract temperature from the thermocouple signal we make the control believe the kiln is cooler than it would otherwise think and it will shut off later and fire hotter. 


  • If the witness cone bent slightly during the first firing, but no more than a little bit, then start by reducing the thermocouple offset setting by 5°F to make the kiln fire slightly hotter.
  • If the witness cone did not bend at all, then you can start by reducing the thermocouple offset setting by 10°F to make the kiln fire hotter.
  • If the witness cone bent a little too much, you might wait and see how it does with a full load, or you could start by increasing the thermocouple offset by 5°F to make the kiln fire a little cooler.
  • If the witness cone bent more than a little bit start by reducing the thermocouple offset settings 10°F which will make the kiln fire cooler.
  • If the witness cone collapsed start by reducing the thermocouple offset settings 15°F.
  • There are beginning suggestions - feel free to experiment outside of this.


  • To determine an good starting point for thermocouple offset (or cone offset) you can fire the kiln a cone or so hotter than your witness cone.
  • Then carefully observe when the cone bends and note the temperature displayed on the control at that moment.
  • Compare this number with what it you think it ought to be using an Orton Cone Chart. You will have to have some idea of the speed of firing to get an accurate number.


  • Turn kiln on with toggle switch. Wait 5 seconds.
  • Press 1, wait 5 seconds. The kiln display will say STOP and then go into IdLE mode.
  • Press OTHER about eight times until you see TCOS
  • Press ENTER. See TC 1
  • Press ENTER again
  • It will flash between °FOS (which stands for Deg F Offset) and 0018 (The 0018 stands for a thermocouple offset of 18°F - which comes preprogrammed into the control to compensate for the ceramic protection tubes. By changing the offset to 0010 we are REDUCING the offset by 8°F- making it fire 8 deg hotter). (NOTE: On older kilns with a slightly different composition thermocouple protection tube the preprogrammed value is 0050).
  • Press 0008 to reduce thermocouple offset by 10°F.
  • Press 0013 to reduce thermocouple offset by 5°F.
  • Press ENTER to accept your input.
  • Do the same for all your thermocouples. The prompts will scroll past in the order of TC1, TC2 and TC3.



  • Access the Thermocouple Offset by getting into the Options.
  • Options are accessed by holding the ENTER button while turning the power onto the control (turn on the kiln with the toggle switch) and continuing to hold onto the ENTER button until EdIt is displayed.
  • This activates the Options Menu.
  • The first thing you will see after turning the power on while pressing ENTER is LL-G or 1t-1 (This is the software version).
  • Then you will see 1288 if it is a Cone 10 control or 1249 if it is a Cone 6 control.
  • Then you will see EdIt and you will hear a beep. You can now let go of the ENTER button.
  • Press ENTER again. You will see °F or °C.
  • Press ENTER again and you will see OFFS
  • Press the UP arrow to enter a positive offset OR press the DOWN arrow to add a negative sign to the offset. All this first button press does is get you in the right range.
  • You can NOW use the UP or DOWN arrow to change the value of the offset. 
  • When you are in the positive range you can only go as low as 0000. The UP arrow increases the offset setting and the DOWN arrow decreases it.
  • When you are in the negative range you can only go as high as -000. The UP arrow, when you are in the negative range, is actually moving the setting in the negative range. For instance if you start at -000 and hit the UP arrow you will be at -001 (which is negative by one degree). If you then hit the DOWN arrow you will go back to -000 (which is actually in the "postive" direction). Some of this is counterintuitive so it make take some getting used to by just experiementing.  
  • Once you are done with your setting hit ENTER and in a few seonds the control will flash between IDLE and some temperature and STOP.
  • If you want to redo the offset setting you have to turn the control off and start over.
  • The control comes with a pre-programmed +18 Deg F offset to compensate for the thermocouple protection tube. 
  • Note that the control does not convert the offset setting to deg C or F. If you are operating in Deg C then the offset setting should be +8 Deg C.
  • Note: if you first press the DOWN button you can only set a negative value or if you first press the UP button you can only enter a positive value. 
  • You can go back and change this later if you make a mistake.


