Kiln Temperature Control Methods

Pretty much everything you ever wanted to know about controlling kiln temperature (maybe more)

Kiln temperature may be controlled by a variety of methods. Control techniques that are suitable for one process are often not appropriate for another.

The oldest method of temperature control was simply adjusting the amount of combustible material (wood or charcoal) and / or (draft) in an earthen kiln based on observation of the kiln interior and the objects being fired. This technique was used for centuries and became sophisticated enough for artisans to be able to produce high quality pottery objects. Considerable skill and practice is required to produce satisfactory results with these techniques and the methods and training often passed through generations of the same families.

The cone method was developed during this time which relies on the slumping of a temperature sensitive clay cone to indicate the maximum temperatures achieved in a kiln. Typically 3 cones are used in a firing: one above the desired temperature, one below the desired temperature and one at the desired temperature. The firing is terminated when the center or (at the right temperature) cone has slumped slightly. The low cone will have completely collapsed and the high cone should have barely moved at all.

Gas fired kilns permitted easier and finer control by varying gas flow and allowed satisfactory results to be achieved more easily.

An improvement in control resulted when kiln sitters were introduced. A kiln sitter allows you to insert a temperature cone in a special bracket inside the kiln which closes a solenoid valve for a gas kiln or a switch for an electric kiln when the cone reaches its critical “set” temperature. This permits automatic termination of the firing when the kiln reaches its “cone” temperature.

The above methods were used very successfully for centuries to fire and glaze pottery,
But newer methods enable much finer and more repeatable results and can be used with much more challenging materials like glass.

The electric kiln is a fairly recent development that has permitted all kinds of things to be easily undertaken that were simply not possible before. Electric kilns are widely available, easy to hook up and do not require the elaborate installation of gas kilns.

Because the electric kiln is controlled simply by turning the electricity on and off it is possible to achieve much finer temperature control than other methods and it is very compatible with Electronic control techniques for the finest and most versatile means of control. The remainder of this document will discuss methods for temperature control of electric kilns as these are by far the most popular.

Electric Kilns

With electric kilns the simplest control method is still turning the kiln on and off coupled with frequent observation of the item(s) being fired.  However, two important and relatively inexpensive items can noticibly improve results and repeatability and should really be considered the absolute minimum for any kiln.

1. The infinite switch is the simplest form of control available for a kiln. This item works just like the dial control for the surface burner on your kitchen range. How far it is turned determines what percentage of the time the kiln elements are on, thus how fast your kiln will heat up. Notice, this is not the same as what temperature your kiln will reach. It is not controlling the temperature, just percent on time of the kiln elements.

2.This brings us to the second item, some way of directly reading the kilns temperature. The dial type pyrometer is the simplest (and cheapest) device available for continuous observation of the kilns interior temperature. While dial type pyrometers have considerable limitations, in conjunction with an infinite switch they provide sufficient information to be able to meet the minimum requirements for controlling a kiln.

3.The infinite switch and pyrometer are in common use in electric kilns and with diligence can be used to normally produce acceptable results even with highly sensitive and troublesome materials such as glass and enamels.

4.Firing with an infinite switch and pyrometer do require continuous supervision and adjustment to achieve optimal results.

5.Dial type pyrometers are both not very accurate and are hard to read as accurately as would be desirable especially for critical items like glass, PMC or enamel.

Dial pyrometers as well as most other kiln temperature measuring techniques rely on the use of a bi-metal “Thermocouple” placed in the kiln which generates a tiny voltage somewhat proportional to the heat inside the kiln. The dial pyrometer is simply a very low power microvolt meter that has a special dial that is non-linear roughly in the same way as the thermocouple to which it is attached.

Modern thermocouples themselves tend to be both highly consistent and repeatable.

However, the un-powered dial type pyrometers gauge is very delicate and extremely difficult to manufacture so that it has significant accuracy. The extremely low voltages (and power) produced by the thermocouple requires that a simple hair spring apply a tiny force against the tiny supplied voltage. These hairsprings are inherently difficult to produce with repeatable accuracy, especially at the very low forces required. They also drift very significantly as they age. Larger dial pyrometers are normally somewhat more accurate as well as being easier to read, but all periodically need to be adjusted to the desired primary target temperature while at that temperature and are presumed to be seriously off at any other temperature (often 100 to 300 degrees off). The considerable absolute and relative inaccuracy and drift make dial pyrometers less than satisfactory for all uses and completely unsatisfactory where close temperature control is required.

