Electric Kiln Maintainence

About the Author

Now I've been working at Clay Planet in Santa Clara, California since February of 2003. In May of that year we decided I should act as the kiln repair-man. I went to Skutt in Portland, Oregon and learned all about their kilns: the basics of electrical theory, how to replace elements, and everything about the control box. Afterwards, I started repairing kilns quite frequently. Now it seems like I've got at least two repair jobs per week.

I'm facinated by electronics and electricity. Repairing kilns is very interesting. A kiln is a very simple machine regardless of whether it has a late-model digital controller with relays or an older-style manual controller with switches and a Kiln-Sitter attached. The main purpose of the kiln is to heat up to a very hot temperature, mature your clay and glazes, and shut off.

Table of Contents

Power Requirements

208 or 240 Volts? Single- or Three-Phase?

If you're in a home you'll have 240V, single-phase. If you're in a heavy industrial area you'll have 240V, three-phase. If you're in a school or a community-facility you'll have 208V, three-phase. Always have your electrician check with his volt-meter to ensure proper voltage before ordering your kiln; more than a couple times an electrical box has been marked 240V but carries only 208V.

The difference between 208V and 240V is in the transformer. In both cases the transformer draws high voltage, 10,000V. In 208V circuits all three live wires ground with 120V potential. In 240V circuits two live wires ground with 120V and the third live wire grounds with 200V potential. Home-owners have the first two live wires to their house; industrial users have all three. Schools and community-facility users have all three live wires from a 208V transformer. Larger kilns will need the 208V or 240V circuits. Smaller kilns may only require the 120V-to-ground provided by both circuit types. You may encounter a 480V circuit; kilns are available for this circuit as well, although it is much less common.

Amperage is the next concern. Typically a residence has 100 or maybe 200 available amps at 240V, single-phase: this is to power all of the lights, the kitchen, the laundry room, the entertainment system, and everything else in the house. Clay Planet has 400 available amps at 240V, three-phase - to run our clay machinery, our kilns, and all of our lights. Your electrician will be able to tell you how many amps you have available and should discuss with you how much amperage you might offer to your kiln or kilns. The same size kiln will require more amperage in 208V circuits and will require less amperage in three-phase circuits. Always provide 120% more amperage rating on the circuit breaker than is required by the kiln. For example: a 48-Amp kiln should have a 60-Amp breaker.

If you're considering a new kiln, find out from your electrician which circuit type and how much available amperage:

• 120V, 208V, 240V, 480V
• Single-Phase or Three-Phase
• 15A, 20A, 30A, 40A, 50A, 60A, or 100A

Sometimes you will hear about 220 Volts. Foothill college has 220V and I'm not quite sure how, but it must be with their transformer. You will have either 208V or 240V. Appliances rated for 220V such as dryers and ovens will work well with either 208V or 240V. Don't let that confuse you: kilns with elements rated at 240V will not get hot enough with only 208 Volts and, conversely, kilns with 208V elements will burn out when fired with 240V. Cress Kilns manufacture models rated at 220V and these are designed to run on either circuit type. However, 208V circuits will not be able to heat those kilns as hot as a 240V circuit will. You will have longer lasting and hotter elements when you order the correct voltage for your kiln.

Elements

Vacuuming Elements: Glaze is a hazard to elements. If a chip of glaze rests on an element when the kiln is fired, it will melt through and break the element. Glaze will also eat holes in the brick. It is a great idea to keep a vacuum close to your kiln so you can vacuum all the elements and brick before each firing. Bisque-ware is not a threat but anything that might melt when heated to your firing temperature will melt right through, and break the elements. Vacuum up everything and keep your kiln clean. You can pull chunks of glaze from the brick with a pair of needle-nosed pliers.

There are two easy ways to test the elements with-out opening the control-box. Turn on the kiln and let it warm up a bit. 1) Take a small piece of paper and touch it to each element - if the paper chars black, the element is working fine. Test each element so you know all the elements are working. 2) Let the kiln run, close the lid and look through the peep-holes. Eventually the elements should all be glowing. In either test if one or a pair of elements are not working, then there might be a broken element or a faulty relay.

