Your kiln shelves need to do a number of things:
- support your pots, without bending under their weight
- be resistant to any glaze drips or runs (aided by bat wash where necessary)
- be light enough to handle easily, and strong enough not to break
- have high electrical resistance, so if an element comes loose it doesn't cause a short circuit
- be of low thermal mass
- be resistant to thermal shock
The first four are pretty obvious, but a bit more info on the last points. Thermal mass is, effectively, the amount of heat energy that something stores per degree of temperature change. A high thermal mass means that a significant amount of the kiln's energy will go into heating up the kiln shelves, so the kiln will take longer to reach the desired temperature - and then it will take longer to cool, as the heat is released back into the kiln. To some extent you can correct for this by reducing the top temperature a bit, but it still decreases the efficiency of the kiln. Also, a high thermal mass generally means a bigger difference between the temperature of the air in the kiln and the kiln shelf, so there will be greater thermal stresses around the bottom of the pot.
The kiln shelves themselves need to be resistant to thermal shock and stresses, as they will be heated unevenly. Put a shelf level with your elements, and the edge will be heated much more than the centre - unless you have a front loading kiln with n elements in the door, when the front edge will be heated much more slowly than the others. And, of course, pots affect things too: the shelf will be cooler on the top under the base of a pot, compared to both the surrounding shelf and the underside. And pots may block radiant heat from the elements, causing cooler patches. To minimise this, two things help: high thermal conductivity, so heat quickly evens out across the shelf; and a low thermal expansion coefficient, so the forces created by differential expansion are minimised. Also, a more elastic material would help minimise the build up of stresses, as the shelf can flex to absorb the stresses, though this doesn't really apply much to the standard flat shelves we use.
There are a number of options for kiln shelf materials, though not all are suitable for electric kilns. Also, unfortunately, many are not available through suppliers here in the UK, but need to be imported. Below is a table summarising the key points, and a more detailed discussion of each option. Pricing is given in US$, and based on US prices, due to the lack of ready availability of some options over here.
|Material||Density kg/m³||Thermal conductivity W/mC||Thermal capacity J/kg C||CTE 1/C||Youngs Modulus GPa||Open Porosity %|
|Cordierite (high Al)||1910||1-4||1100||1.5 - 3.3x10-6||34||30|
|Oxygen bonded SiC, 70% SiC||2100||7||5x10-6||25|
|Oxygen bonded SiC, 90% SiC||2200||25||5x10-6||25|
|Crystolon RC-4128, 83% SiC||2450||10||5.6x10-6||18|
|Recrystalised SiC, >99.5% SiC||2700||25||5x10-6||250||15|
|Nitride bonded SiC, 65% SiC||2850||12||4.6x10-6||200||1|
|Cryston, 70% SiC||2750||20||8|
|Advancer, 70% SiC||2800||18||4.3x10-6||235||<1|
|Reaction bonded SiC, 78-92% SiC||30800||25-45||5x10-6||325||0|
Comparison of 355x711mm (14x28 inch) kiln shelves
|Material||Thickness mm||Weight kg||Cost US$|
|Cordierite (high Al)||25||13.3||82|
|Oxygen bonded SiC||19||11.8||111|
|Crystolon oxygen bonded SiC||19||11.8||166|
|Nitrogen bonded SiC||19||11.8||111|
|Cryston nitride bonded SiC||8||5.0||221|
These are produced in the USA as a low cost option, but are only really suitable for firing to earthenware temperatures. As such, they won't be considered further.
This is the standard kiln shelf. It is robust, and well proven in electric, gas and wood fired kilns, though not the best option for salt, soda or raku kilns. Its porosity is high, at about 30%, so will readily absorb any glaze spills, which will spread within the kiln shelf if not removed. Thus protection with batt wash is essential.
Composition is variable, but generally a high aluminium composition is used, that can be used at higher temperatures. Generally it is usable to 1300°C, above which point it slowly starts losing strength, and an upper temperature is 1350°C, though these temperatures can vary a bit according to the precise composition. The coefficient of thermal expansion is low, at about 3x10-6, giving good resistance to thermal shock.
Shelves should not be taken out of the kiln until below 200°C, to avoid cracking from thermal shock. Also, care should be taken to avoid chips on the edges, as these will act as stress raisers from which cracks may propagate, either from mechanical or thermal stresses.
In time, the shelves may begin to bow. Once out of true by 2mm they should be turned and used the other way up, which obviously means removing the bat wash and applying it to the other side.
The shelves should be kept dry. As with clay, if they get wet they should be dried out before putting in the kiln, to avoid cracking or explosions; also wet shelves should not be allowed to freeze, as they may be damaged by water expansion. A typical drying cycle is 5 hours at 90°C.
Shelves are best stored on edge, ideally resting on softwood battens or similar. Flat storage makes them more prone to cracking, as slight bowing in the shelves creates point loads in the centre of a shelf.
Perforated shelves are available which allow greater heat flow through the shelf, and also reduce weight by about 10%, but these may need to be thicker to offset the weakening effect of the holes.
Hollow shelves are also available (such as the Corelite brand in the USA), and these are much stronger for their weight. They are about a third of the weight of a solid shelf, but they are also thicker and so take up more space in the kiln. Also, if at the bottom of a tall stack of heavily laden shelves there is a risk of the pros squashing the shelf, so some care may be needed to avoid this. And these shelves are not suitable for wood firing.
There are a number of different types of silicon carbide used for kiln shelves. These vary quite a bit in their properties, as may be seen in the table above.