  • There is no Cone Offset Option on the One-Touch Control


  • Note that you can use different Thermocouple Offsets for the three (or two) different thermocouples. 
  • This allows you to make the top or bottom hotter or cooler to even out the temperatures in a kiln.


  1. Tune your kiln using the Thermocouple Offset for your most critical firing (typically glaze firings). 
  2. Typically bisque firings are not very critical. 
  3. CNOS (Cone Offset) - is used to fine tune what the Dynatrol thinks the final cone temperature should be in EASY-FIRE programs. The final cone temperature can be raised or lowered a maximum of 99°F (or 55°C). When entering the offset temperature the following code is used: the left two digits designate whether to raise (00) or lower (90) the cone temperature, that is, “00” means plus (+) and “90” means minus (-). The right two digits are the number of degrees the cone temperature will be raised or lowered. This offset will remain programmed only for the specific cone number until you reprogram the cone offset differently





Raise the final cone temperature by 20°F


Raise the final cone temperature by 40°F


Raise the final cone temperature by 15°F


Lower the final cone temperature by 30°F


Lower the final cone temperature by 5°F


Lower the final cone temperature by 45°F

NOTE: This option does not affect the VARY-FIRE (Ramp-Hold) mode but it will show up on the menu.


  • The Cone Offsets come preprogramed. From cone 022 to cone 017 the cone offsets are set at 9020. All other cones are preset at 0000.
  • Note on Blue DynaTrols made before Oct 1 2004 the cone offset was 9030 for cones 022 to 017 and 9020 for other cones. (The offsets were changed when we switched to a more responsive thermocouple protection tube).
  • You can always change offsets.
  • The RESET option in Other menu will NOT reset these cone offset settings.
  • This is part of the compensation necessary for the mullite thermocouple protection tubes.

Cone Offset Example: Adjust cone 07 to shut off the kiln at 20°F below Orton’s prescribed cone temperature.






If CNOS does not show on the display, press the Other key until CNOS displays.


Alternately flashing:
CONE & #

Cone Offset has been selected; the word CONE and the last entered cone number will alternately flash on the display. Now enter the cone number which you want to adjust (in this example cone 07)


Alternately flashing:
CONE & 07

The word CONE and the entered cone number (07) will alternately flash on the display. If you type a wrong number, press 0 three times, press ENTER, then type the correct number.


Alternately flashing:
°F0S & 0

°F0S and the previous offset setting alternately flash. Enter the new offset temperature using the rules above, in this example, 9020



The selected offset temperature is displayed. If you type a wrong number, press 0 four times, then type the correct number.


IdLE flashes then the current temperature

IdLE appears indicating the offset temperature adjustment has been accepted. The current temperature then flashes in the display.



See our various instruction sheets about cones, specifically troubleshoot-cones.pdf.

  1. Make sure you have programmed the kiln properly and it is supposed to be firing.
  2. Read the Operation instructions.
  3. Do a Program Review as soon as you start firing. (For DynaTrols - see this video)

Use cone packs in all sections (top, center, bottom) of the kiln and keep records of what happens.

  1. Unplug kiln.
  2. Remove or hinge open the control box.
  3. Remove the Thermocouple Lead Wire from the Thermocouple.
  4. Unscrew the Thermocouple from the kiln.
  5. Remove Thermocouple.
  6. Install a new Thermocouple and screw in place.
  7. Replace Thermocouple Lead Wire and tighten. Be sure to get red matched to the MINUS (Negative) sign and the Yellow matched to the PLUS (Positive) sign.