Element Pins: The elements are held in the grooves of the kiln brick with straight pins. After-market U-pins are available but I only use straight pins myself. These pins are made of the same Kanthal wire as the elements. ConeArt Kilns don't use pins - instead the bricks are cut with deep grooves to keep the elements in place. Similarly, L&L Kilns have ceramic element holders and no element pins. But, for most other manufacturers: at each corner of a circular kiln, one pin holds the element down inside the groove. In a square kiln, there is a pin in the corner and there are a few pins along the length of the wall. As the kiln heats, the elements expand faster than the brick - it pushes against the wall and the pin keeps it down. As the kiln cools, the element is contracting faster than the brick - now it pulls against the groove but it will slip out if the pin does not hold it in place.

Re-pinning Elements: Sometimes the pins don't hold the element and it ends up hanging into the kiln. Some people will keep firing the kiln until the element breaks. Instead, heat up the element with a torch to a glowing red: then use two pair of needle-nose pliers to stretch or compress and put the element back into the groove. Take out the old pins that might get in the way. When you place a pin to hold the element, angle it downwards so that gravity keeps the pin from sliding back out. Pin the element down in the corners and the pin it some more along the length of the brick so that it will not fall out again. If you have a broken groove, pin the element from underneath so that it has a place to rest. Just remember: heat the element to a glowing red just before you start to work it, otherwise you may end up with a broken element instead of just a hanging one.

When to Replace the Elements: If you use your kiln on a regular basis, there will come a time when you must replace your elements. You can tell that they need replacement because the kiln will no longer reach temperature in a reasonable amount of time: it will either time out on a manual kiln or it will report an error on a digital one. You may choose to replace them earlier so that you will not be using a struggling kiln. Sometimes you can tell by looking, if the elements are really thin, they probably need to be replaced. Elements are typically 14-guage or, on larger kilns, they are 12-guage. If they look worn down to 16- or 18- guage then you are not going to be able to high-fire and it's time to replace your elements. The test I learned at Skutt is the resistance test and you need to understand electricity: when you open the control box, measure the resistance of each element: if the resistance is more than 10% greater than that the new element, it is time to replace. Typically, except for special broken-new-element circumstances, when you replace one element, it is time to replace them all.

Cones, the Kiln-Sitter, and the Thermocouple

Glazes and clay-bodies define their maturing temperature by pyrometric cones. The Orton Ceramic Foundation has a standard set of cones. Each cone is composed of certain oxides and ceramic materials so that it will melt after absorbing just enough heat energy. Orton provides pyrometric cones from cone-022 up to cone-36. This is a list of the maturing cones to which I have been exposed:

Use	Cone	dF @ 108 dF/Hr
Enamel	020	1159
Enamel	018	1319
Glaze	06	1828
Glaze	05	1888
Bisque	04	1945
Bisque	1	2079
Glaze	5	2167
Glaze	6	2232
Fuel-Burning	10	2340
Fuel-Burning	14	2530

There are essentially two types of cones: witness cones and Kiln-Sitter bars. The witness cones are usually lined up three or four in a cone-pack: the indicator, the witness, and the guard cones. So if you are glazing to cone-5, you would put a cone-4 indicator, a cone-5 witness, and a cone-6 guard. In a fuel-burning kiln the cone pack will help determine when to stop firing. Witness cones are seldom used in electric kilns but they can provide a great indication that the ware did in fact reach the correct temperature.

Manual electric kilns, equipped with a Kiln-Sitter, need to used the Kiln-Sitter bars. These are made of the same material as the witness cones and are designed to be placed into the Kiln-Sitter controller. Once the bar absorbs enough heat energy, it will bend by the weight of the rod, the latch will release, and the kiln will shut off. Modern Kiln-Sitter assemblies come equipped with a 20-hour timer to ensure shut-off in case the manual latch fails to release.

Kiln-Sitter Tube Assembly Replacement: When the sensing rod starts to wear very thin or when the cone supports are broken and can no longer be held in place, it is time to change the Kiln-Sitter tube. 1) Disconnect the kiln. 2) Either open the control box or remove the Kiln-Sitter face-plate by unscrewing the four corner screws. 3) Remove the claw-guide by opening the two screws on the face. 4) Remove the tube-assembly by the two screws on the face underneath the claw-guide. 5) Install the new tube assembly. 6) Do the claw adjustment. 7) Do the latch adjustment.

Kiln-Sitter Latch Adjustment: If your witness cones are not bending properly to match the Kiln-Sitter bar, your latch might be out of adjustment. The latch has a slide that can go up and down. You will need the coin-like Kiln-Sitter tool that came banded to the tube assembly. 1) Raise the latch and hold it with the claw. 2) Slide the tool with the hole around the rod and the two slots on the cone supports. 3) The claw should hold about 2-mm of the latch: slide the latch longer if the claw doesn't hold or slide the latch as short as possible so that the clow still holds.