The next step up is a digital pyrometer. There are several brands of these available, usually in the form of a hand held test instrument. If they are paired with an appropriate kiln type thermocouple (such as the one that came on the dial type pyrometer) they can provide greatly increased accuracy and repeatability over a dial pyrometer. They use electronic amplification and programmatic compensation for the non-linearity of the thermocouple. They tend to be accurate within 10 – 20 degrees or better. However, digital pyrometers have not proven to be very popular for a variety of reasons. They are not designed for continuous use and often run on batteries and incorporate a built in turn off feature both of which are less than ideal for continuous firing. Also, quite often, the thermocouple supplied with the digital pyrometer is inadequate for maximum kiln temperatures or for continuous kiln firing. Finally, the best of them are quite expensive getting up into the range of true electronic kiln temperature controllers.

It is worth mentioning the optical or infrared pyrometer which is becoming more widely available lately, but ones that will work at kiln firing temperatures are still very expensive and they require you to open the lid or door of a kiln, point the pyrometer at the kiln chamber and push a button to take a reading. This is considerably less than ideal for continuous temperature monitoring use. Using one actually considerably changes the temperature in the kiln as well as providing a significant heat hazard. Not what you need!

Electronic Kiln Temperature Controllers

Electronic Kiln Controllers represent the most significant advancement since the invention of the kiln itself. A controller controls, an electronic controller, controls tirelessly, accurately and without requiring continuous monitoring.

The digital “Set Point” controller is the simplest kind of electronic temperature controller available, but even it provides a quantum leap over non-controller based methods. The set point controller allows you to set a single “set” temperature and then have the kiln go to and accurately maintain that temperature until you shut it off.
Set Point controllers usually have a digital display which can show either the kilns current temperature or the desired set temperature (the best ones show both simultaneously). Your only interaction is if you wish to adjust the set temperature or end the firing. They are precise and will let you achieve the desired results consistently. For more complex firings you will need to adjust the controller during firing. An infinite switch can be used in conjunction with this controller to slow temperature rate of rise if desired.

The set point controller gives you the precise control you need for simple firings of even troublesome materials. Glass and bead annealing require precise holding at the annealing temperature (preferably within 10 or 20 degrees Fahrenheit) which is simply not achievable without a controller.  Fused glass requires tight temperature control for a variety of effects: fusing, slumping, fire polishing, tack fusing, etc and these temperatures vary with the type of glass, its coefficient of expansion and even its color. Precious Metal Clay requires accurate temperature control for proper “sintering” as do enamels and cloisonné, polymer clay and many of the fancier pottery glazes. A controller, even the simple “Set Point” will give you the kind of temperature control that you need to achieve desirable and repeatable results which are simply not possible without a controller.

In many kiln use projects there is a need to fairly closely follow an idealized firing sequence or schedule, normally referred to as a profile. A profile generally consists of one or more (steps, segments or cycles), each step consisting of a:

1.Set or destination temperature.

2.Ramp time from the previous temperature to the set temperature.

3.Hold time at the set temperature before proceeding to the next step or ending the firing.

Using the previously mentioned control methods, these ramp and hold multi-step profiles are implemented manually by the user performing frequent adjustments during firing.

Virtually all kiln projects benefit greatly from using the profile method and in the case of most glass projects are absolutely mandatory due to the need for timely and accurate temperature rise, fall and annealing requirements.

Ramp and Hold Kiln Temperature Controllers

Digital Ramp and Hold Kiln controllers have proliferated in the past 15 years or so and are now quite common although still found on a minority of the kilns in use. These controllers are designed to implement the Profile method and are normally capable of supporting multiple profiles each with multiple ramp and hold steps.

Most Ramp and Hold controllers provide for user entry of multi-step ramp and hold profiles and provide some monitoring of the project during firing. Ramp and Hold Kiln Temperature Controllers that do not provide for user profile data entry of multi-step profiles should be avoided as they do not have nearly the versatility or adjustability of those that do.

Programmable Ramp and Hold Kiln Temperature Controllers

Controllers that do offer user programmability and multiple steps are collectively referred to as: Programmable Ramp and Hold Kiln Temperature Controllers.

Most Programmable Ramp and Hold Kiln Temperature Controllers can automatically provide accurate reproduction of complex firing schedules and generally make using a kiln a more rewarding and satisfactory experience than by any other method.

That having been said there are many variations of the Programmable Controller and their differences are very significant in the overall ease of use and versatility of the controller.

The most common type of programmable controller which was originally developed 15 to 20 years ago has a single 4 digit LED numerical display and a keypad for data entry. Generally they are in a square or rectangular metal box and come pre-attached to the kiln. Normally they will accommodate 4 to 10 user installed profiles of 4 to 8 steps each.  Some of them also come with 4 or 5 preinstalled simple profiles as well.