See this video:

Resistance and Error 1

  • The most common cause of kiln slowdown, E-1 messages, and failure to reach temperature is element wear. As your elements age they generally increase in electrical resistance.
  • When resistance, measured in Ohms, increases, both Watts and Amperes (amount of power) will decrease, assuming Voltage remains constant. If you don't have enough power, your kiln will fire slowly and might not even reach the desired temperature.

What does this mean?

  • Using resistance, we can tell exactly how much power your kiln has lost over the course of your element's life.
  • For example on an e23T that uses 240V, a brand new kiln section would read about 14.5 ohms. If you measured this same kiln section after many cone 6 firings and the reading was 16.5 Ohms, you would know that this section of elements has lost approximately 14% of it's power (16.5/14.5=1.138, or close to 14%).
  • A very general rule of thumb is that most people will typically begin to notice some slowdown once you've lost more than 10% of your power.
  • It will certainly vary based on the kiln you have, your voltage, as the types of firing you do. People only doing low fire work will continue to get by on lower power than those needing to go to higher temperatures (cone 6).


In these series' of kilns a piggy-backed control panel covers up the element terminals.

  1. Turn the power to the kiln completely OFF and unplug it if possible. If it is direct wired, then you should at least turn off all power at the disconnect switch or circuit breaker.
  2. Open the outermost control panel by unscrewing it either from the element cover box in the case of Easy-Fire, eQuad Pro, School Master and Liberty Belle kilns or from the kiln body in the case of Doll kilns.
  3. Once you open up that control panel you will see the element power wire terminal strip. See the picture. It will have numbered wires coming from the element terminal blocks and wires connecting to the power relays. There are two wires per kiln section/ring, so numbers 1 & 2 are for the top section, 3 & 4 for the middle, and 5 & 6 for the bottom section on a three ring kiln.
  4. Set your multimeter to Ohms (Omega symbol Ω) and using your testing leads, place one in between the two tabs/terminals w/ #1 wires connected. There is a small circular divot that the lead fits into (see picture). Put the other lead on terminal #2 and make note of the reading. Repeat the process for 3 & 4 and then for 5 & 6. Remember that each pair of wires represents one section.
  5. Compare your readings to those on the wiring diagram in your instruction manual. Keep in mind that the ohms on the wiring diagram are per ELEMENT while your reading will be per SECTION. See above for more info on understanding the readings.



In these series' of kilns, the control panel is separated from the kiln body and the element terminals are connected to the controls via external jumper cords or plugs.

  1. Turn the power to the kiln completely OFF and unplug it if possible. If it is direct wired, then you should at least turn off all power at the disconnect switch or circuit breaker.
  2. Unplug the first jumper cord from the control panel.
  3. Set your multimeter to Ohms (Omega symbol Ω) and using your testing leads, place one lead on each of the "hot" prongs. They will be the flat ones.
  4. Make note of the reading and move on to the next one.
  5. Compare your readings to those on the wiring diagram in your instruction manual. Keep in mind that the ohms on the wiring diagram are per ELEMENT while your reading will be per SECTION. See above for more info on understanding the readings.


See this tutorial on how to use a multimeter.

Ohms Per ELEMENT VS Ohms per SECTION (or Circuit).

  • How you figure out the section ohms depends on whether the elements are wired in Parallel or Series.
  • Most kilns are wired in Parallel except for JD230V and most 18" kilns like the e18T.
  • For a parallel kiln you take the per element ohms listed on your wiring diagram and divide by the number of elements per section.

Example #1 (2 Elements in Parallel): e23T 240V 1 Phase: Elements are 28.9 Ohms each. Divide by two because they are in parallel and you will get a reading of 14.5 Ohms per section.

    Parallel element connection for a kiln with two elements

    Example #2 (2 Elements in Series): e18S 240V 1 Phase: Elements are 9.6 Ohms each. Multiply by two because they are in series and you will get a reading of 19.2 Ohms per section.

    Series element connection for a kiln with two elements

    Example #3 (3 Elements in Parallel): JD2927 240V 1 Phase: Elements are 36.5 Ohms each. Divide by three because they are in parallel and you will get a reading of 12.3 Ohms per section.