Kiln-Sitter Claw Adjestment: If the claw is spinning around, adjust the claw inward so that it slides within the metal guide. If the claw is outside of or just barely within the guide, it can catch on the outside of the guide and not release the latch. If this happens, your kiln will over-fire and you could have a big mess. Realize that the rod will expand and lengthen slightly when it gets hot, so make sure that the claw is well within the guide.

Digital kilns are equipped with a thermocouple. Whereas the pyrometric code measures heat-work, the thermocouple measures temperature. A thermocouple is made from two different metal-alloy rods, welded together at the tip. When heated, the metals produce a tiny voltage rated in milli-volts. Thomas Seebeck discovered this principle in 1821. This voltage is converted to temperature and displayed on a modern digital controller. The controller decides when to operate the elements based on the measured temperature and the programmed firing cycle. As the thermocouple ages, the temperature displayed will be slightly higher than the true temperature of the kiln. When the thermocouple eventually breaks, the controller will usually identify this with the FAIL message.

There are two thermocouple types used in ceramics. The common is the type-K thermocouple. It is very durable and provides accurate temperature measurement over many uses. You may see the type-K with the weld visible and two metal rods in a twin-porcelain sleeve; otherwise it will be one metal tube encapsulating the weld within. Both are type-K and are interchangable. The rarer, type-S thermocouple is even more durable and will maintain the true temperature even more accurately. It is made with different metal-alloys which make it more durable yet more expensive: but it produces a different reference voltage. If you swap thermocouple types, your controller will have to be reprogrammed. Reprogramming may not be possible on all conrollers. Unless you know otherwise, you have a type-K thermocouple.

Thermocouple Replacement: When the digital display on the kiln reads FAIL, it is time to replace the thermocouple. You may choose to replace the thermocouple earlier if it appears to be aging. 1) Disconnect the kiln. 2) Open the control box. 3) Remove the thermocouple and porcelain holder. (First, if the thermocouple is bent, heat it to glowing red and bend it straight with two pair of pliers.) 4) Remove the thermocouple from or disconnect the thermocouple lead wires from the porcelain holder. 5) Connect and install the new thermocouple making sure the yellow wire connects to positive and the red wire connects to negative. 6) Close the control box and reconnect the kiln. 7) Ensure proper polarity from step (5) by starting the kiln and watching the temperature rise.

Switches and Relays

Infinte switches allow you to adjust for how long the contactors are engaged. With the switch on low the elements are running at about 25%. With the switch on high the elements are running at 100%. And the switch is variable in between. These switches use a bi-metallic contactor and will disengage when it gets warm. After it cools, it re-engages and the process repeats. Turning the knob to a higher setting shortens the contactor length so it must get hotter before switching off. Simply put: Infinite switches connect the main power from the Kiln-Sitter assembly to the elements. You can control the rate of heating with the knob from low to high.

Digital controllers use relays. Like switches, relays connect the main power to the elements - but there is no Kiln-Sitter in between. The relays are controlled by a 12-Volt DC electro-magnetic contactor. When the controller signals the relays to engage, the contactors close and the circuits are complete: the elements are on.

There is another, otherwise identical, relay in use as well. The Cress FX Series relays switch with 240 Volts AC. The FX controller is a manual controller, as it has a Kiln-Sitter assembly and no thermocouple, but it is automated with a motor to advance an infinite switch that controls the elements. One set of elements is connected to the infinite switch. The other elements are connected through 240V AC switched relays which are signaled by the infinite switch.

Each time a contactor opens or closes, a spark is generated, the contactor is charred, and a tiny piece flies off. Listen to your kiln and you will hear how often the contactors spark. As the contactor wears out, it creates additional electrical resistance: instead of providing energy to the elements, some energy is wasted trying to go through the switch. Typically when you replace your elements, it is time to replace the switches as well.

If you find that an element pair is not working, the problem may be a broken element but, otherwise, it may be a faulty switch or relay. If the switch is worn too much it may not be able to complete its circuit and that element or element pair cannot energize.

A worse problem occurs when a relay gets stuck closed: on old relay may generate so much heat that it can weld itself stuck. Infinite switches have the Kiln-Sitter to inhibit this stuck situation but if a relay gets stuck closed, in a digital kiln, the element will stay on until you manually disconnect power to the kiln. For this reason it is always good to ensure you can pull the power or shut-off the breaker with-out having to crawl around the kiln.