These are very powerful devices and they are “capable” of providing adequate control over your kiln for almost any purpose.

However, the traditional style controller does suffer from one very significant limitation. They are all truly hard to “program”. This is a direct result of anemic microcontrollers and especially the single 4 digit numerical LED displays’ inability to be able to provide the user with any reasonable feedback as to what is going on in the programming cycle.

They often resort to obscure hieroglyphic characters that can be constructed out of non-numeric codes displayed on a numeric display and requiring the user to plow through the voluminous manual during each attempt at programming.

An unstated reality of this traditional style of controller is that few people go through the agonizing process of actually programming a profile into it more than 10 times in the entire life of the kiln / controller. In fact, many just give up and use the profiles that came preinstalled from the factory even though they do not provide an ideal schedule for them.

Originally these controllers were designed for use on clay firing kilns which do not require as complex firing schedules or the extreme accuracy of more difficult processes. Glass being the most notable example. Since clay was the initial intended market they were generally only offered on larger 220 volt kilns and were priced accordingly (High).

The traditional controllers are generally of excellent industrial quality construction, but the design is now seriously dated and they have stood still in the midst of the electronic revolution. Their control methodology is outdated, they are very hard to use and they provide minimal feed back during firing.

Fifteen years ago, microcontrollers were expensive, very limited and re-programmable storage memory for them was very expensive. Small red LED 4 digit numeric only displays were all that were cost effective.

Thermocouple amplifiers had to be built from discreet parts and required complex set up and factory tuning procedures for each controller.

Low duty cycle mechanical relays with arcing points were all that were affordably available, but they required slow switching times due to the limited number of times a relay can be switched in its life. This resulted in long on / off cycle times and required complex algorithms to compensate for kiln temperature over and under shoot.

The traditional programmable controller design was introduced over 15 years ago and represented a quantum leap at that time. However, for the most part relatively little significant innovation has been put in to them since, each new one being a slightly “improved” copy of others already in existence.

Since that time, advances in microcontrollers, relay design, display technology and even in the electronics for reading thermocouples make the possibility of creating an easier to use, more responsive and more reliable kiln controller a real possibility.

My wife is a glass artist. I am an electronics (microcontroller) engineer. When Barbara started in glass art, the deficiencies of the dial pyrometer and infinite switch became quickly apparent (terrible accuracy, repeatability and very inconsistent results).

So we got a kiln with a traditional controller. This gave us the accuracy and repeatability that Barbara needed, but the programming difficulties were a major roadblock even to me. Experimentation and refinement were very difficult.

I decided I could do better so I analyzed what was already being done and with Barbara’s help set out to design from scratch a brand new type of user friendly kiln controller that took full advantage of every advance in technology.

I know this is a guide and not supposed to be a sales plug, but I honestly feel that the controller we have designed provides a whole new level of usability, user friendliness and capability and truly does deserve to have its own category.

You can get perfectly good results with good traditional style kiln controllers, but they can’t really touch the EZ2Set in usability, reliability, capability or value.

The User Friendly Programmable Kiln Controller: EZ2Set

The EZ2Sets powerful microcontroller and multi line character type LCD display allows personal computer like menu navigation and screens are presented in plain English. There is no need to try and figure out what to do next. It’s on the screen and it’s obvious.

Larger user storage memory provides space for up to 64 User programmed profiles of up to 10 steps each in addition to providing 10 preloaded profiles for a variety of applications.

Quick Fire and Quick ramp modes enable simple single fire sequences to be immediately entered and executed while the preloaded and user profiles permit very complex profiles to be quickly loaded, edited as desired and fired. Previous profiles can all easily be used as the basis for new ones. Profiles can even be easily modified or skipped ahead during firing.

Solid state relays are more reliable and permit much faster cycle times resulting in greatly reduced temperature variation. An improved thermocouple interface system provides drift free accuracy, improved linearity and better reliability. A desktop sloped computer style enclosure makes programming the controller easier and more enjoyable and the stand alone system makes it easy to move between kilns.

The EZ2Sets’ ease of use encourages experimentation and refinement. Repeatability and satisfactory results are virtually guaranteed.  We truly feel that we have now produced the finest and most user friendly kiln temperature controller ever made.

The EZ2Set is currently being made for 110 volt and up to 15 amp kilns and will soon be released in models for 220 volt and up to 60 amp kilns. FYI in the US single phase voltage is 110 volts and 220 volts, NOT 115, 120, 230 or 240 volts as are sometimes advertised.

At mccraystudios we have such great confidence in its capabilities and reliability that we are offering it with a 30 day money back guarantee and a 1 year warranty.

You can also contact us at McCray Studios EBay Store