Kiln shelves & slabs

Kiln shelves

The type in most common use is made of a dense pressed refractory mineral called silimanite (sometimes spelt sillimanite) which is capable of withstanding temperature to above 1300ºC. They come in a wide range of size and thickness; are commonly known as ‘batts’. Currently the largest size commercially available in Australia is 650 x 650 x 20 That’s millimetres for those who haven’t read ‘How long is that’. 

They are a reasonably good conductor of heat, but are still likely to warp and crack if too great a temperature difference between their two faces is allowed to develop: Never put a kiln shelf flat onto a surface so that the flow of heat to the underside will be restricted.

Always prop shelves up off a kiln hearth or floor. The larger the shelf size the greater the clearance, but a minimum clearance should be about 20mm.

Except when making glass castings using heavy moulds, the load on the shelf when glass firing is usually much lower than for pottery firing, so kilnformers can use thinner shelves. This lowers the thermal mass and makes for improved kiln efficiency – reduces the power bill.

If purchasing new shelves, the thinnest available of the size you need will be adequate.

They can crack, through thermal shock, or through an accidental knock. Cracks not visible to the naked eye can let go when stress is greatest. Check for cracks regularly; when tapped they should give off a sharp sound, not a dull ‘muddy’ noise.

When not in use, always store them on edge. To avoid the battwash taking up moisture and sticking them together, put a sheet of plastic between them: or maybe small chocks so that they don’t touch one another.

Silimanite shelves are all imported, so the particular size you need may not be readily available. Fortunately they can be cut with a diamond saw; so most major suppliers can cut to order.  They can also be cut with an abrasive stone cutting disc in an angle grinder or similar power tool; be careful, as this can be a hazardous operation.

Silimanite shelves rarely warp or sag at glass firing temperatures, but it’s a good idea to follow the potter’s practice and turn them over regularly. This ensures that sagging is unlikely to occur.

Some makers embed brand marks into one side. They have to be used with the same side up all the time or the marks have to be filled.

If shallow they can be filled with battwash; if deeper with a mixture of battwash powder and colloidal silica. Sand the patches smooth when they are dry and then battwash as normal. The patch may loosen with use and need to be replaced.  

Another shelf material sometimes used by potters is Silicon Carbide. This is much stronger than Silimanite and a much better conductor of heat, but much—much more expensive. If you get them with a pre-owned kiln—so well and good—but don’t consider buying them new, as they would be a waste of money.

The kiln shelf materials mentioned above have a high density, usually being many times the weight of the glass you are firing.

Low-density materials which can be used to support glass, albeit with a considerable degree of risk of cracking or warping, include:

• Vacuum formed ceramic fibre board (fibreboard).
• Calcium silicate board.
• Compressed vermiculite board.

Unfortunately, each of these materials have weaknesses which make them less reliable as a shelf material and more usable as an underlay or mould material: so more information on them is given elsewhere.

They are all good heat insulators and restrict the flow of heat to the underside of the glass more so than silimanite does.

Props

In pottery kilns are usually made of silimanite but, for glass temperatures they can be cut from IFB; this is lighter and lowers the thermal mass.

If using more than one shelf, always position the props one above the other.

Use only 3 props to a shelf. If you use four props, especially with stacked shelves, there is the risk that a shelf will warp during firing, a prop will fall and damage the work or, catastrophe, the whole load goes down. Less likely at glass temperatures, but a good habit to get into.

Slabs

Extruded slab hearths

Fig 1

Recently developed in China these hearths are a form of extruded shelf made of cordierite; a material functionally similar to silimanite but of a paler colour. They allow much larger shelf areas without joins.

The one illustrated is 1300 x 675 x 38mm (4’ 3” x 2’ 2” x 1 ½”) and forms the full hearth of the Riley FS-1 Top hat kiln. Note the clear space underneath for air circulation. The size mentioned is the only one readily available in Australia.

Both faces are machined parallel after firing, but the surfaces are not always as smooth as with the pressed silimanite shelves. One user wanting a superior surface had his surface ground by a monumental mason.

Multi batt hearths

Fig 2

It is sometimes necessary to make up a large hearth using a number of kiln shelves joined together. The one illustrated in figs 2 & 3 is made up of eight batts.

Fig 3

They were all individually packed up on props to get a surface as level and true as possible, and then covered with thick ceramic fibre paper.

An appropriate texturing powder is applied before each firing, with care being taken not to damage the fibre paper when removing the bulk of the old powder. A very soft brush is best for this.