Brick and the Shell

If you're careful with the kiln - don't let the lid slam closed, and don't hit the sides of the kiln with shelves or ware pieces - the interior brick can last through several element replacement cycles. Be very careful moving the shelves in and out by not dinging the walls or hitting the thermocouples. The element grooves are prone to breaking when hit with a shelf or ware-piece. Remember, the grooves keep the elements from sagging into the kiln; so keep the bricks in good working condition.

Don't throw away your broken brick pieces. You can use XL-Plus and some element pins to rejoin the bricks. 1) Make sure you know where each brick piece fits. 2) Apply XL-Plus on both the brick and the brick piece to be joined. 3) Press the piece and fit it onto the brick. 4) Wipe away excess XL-Plus, it will damage the elements in the same way glaze will damage elements. 5) Pin the piece with an element pin, making sure gravity holds the pin into the brick. 6) Clean up the XL-Plus with water as it will be most difficult to remove once it has hardened.

If you want to replace an entire brick or set of bricks: it is possible to order the brick for current model kilns; it is best to replace bricks only when replacing elements; you must heat the band with a torch when retightening to ensure a snug fit at high temperatures.

The heat from the kiln will oxidize and rust many of the stainless steel parts affixed. Keep the stainless steel clean of rust with scotch-bright. Otherwise, handles and hinges and other hardware will eventually rust on your kiln and may be reordered, if desired. Most of the hardware will endure even though it is rusted. When you open your control box, however, it is a good idea to have new stainless-steel sheet-metal screws ready to replace the old, rusted-out screws you remove: the screws can rust away to practically nothing on an older kiln. The worst effect of rust is when a piece of hardware's straight edge rusts against and breaks the band that holds the kiln together. When this happens, it is probably bets to order a replacement piece for the section with the broken band.

Venting

Modern ventilation systems are of the down-draft variety. They affix a cup to the bottom of the kiln and pull a slight draft, down through the kiln. Toxic fumes are never released from the kiln except through the vent. In addition, the down-draft systems provide even heating through-out the kiln my acting against the natural rising of heat. These systems require holes to be drilled in the lid and floor of the kiln. Only a small fraction of the air flow generated by the vent travels through the kiln - the rest of the air is pulled from larger holes in the cup.

When you fire with a down-draft ventilation system, always leave the lid shut and keep the peep-hole plugs closed. To test the system: hold a lit match or a lighter over each of the holes in the floor and each of the holes in the led. You should see the flame being pulled down into the hole. If the flame does not pull, either you've left the peep-hole plugs open or the cup is not properly placed.

Regardless of your ventilation system, a digital kiln cannot be exposed to an atmosphere exceeding 200-F. The control board can fail in this event. Provide adequate airflow around your kiln.

Summary - How to Keep your Kiln Healthy

Elements: Remove any glaze or clay particles before firing: the easiest way is with a vacuum. Make sure they are pinned in their grooves and not hanging. You can check that all of the elements are working properly with either the paper or glow tests. If all the elements are working properly, you must replace them if the kiln still cannot reach the desired firing temperature.

Thermocouple: The digital display should not read FAIL. The best way to determine that the thermocouple is working properly is by using witness cones to ensure the proper temerature has been reached.

Kiln-Sitter: Witness cones will help ensure the Kiln-Sitter is adjusted properly. Make sure the claw is within the guide, the cone supports are not broken, the rod is not too thin, and that the latch is adjusted properly.

Relays: Make sure you can pull the plug or flip the breaker with-out having to crawl around the kiln. If a relay gets stuck on, the kiln will be hot when you try to disconnect power. You're smart to replace the relays when you replace the elements. A faulty relay can be one reason your elements are not working.

Switches: Old switches produce additional resistance, taking power away from the elements. So replace the switches when you replace your elements.

Bricks: You can glue and re-pin broken bricks with XL-Plus and element pins. Long cracks in the floor and lid are normal. Be careful when stacking and close the lid gently.

Shell: Rust can be the culprit. The sheet-metal screws will eventually rust, but long lines of rust along the bands can be disastrous. Scotch-bright can help keep your kiln rust-free.

Venting: Use a match or lighter to check your down-draft system. Make sure your kiln room stays cool during firings especially for digital kilns.

Kiln Manufactureres, etc.