The traditional material for kiln insulation is refractory firebrick, a tough and dense material. You will find older kilns made of this, and it durability also means it is well suited to wood, salt and soda firing as well as being used in some larger kilns intended for high usage and a long life. Apart from the weight of the bricks, which makes moving the kiln a major exercise, their other downside is that they absorb a lot of heat from the kiln. This means that, for a given size and power of kiln, it takes longer for the kiln to heat up, but the energy is largely stored in the bricks and released back into the kiln on cooling, so effectively you have a longer hold and a longer cooling time.
Nowadays most electric kilns use insulating firebrick for their primary insulation. This is much lighter and absorbs less heat, but physically it is weaker and quite friable. These bricks absorb much less heat, so increase the efficiency of the kiln, allowing it to heat up and cool down more quickly. Whilst older kilns, especially in the USA, may have just 2½ inches (63mm) of insulation, the standard is generally 3 inches (76mm). Because of the low strength of these bricks, if the elements are recessed into the bricks then hard brick element holders may be used to prevent damage to the kiln when the elements are changed. For the same reason, a hard brick tube may be used to take the element tails out of the kiln.
Some kiln manufacturers are now adding an additional layer of insulation outside of the bricks, in the drive to increase energy efficiency, which is good for your electricity bill and for the planet. Because the outside of the brick is relatively cool, this insulation doesn't have to withstand the same high temperatures, so highly effective but lower cost insulation can be used.
|Material||Density kg/m³||Linear expansion %@1000°C||Specific heat capacity, J/kg/°C||Thermal conductivity W/m°C@1000°C||Diffusivity m²/s x10¯³||Effusivity J/(m² °C √s)|
|Refractory firebrick||2142 - 2270||1000||1.37 - 1.54||60 - 70||54 - 59|
|Insulating firebrick, 26 grade||770||0.6||1100||0.27 - 0.37||30 - 40||15 - 18|
|Ceramic fibre||96 - 160||1130||0.074||40 - 70||3 - 4|
|Stainless steel, 304||7800||450||14.4||400||225|
If the lid or door is just a flat fit then this will leak heat, but a number of things can be done here beyond just making sure that the hinge is properly adjusted so that everything lies up properly and is flush. First, some lids and doors have a central boss that fits into the hole of the kiln - on some kilns it is designed so that as the kiln heats up the expansion makes the door and body lock together, and the kiln cannot be opened until the kiln has cooled down. On other kilns, a soft rope of ceramic fibre is put into the door, squashing in to a groove in the body to seal any gaps.
Bungs going into peep holes and ventilation holes are another potential heat loss, especially the thin slip cast type. They will quickly heat up, and leak heat out of the kiln both by convection and by radiation. The better type are made from insulating firebrick, with just a very slight taper to minimise any air gap.
You may feel that your kiln could have improved insulation. So what can you do, without it becoming a major re-engineering project?
First, check your lid or door hinge is properly adjusted so that everything is flush when closed. If you don't have a rope seal, then you may want to consider cutting grooves in the lid and top of the kiln to take one - get some bricks to test the size of groove before starting on your kiln. Also, check your bungs, and make some out of IFB if yours are slipcast.
Next, consider the insulation on the kiln. Do not try adding insulation on the outside of any metal casing on your kiln, as with the metal sandwiched between layers of insulation it will get excessively hot. But many top loading kilns do not have any metal on the top of the lid or on the base, so these are places where you can add an insulating board. Check the temperature on the outside of your bricks, and add a good margin as this will increase as you increase the insulation, to see what types of board may be suitable. These can be attached to the kiln using suitable high temperature adhesives.
The table below (based on data from Vitcas) shows the outside temperatures that can be achieved for a number of materials, for a surface temperature of the kiln of 200°C before adding additional insulation.
|Material||Thickness mm||Temperature °C||Thermal Conductivity W/m°C||Specific Heat Capacity kJ/kg°C||Cost £ inc VAT/m²|
|Fireplace Construction board||30||52||0.2||1.15||59|
|Ceramic fibre board||25||56||0.074||1.13||50|
|Ceramic fibre board||50||38||0.074||1.13||70|
|High Temperature Plasterboard||12||100||0.8||37|
|Ceramic fibre paper||5||120||0.05||1.07||20|
|Insulating firebrick 23||114||32||0.37||1.1||137|