Provided that it isn’t disturbed, the paper will last for very many firings. In this case a second layer has been installed over the first as it was getting a bit ‘ratty’. Certainly no point  in throwing away the first layer.

The props here are longer than normal, to allow access to the centre for levelling.

Battwash

All of these shelves require battwash. See Battwash for full details on the materials and application methods.

Thermal shock

Thermal shock is the effect of excessive stress caused by too rapid a change of temperature or too steep a temperature gradient through a material, resulting in it breaking.

In the case of silica based materials such as silimanite and cordierite an additional risk is posed by the quartz phase change phenomenon, the most significant of which is the alpha-beta shift which occurs at 573°C (1063°F).

The effect of this is best explained in a bulletin issued by Riley Glass Kilns to owners of FS-1 kilns. Please note; the bulletin as shown below has been edited in minor detail from the original.

Bullentin

Issued in September 2007 by Riley Glass Kilns.

To owners & users of FS kilns

There have been reports of 1300mm x 675mm x 38mm extruded hearth slabs cracking during a firing.

In some cases they have been used in our (RGK) model FS-1 kiln, in other cases in kilns by other makers.

The slabs have been imported by Refractory & Ceramic Pty Ltd  of Melbourne (R&C) and supplied either by us with our FS-1 kilns or by R&C direct to various consumers.

Recommendations

• Glass firing schedules be amended to progress from the transition zone to above 600°C at much slower rates than is appropriate for the glass. In this range the glass is able to withstand unlimited rates of rise, whilst the kiln furniture cannot.
• Additional support for the slab be built in on both sides at approximately the half way point. These are not to support the slab in normal operations, but to minimise loss of product should a crack occur, (cracking has occurred at about this point).
  

Editor comment March 2010.

A more recent report tells of a slab cracking longitudinally from end to end, again about in the centre.There is a limit to the number of safety props which can be installed.

• Tuition in the proper handling and use of kiln furniture be included in glass training courses. This comes naturally to potters, as their ware is subject to the same phase change and other stresses as the kiln furniture. Their training includes instruction on the quartz phase change phenomena.

In some instances the slabs were placed directly onto a refractory base, without being packed up on props to allow adequate air circulation. It has long been known that not allowing free air circulation all round a kiln shelf is a recipe for disaster, (slabs in FS-1 kilns were NOT installed in this manner).

Replacement slabs, installed with appropriate clearance, functioned satisfactorily.

Technical

The slabs are primarily Cordierite, but with an inevitable and significant quartz (silica) content.

Cordierite itself is a robust refractory compound with low thermal expansion and reasonable resistance to thermal shock.

Quartz (silica), both in free form and in simple compounds, is subject to a number of phase changes during a firing cycle, the most significant of which is the alpha-beta shift which occurs at 573°C.

At this point the crystal structure changes. When being heated it changes from being symmetrical to skewed. Energy is absorbed without change of temperature and the volume  increases.

On the way down the opposite occurs. The structure changes back from skewed to symmetrical, energy is released without the temperature of the body decreasing and the volume decreases.

This change in volume takes place only where quartz particles with a crystal lattice structure exists, and not where individual silica atoms form part of a complex compound structure.

They may make up only a small part of the whole mass of the slab, but small clusters distributed at random may cause localised stress which could become excessive.

In refractory and pottery firing the commonly accepted method of coping with this phenomenon is to progress the firing through the zone at a slow rate when both heating and cooling.

Starting the glass annealing process at a slightly higher temperature will cater for the cooling stage.

General

The above comments apply not only to the extruded slab mentioned, but also to all other pressed or extruded kiln shelves, whether they be silimanite or cordierite.

It is nigh on impossible to produce kiln furniture which is completely free of silica/quartz, so its presence must be allowed for when preparing a firing pattern.

A general rule applying to the firing of pottery or glass could well be applied to the kiln shelves: the thicker the material the slower the rate of heating.

A cracked slab can continue to be used. (in one case for a number of years and still going in 2010. Ed).

• Prop the two halves at the crack so that the top surface is true and level.
• Force the halves tightly together.
• Cover the area to be occupied by the glass with CF paper. About 1mm or so thick works OK. Apply bedding powder or other treatments as normal.

The paper is needed to prevent the crack line showing on the glass. The parts of the slab will expand during the firing and as they cool will contract so that they may be apart when cold. There's no great need to push them together before each firing; the gap will only open up again.