    Parallel element connection for a kiln with three elements

    Example #4(3 Elements in Series): Doll DLH11-DBX 240V 1 Phase: Elements are 6.6 Ohms each. Multiply by three because they are in series and you will get a reading of 19.8 Ohms per section.

    Series element connection for a kiln with three elements

Fix lid seal if a bright red glow is visible around the seal when kiln is operating and/or excessive heat loss can be felt around seal.

  1. Rub seal high points down with sandpaper until no more than 1/16 of an inch gap is found at any point along seal.
  2. Check for unevenness in the gap that will cause an excessive heat loss.
  3. Replace lid if it is excessively cracked or worn or has holes in it. Replace lid.
  1. Unplug kiln.
  2. Remove the Control Box.
  3. Using a 3/8" nut driver or ratchet wrench or adjustable wrench, remove the nuts that hold the element end onto the Element Terminal Bolt. Note that the terminal bolt head is held in place by an inset shape on the underside of the ceramic terminal block and it will not turn much.
  4. Untwist the element end from around the Element Terminal Bolt. Straighten it out as much as possible.
  5. In most cases the element can be lifted out of the holder at this point. Sometimes, if the element has really disintegrated, you need to remove it in pieces with needle nose pliers.
  6. If element is hard to get out of the holders (because of growth of the element) you can try heating up the kiln slightly so as to heat up the element slightly to just the point where element is slightly pliable–don't let it get red. This will soften the wire. Then turn off the kiln and disconnect all power to the kiln. Using heat protecting gloves and a pair of needle nose pliers pull out the softened element.
  7. From the inside of the kiln, using needle nose pliers, grab the element as close to where it goes through the brick wall to Terminal Block. Pull the element end through the hole. Be careful not to enlarge the hole in firebrick. The brick is soft and will not take much abrasion.
  8. Be sure to check for failure points for evidence of contamination on the element and the element holder. If the element holder is contaminated it will cause rapid failure of the new element. Replace contaminated holders with the new ones.
  9. Using your multimeter check the resistance of the new element.
  10. Install the twisted ends of the elements through the holes in the wall of the kiln. Element ends should be straight at this point.
  11. Pull them up tight up to the wall of the kiln by pulling from outside the kiln.
  12. Lay the element into the groove. Note that the unfired element is going to have some springiness to it before it is fired for the first time. You may need to use a screwdriver to press the element into the holder. YOU DO NOT NEED PINS.
  14. Consulting your picture or labeling, wrap the appropriate element tails around the appropriate element connection bolt, clockwise, one around and cut off the excess tail.
  15. Install the elements and hardware: Place the wires from the jumper cord or connecting wires onto the appropriate bolts and tighten with stainless steel nuts.
  16. A washer goes under the first element.
  17. Twist the first element end CLOCKWISE around the Terminal Bolt.
  18. The next element gets twisted around the Terminal Bolt on top of the first element.
  19. Another washer goes over the Terminal Bolt.
  20. Place a nut on top and tighten it.
  21. Put another washer on.
  22. Put on the Ring Terminal of the Power Lead Wire.
  23. Put another washer on.
  24. Put another nut on and tighten it. How much the nut can be tightened is dependent on how tight the element connection bolt is on the element connection board. A tight connection is very important, but if you tighten too much and twist the element on the bolt too far you could break the element, the bolt, or the insulator.
  25. Reattach the ground wires and the element box if the kiln has them. DO NOT FORGET TO ATTACH GROUND WIRES. IF EACH KILN SECTION IS NOT GROUNDED THIS CAN BE VERY DANGEROUS.
  26. Test the resistance at the jumper cord's plug head or at the other end of the connecting wires.
  27. Reattach the control box, turn on the kiln and make sure all the elements come on.

See this tutorial on how to use a multimeter.

See this video: