SELECTING AN ELECTRIC KILN

Transcription

1 SELECTING AN ELECTRIC KILN David Coggins INDEX 1.0 Introduction 2.0 General Considerations 2.1 Power Supply 2.2 Electricity Tariffs 2.3 Insulating Material 2.4 Kiln Location 3.0 Kilns for Pottery and Sculpture 4.0 Kilns for Slipcast Ceramics 5.0 Kilns for Porcelain Dolls 1.0 INTRODUCTION Choosing a kiln to suit your purposes requires care. Whether you are buying new or used, you will need to choose a kiln which will reach the temperature you require without stress and strain. The broad categories of firing requirements are listed below, together with suggested styles of kiln for that purpose, with reasons for that choice. 2.0 GENERAL CONSIDERATIONS 2.1 Power Supply. Power requirements for a kiln must be the first consideration, as the cost of installing extra power for a kiln to your house can be considerable. Electrical appliances, including kilns, are rated according to the power consumed and/or the current drawn by the appliance. For the technically minded, power in watts equals the rated voltage (eg. 240 V) times the current drawn in amps - or P = V x A. Therefore, A = P / V, or the current drawn in amps = power in watts divided by the voltage. An example is a kiln rated at 7200 watts at 240V supply. Amps = 7200 / 240 = 30 amps.

2 Therefore this kiln will draw 30 amps from a 240 volt supply. From this point, kilns will be referred to using their current rating on a 240 Volt supply. Almost all kilns require special power supplies, only the smallest kiln can run off a standard 10 amp power point as you find in your house or shed. Many kilns require a larger power point, such as a 15 amp, 20 amp, 25 amp or 30 amp single phase outlet. Usually, the larger sizes will be "hard wired", that is, connected permanently to a wall switch. This is done to reduce costs and a possible point of overheating, and also for reasons of special tariffs (see below). All the above are "single phase", that is the power connection you find in most houses. You can check that you have single phase by looking at the power feed from the pole to your house (assuming that you don't have underground power). A single phase feed will have only 2 wires coming into your house. Another way to check is that you have one meter in the meter box, or two if you have off peak hot water supply. Single phase kilns up to 30 amps are commonly available and can be connected to many installations by fitting a changeover switch to your electric stove circuit (assuming that you have one!). In other words, you can use the stove or the kiln, but not both. This avoids the cost of bringing in additional power from the street supply. Most electrical authorities won't allow connection of a kiln larger than 30 amps to a single phase circuit. You will need to consult your electrician to discover what power your installation can deliver. Larger kilns will require 2 or 3 phase power. This may require more wiring from the street, a new power switchboard, plus additional wiring - a quite expensive exercise, not to be undertaken lightly. Some 2 or 3 phase kilns can be connected as a single phase unit, but the limitation of 30 amps still applies. The number of wires from the street, or the number of meters in the box will also give a clue to how many phases you have. 2.2 Electricity Tariffs If you propose to use the kiln frequently, ie. more than 20 times a year, it may be worthwhile to consider having the kiln connected to a special tariff. This means that a special meter will be installed in your power box for the kiln, and you will be charged less per Kilowatt/hour (Kw/h) for your power. There are several types of special tariff available, but the safest is one where you pay much less than the normal rate during off peak hours (usually about 20 hours) and more during the peak times. The advantage of this tariff is that if a firing is running late, it can be completed, even though the last hours may cost more. Another tariff commonly used is the same as the hot water supply, that is the power is actually cut off in peak time. This is usually cheaper still, but you have to be very disciplined to avoid the frustration of the power going off 10 minutes before the completion of the firing.

3 A common feature of special tariffs is that the kiln must be wired direct, that is not plugged in, to avoid the possibility of running other appliances from the same source. The installation of equipment for special tariffs can be costly, so you should check the cost advantages carefully before considering it. 2.3 Insulating material. BRICK LINED KILN FIBRE LINED KILN All kilns use two basic types of insulation, brick or fibre. Refractory Insulating Brick (RI Brick, or just brick) is usually buff in colour, and grooved to mount the elements. These are fairly strong but still require gentle handling. Their insulation is good, but they absorb a lot of heat (power) during firing. RI brick insulation can be from 65mm (2 1/2") to 115mm (4 1/2") or more in thickness, depending on the application. Refractory Ceramic Fibre (RCF or just fibre) is usually white in colour, and comes in the form of a blanket 25mm (1") to 50mm (2") thick. Occasionally, a formed ceramic fibre board is seen - this is the same material, mixed with a fixative and pressed into a mould. Fibre is very fragile, and needs careful handling to avoid breaking the surface and releasing fibre particles. It is a very efficient insulator, and absorbs almost no heat. The elements are usually mounted on separate ceramic suspension systems, except in one case where there are moulded element grooves in ceramic fibre board. Fibre insulation is usually 125mm (5") to 150mm (6") thick in total, made up of various layers of blanket. 2.4 Kiln Location. The electric kiln may be located inside or under a house, so long as there is some cross ventilation available to take away any fumes or gases. The degree of ventilation required depends on what the kiln is used for. Any kiln firing mined clays, all of which contain some sulphur, will cause sulphuric acids to condense on nearby metal surfaces, with nasty results to the paintwork and metal. These gases must be removed altogether by positive ventilation if the kiln is to be located inside. Kilns firing cast ceramics, china paints or glass, will produce irritating but not harmful gases, and natural ventilation may be used. If ventilation or heat is a problem, the kiln should be located in a separate shed, or on a verandah.

4 Consideration must be given to wiring for the kiln - a direct path to the switchboard should be available to minimise installation costs. Also, don't forget that kilns are heavy - access to the selected location must be suitable for the entry of carrying equipment, ie. level access, wide doorways, concrete paths etc. There is nothing worse than trying to manhandle a 300 Kg kiln into position - I write from painful experience. 3. KILNS FOR POTTERY AND SCULPTURE Pottery and sculpture requires temperatures up to 1280 degrees C, which is the maximum permissible temperature for element wire. If you plan to fire any kind of pottery, you will need a kiln which can achieve that temperature without strain, and which can be carefully controlled during the firing cycle to avoid explosions in the kiln. The kiln for pottery or sculpture needs to have at least 115mm (4.5") of insulation all round. Any less will make firing to high temperatures difficult, if not impossible, in spite of claims made by US manufacturers of cone 10 (1280 deg C) firings on their round ceramic firing kilns. These are not capable of reaching cone 10 on the slow firing cycle required by potters and sculptors, and are not up to the hard work required to fire to that temperature. Suitable kilns for pottery or sculpture will be front or top loading, depending on preference. Top loaders tend to be cheaper to buy. Brick or fibre insulation of 115mm (4.5") minimum is required. Brick lined top loaders have elements all round for even firing, larger front loading brick lined kilns (over 4 cubic feet) should have elements on the door to give even firings. Fibre lined kilns are more expensive to buy, but cheaper to run. Fibre lined kilns usually have elements only on the sides but this does not cause uneven firing, as fibre does not absorb heat, but reflects it. The kiln for pottery or sculpture should be fitted with a sophisticated control system, to allow close control of the temperature rise, cut-off temperature and soak time. This can be done manually, but this can be very laborious and not always satisfactory. The minimum requirement would be a digital cut-off controller perhaps with soak timer, the best is probably a multi-stage programmable controller.

5 4. KILNS FOR SLIP CAST CERAMICS Slip cast ceramics require temperatures up to 1150 degrees C, with fast firing and cooling cycles. These requirements can be met by using one of the range of round top loading kilns, mainly from the US, with 62mm (2 1/2") or 75mm (3") thick brick walls and lid. Most kilns will fire slip cast ceramics, but a kiln designed for pottery may have a cooling rate which is too slow for the purpose. However, a pottery kiln uses energy much more efficiently due to its better insulation. A decision will need to be made to suit your needs. The control of kilns for slip cast ceramics is much easier, with a simple controller or kiln sitter usually sufficient. Many of these type of kilns feature an automatic firing cycle, which is satisfactory for firing ceramics. 5. KILNS FOR PORCELAIN DOLLS Porcelain dolls require temperatures up to 1205 degrees C, with fast firing and cooling cycles similar to slip cast ceramics. The temptation may be to use one of the kilns designed for slip cast ceramics, but in my experience, these don't give satisfactory results in all cases. The problem is that many doll makers like to have a slow bisque fire to give a translucent look to the doll, and the ceramic kilns tend to slow down too much under these conditions, and not reach maturity. Also, I have found that a small drop in the supply voltage to 225 volts, for example, (normally 240V) can cause some ceramic kilns to fail to reach 1205 deg C or take up to 16 hours to do so. I recommend that a pottery style kiln be used for dolls if you prefer a slow firing cycle. This will cost you more to purchase, but the results will be worth the difference. If you are happy with a fast firing cycle, and have no doubts about your power supply, then a ceramic style kiln will proably be suitable.