The kiln: A closer look

Kiln  choices

As was seen in ‘The kiln. A brief outline’, kilns come in a wide variety of shapes and sizes.

The choice will depend largely on the type of work to be fired, and on one’s budget.

For the newcomer to kilnforming, a low priced mass produced unit in which to do only the occasional firing may be adequate.

For the dedicated glass artist who has identified their area of specialty a more efficient or custom built kiln may be their best option.

Still others may opt for a pre-owned pottery or glass kiln and modernize or upgrade as finances permit. For those traveling this path, see 22. Evaluating a pre-owned kiln.

For the commercial operator considerations will be the size of work. energy efficiency and and unattended operation.

 Desirable features of a kiln for kilnforming include:

• Even heat distribution

• Ease of loading & unloading

• Ease of monitoring the progress of a firing

• Precise temperature control

• Low operating cost

• Unattended operation

Each owner must make their own value judgement as to which are the more important features.

This brief introduction of what makes up a kiln may be sufficient for those needing only an overview. For a more in-depth coverage, see the section dedicated to kilns.

All types of kiln, from the smallest to the largest, have components performing similar tasks; although they can look and be arranged in many different ways.

Apart from the shell and a means of gaining access, they should all have:

1. Insulation to slow down the loss of heat.

2. Electric elements to introduce the heat.

3. Some form of temperature control. Desirable, but not always fitted.<

4. Some form of power switching device to control power to the elements.

5. An interlock switch to ensure that power cannot be supplied to the elements when the kiln is open.

6. A shelf or hearth to support the glass.

7. Spyholes for monitoring the firing and vents for ridding the kiln of fumes.

8. A label showing the maker’s name and the power rating of the kiln. (A compliance plate)

9. A means of connection to mains power.

Insulation

This can be either Insulating Fire Brick (IFB) or Ceramic Fibre (CF). Glass kilns available through Art Glass outlets in Australia are almost exclusively top loaders of American manufacture with IFB insulation in the walls, floor and lid. The major makers include Evenheat, Paragon, and Skutt.  

Traditional Australian front and top loader pottery kilns use a similar brick but the walls and roof often had two layers of brick to give much better insulation and vastly improved energy efficiency.

All refractory materials are subject to thermal shock and the continuous thermal cycling from room to soak temperature will gradually cause the IFB to develop cracks. This will especially occur where the brick is machined to house elements and at sharp edges or thinned sections.

Kilns with larger hearth area will usually have CF wall and roof insulation. This can be either CF blanket or fibreboard. There are two main methods of CF blanket installation; commonly referred to as ‘wallpaper’ or ‘stackbond’. Both methods are satisfactory at glass temperatures, but stackbond is far superior at higher temperatures.

Fibreboard is a good insulator but is subject over time to slight shrinkage at glass temperatures. When used as a wall or roof hotface lining it can sometimes dish or warp because of differential shrinkage between the hot-face and the cooler back-face; giving a shorter working life than the blanket linings. 

Electric elements

They are made from Kanthal high temperature resistance wire. In brick lined kilns they can be housed in slotted grooves, cut into the IFB wall or lid lining, whilst in CF lined units they are supported on ceramic tubes fixed below the roof.

They are usually wound as a closed coil and then stretched to whatever length is required. When heated repeatedly the coils try to ‘creep’ closed again; the coils get shorter. They do this randomly, bunching up in one spot and getting further apart in another.Coils in grooves can sometimes get ‘drunk’ and lean over on top of one another.

In square kilns they can pull out of the corners, in multi sided units they can ride up out of their slots. Coils in grooved lids can also fall out of their slots.

The element wire crystallises and becomes brittle after it has been in use and can easily snap if excessive strain is put on it.

Elements should be inspected regularly to ensure that they remain seated in their grooves.

See Heating elements  on how to pin them or otherwise keep the elements in place.

Temperature control

Most new kilns will come with some form of programmable controller. Some can be quite complex to operate and all should come with detailed operating instructions.

There are two components to the controller:

• The controller/indicator mounted on the outside of the kiln, and

• The thermocouple which measures the temperature in the kiln.

The thermocouple is a probe which protrudes through the wall. It senses the temperature at its tip and transmits it to the controller.

Before the arrival of low cost programmable controllers, smaller glass kilns were sometimes fitted with an instrument providing ‘single ramp and soak’ control. This would accept a pattern to take the kiln to a soak temperature at a set rate of rise and hold it for a set soak time. The kiln would then cool down naturally.

They do not provide full control and require supervision. They are still widely used on pottery kilns.

Many pottery kilns are fitted with a device called a ‘Kiln Sitter’. It is still fitted to new kilns. It uses a ‘pyrometric cone’ to sense when the clay has been properly cooked, at which time it turns off the power. It is totally unsuitable for firing glass as it gives no indication of temperature. 

Other devices used on pottery kilns in the past include;

• temperature switches, where a soak temperature is selected on a dial;

• indicating temperature switches which function in a similar manner but also give an indication of temperature as the firing progresses.

None of these devices allow for a soak time; turning off the power as soon as the set temperature is reached. Nor do they give any control of the rate of rise of temperature during the firing.This latter is provided by devices variously called an Energy Regulator, Infinity switch or Simmerstat.

Whilst experienced operators have done and continue to use pottery kilns with various levels of control for kilnforming, the task can be greatly simplified by updating the controls.

There is more on this in Kiln controls

Power switching

This can be done in one of two ways;

• Mechanical contactor or power relay

• Solid State Relay.(SSR)

A contactor or power relay works like a normal light switch with contacts which close and open as required by the firing pattern. They make a banging or tapping noise when doing this.