Most of these shelves are good conductors of electricity, and so are not considered suitable for electric kilns. The exception is the Advancer shelf, which has a high resistance glassy outside that does not conduct readily (and possibly Thermal-Lite, though I am not sure about this). In general, if used in an electric kiln it must be unplugged, or switched off at the isolator switch, whenever loading or unloading. Although modern electric kilns (at least in Europe) need an electrical cut-out when the door is opened, this can fail if present, and elements may pop out and touch the shelf.
A benefit of these shelves is that they have much lower porosity (though this can vary), so any glaze runs lie on the surface, rather than being soaked up into the shelf - though a light coating of batt wash, or firing on a saucer, is good practice. The kiln wash should not contain silica, as this will stick to the shelf - an aluminium oxide/kaolin type is preferred.
They are also stronger, lighter and thinner than cordierite shelves, especially the nitrogen bonded versions. But still most of the types of SiC shelves can still warp with time, so they should be turned over periodically.
Silicon Carbide shelves are more susceptible to failure through thermal shock. Some have slits cut in them to act as stress relievers. They are not suitable for raku kilns, and caution is needed in wood, salt and soda firing to avoid thermal shock from flames, sprays of salt or soda, or other differential heating. They should not be exposed to very high heating or cooling rates in the kiln - Advancer recommend a maximum rate of 135°C/hr. Nor should they be placed at the same height as a heating element, as then the edge of the shelf will be heated much faster than the rest.
They are also more brittle than traditional kiln shelves, so may shatter if dropped.
Another downside to SiC kiln shelves is that if they absorb water they may explode, just like a damp pot being bisque fired - particularly the less porous types, as it is harder for water to escape. Thus they should not be left in a wet or humid environment for a long period of time, nor should they be left on a surface such as a cement or concrete floor, through which they may wick up water - though brief exposure, such as wiping with a wet cloth or applying batt wash, isn't a problem.
The recommended drying programme for Advancer shelves is given below. As they are amongst the least porous of shelves, the same cycle should be safe on others.
- To 100°C at 28°C/hr
- To 110°C at 14°C/hr, then hold for long enough for all shelves to reach temperature, and then an additional 12 hours
- To 175 - 260°C at 55°C/hr, then hold for long enough for all shelves to reach temperature, and then an additional 16 hours
- Switch off for a slow cool
This is the basic silicon carbide shelf. Mostly used in gas kilns, its main benefit is resistance to glaze spills. Crystolon shelves are made this way.
One step up from oxygen bonded SiC, this offers greater strength as well. In an electric kiln it needs an initial firing in oxidation in an empty kiln, to let organic matter burn out that would otherwise cause the firing to go into reduction in its first use. Cryston shelves fall into this category.
Therma-Lite shelves are a higher quality of Nitrogen bonded SiC. They are stronger than standard types, and so can be made thinner, saving weight and allowing a bit more in the kiln.
Porosity is less than 1%, and the shelves have a smooth, glassy surface, making it almost impossible for glaze to stick, but high flux ceramic bodies like porcelain may stick a bit to the shelves, so a light silica-free batt wash is recommended to avoid this.
Whilst they are similar to Advancer shelves, no information is given on their electrical conductivity. As such, they must be treated as cautiously as other silicon carbide shelves in electric kilns.
This is the brand name for Saint-Gobain's nitrile bonded kiln shelves. They are about 19% stronger and half the weight of 25mm thick cordierite shelves.
Unlike other silicon carbide shelves, they have a glassy surface layer with high electrical resistance, which gets thicker with repeated firings. This makes them suitable for use in electric kilns assuming the elements are kept in good repair and the kiln is never opened with power on. Tests have shown that short circuits from elements touching the shelves will not result in damage to the kiln, controller or shelf, and the element will not get stuck on to the shelf. They also say that any electrical shock is unlikely to be lethal, though the tests were carried out in the USA which widely uses 110V power, inherently safer than the 230V used in Europe.
Kiln shelves are best stored on edge, as if stored flat the cumulative weight may crack lower shelves in the pile. A wooden rack for the shelves is ideal, as there is no risk of impact damage to the shelves. Also, they should not be stored in a dry environment, not in contact with water, or with substances like concrete that may be holding water.
Mix to a thin cream consistency. Apply 2 or 3 thin coats by brush to the top surface of all kiln shelves - leave the edge and underside uncoated, to avoid bits dropping off onto the pots below. Also apply it to the floor of your kiln, to protect it from any drips, but keep it well clear of your elements as it will corrode them.
Remove damaged or glaze-encrusted small areas with a silicone carbide abrasive block. For large area, I use a diamond grinding disk in an angle grinder. This gets very dusty, so ideally do it outside with a respirator, as well as safety glasses and gloves. Make sure all of the glaze is removed, otherwise it will eat further into porous shelves when it melts in each firing, weakening the shelf.
There are many recipes available for batt wash. Generally they use refractory materials such as silica, zircon, kaolin or aluminium oxide, with a small amount of flux so it adheres lightly to the kiln shelf, such as feldspar or using some ball clay instead of kaolin.
Personally I avoid silica in batt wash. It can dissolve into glaze runs, which defeats the purpose. If you fire above earthenware temperatures, there is a greater risk of silica sticking to the pots, and if using silicon carbide shelves it may react with and bond to the silica on the surface of the shelves. And, of course, breathing in silica dust is not a good idea.
My preferred recipe is 50% hydrated aluminium oxide, 25% kaolin and 25% calcined kaolin, plus 1% feldspar. If you don't have calcined kaolin, just put kaolin into a bisqued bowl and put it into your next bisque firing. Mix to a thin cream consistency and brush or roll two coats onto the kiln shelf, letting it dry between coats. If applied too thick, it will tend to crack.
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