Heat is generated when mechanical contacts open to interrupt current flow and it takes time for the contacts to cool. Should they close too rapidly there is the risk that they will be hot enough to weld together; maintaining power to the elements and overheating the kiln.  Safe practice dictates that the contacts should remain open for a finite time before closing again.

The temperature controller or other device should ensure that this safe period is not compromised.

Chattering contacts sometimes heard during kiln operation are a sure indication of ‘troubles to come’.

The SSR does not have any moving parts. As a result it can switch extremely rapidly; down to once per second.

This allows power to be fed to the elements in very small packages and permits closer matching of kiln temperature to firing pattern.

Whilst being in use for over 40 years, SSR’s have been slow to gain acceptance with kiln builders. This is undoubtedly because of their lack of understanding of instrumentation and power control. It is pleasing that their use is finally becoming more widespread.

There is more on this in 55. Power switching. Contactor or SSR

Interlock switch

(Not all kilns have this, but they should!)

For the owner of a new kiln, the interlock switch will possibly be the first item to make itself known in an annoying way.

On most kilns the interlock switch is a mechanical device mounted on the body of the kiln with an actuator mounted on the door lid or hood. Unless it is correctly adjusted it can stop power going to the elements.

Sometimes it can be correctly adjusted when the kiln is cold, but expansion of metal parts from the heat can put it out of adjustment during a firing: with the result that the kiln cuts out sometime during the firing; usually before reaching soak.

At other times, especially on top loading kilns with a lid hinge system rather more flimsy than it should be, the lid can close without the actuator making proper contact.

First item to check if the kiln won’t start; make sure the interlock switch is closed, and stays closed.

Implicit in this is that the lid actually be hinged to the body. Whilst not the practice of Australian kiln builders, some imported top loading kilns had a loose lift-off lid with no firm attachment to the body.

This is potentially extremely hazardous.

On top loading kilns, especially those with a hinge system which allows sideways movement of the lid, slight misalignment of actuator and switch can prevent the kiln heating. Lifting the lid far enough to clear the actuator, moving it slightly to one side or the other and lowering it again should fix the problem.

On front loading kilns, especially those with the interlock switch on the side away from the hinges, the door may not be clamped shut tight enough to trip the switch.

Kiln shelf or hearth

Most small kilns support the glass on one or more kiln shelves made of silimanite or cordierite. Like all refractory materials, they are brittle and susceptible to thermal shock, so need to be treated with reasonable care.

Glass will stick to them, so they must be battwashed.

They should be supported on props clear of the kiln floor so that hot air can circulate all around them.  This minimises the risk of uneven expansion and cracking.

Hearths are usually of IFB. They should be battwashed, as otherwise the hot glass will fuse to them and could pull out lumps when removed.

Spyholes and vents

Most kilns will have one or two holes somewhere in the side or door to allow viewing of the glass during a firing. These may be covered by swinging metal flaps or have ceramic plugs to minimise loss of heat. 

Some kilns may also have one or more vents in the lid or roof to allow for the escape of fumes from paper or paint, or for the loss of hot air when cooling. Again, these may have ceramic plugs or caps for sealing. 

Compliance plate

This is required by Australian law to be fitted to all appliances intended to be connected to mains power.

It should identify the maker of the appliance and show the Voltage and current rating of the unit.

A typical label on a small kiln suitable for connecting to any of the normal power outlets in a dwelling may show the following:

Made by XYZ Kilns.

10Amps 240Volts single phase (or 1 ph) 2.4kW.

This is the maximum load which can be connected to a normal power outlet.

Any higher power rating outlet must be especially installed by an electrician.

Means of connection to mains power

All small kilns should be fitted with a flexible power cord and three pin plug appropriate to the rating of the kiln.

Larger kilns may require direct wiring to a special outlet. These will usually be in commercial or large glass studios.

The standard power outlet used in Australia is rated at 10Amps 240Volts.

Fig 25-1 View of standard GPO with alternative earth pins for Special Power Outlets.

It has a three pin socket and a switch marked with a red dot to show when the switch is turned on, but no other markings. The socket for the earth pin will accept only the smallest flat pin. 

Other outlets of higher power rating are available, including 15Amp, 20Amp, 25Amp and 32Amp.They are of similar overall appearance to the 10Amp version except for having an earth pin of a shape unique to their rating.

These must all be installed by a licensed electrician using cable of the appropriate size which is protected by a suitably sized circuit breaker on the switchboard.

This to ensure that the kiln has exclusive use of the circuit and that turning on another appliance could not overload the circuit and interrupt the firing.

Shell and frame

The shell is the part of the kiln which encloses the insulation and is subject to changes of temperature during a firing. It is usually made of stainless steel; that being more resistant to corrosion and heat damage than other metals.

Heat affected components

All the components inside the kiln, including the insulating bricks, the elements and the thermocouple are subject to high temperatures and will gradually deteriorate

Molten glass can stick to any of them and do severe damage. To prolong the life of your kiln the following precautions should be observed.

Whatever the power rating of the kiln itself, make sure it is connected to a power outlet of the same rating or higher.

Troubleshooting/ Maintenance issues

Here are some common issues that you might encounter when using kilns over a long period or time, or when you have inherited an older kiln.

Sagging elements in lids

As the element wire ages it can stretch and the coils can sag out of the groove in the lid. As it occurs randomly, and fairly gradually, it is good practice to check periodically for signs of it happening.

Whilst it is soft when hot, the wire can become brittle when cold, so care should be taken when trying to put it back in place. The element can be pinned back into place, and instructions to do this are given in 28. Heating elements.

Elements pulling out of grooves in walls

As well as stretching, the coiled element can also shrink and pull out of the grooves in walls of rectangular or multi sided kilns. Again, it occurs gradually, so periodic inspection will detect it before it has gone too far. Again, see 28. Heating elements for how to fix.