Kiln and Firing Advice
What is an electric pottery kiln?
Kilns are chambers designed to contain heat so that temperatures can be achieved sufficient for clay to be converted to ceramic. The source of heat in electric kilns is the heating elements, wire coils that produce heat as a result of resistance to the flow of electricity. They are similar to the elements in an electric fire. The kiln chamber is constructed from insulation materials that contain the heat reflecting it back into the kiln. A kiln is really a box full of electric fires.
Potterycrafts Kilns have the following safety features.
Safety Advice for Electric Kilns
Electric kilns are designed to contain heat; even in extreme situations where a kiln malfunctions and over-fires the heat will be contained within the kiln. Electric kilns are therefore not a fire hazard and are safe to operate providing common sense working practices are followed. We hope that the following notes will allay any worries, if you have any concerns please do not hesitate to contact our technical staff at the above address.
Legal Requirements
Electricity at work Regulations 1989 apply to both employed and self-employed persons. In essence the following statuary obligations apply.
Maintenance
Kilns do not require very much maintenance, it is important that element grooves are kept clean by gently vacuuming up the accumulated dust. Take care not to chip oxide layer on the surface of the elements. Wiring Inspection should be carried out once or twice a year. After isolating the power open the inspection panel and look for blackened wires that might indicate loose connections. Ensure all element connections are tight and that the porcelain isolators are not cracked. Glaze spots should be removed by gently scraping or digging out; if left glaze will melt on each firing and will eat into brickwork and elements.
Positioning the Kiln & Ventilation Please see our seperate pdf download for the installation and ventilation guides available to download here:
Element Wear
Heating elements supplied by Potterycrafts are all made from the industry standard, Kanthal A1 type wire and are wound in our Stoke-on-Trent factory. There is no accepted guideline on the number of firings to be expected from elements as there are so many variable; temperature, firing profile and kiln atmosphere radically affects wear. For example, one might get in excess of five hundred firings from elements if fired at low earthenware temperatures in a well ventilated kiln; in contrast, higher stoneware firings with long soaks could dramatically reduce the number of firings to fifty or sixty.
The element wire that we use is an alloy of chromium (22%), aluminium (6%) and iron (72%); approximate percentages.
During firing a grey crust of alumina oxide is built up of the surface of the element, this crust protects the metal beneath from oxidation so it is important that this is not damaged. Damage can occur if the element is bent or knocked; also a glaze spillage or glaze spitting during firing will attack the surface and can cause it to burn through and break.
When an element burns out molten slag is produced and this can contaminate the element groove, this must be removed before replacing the element as it will melt and eat into the new element as the kiln is fired. The surface crust can be corroded by some fumes especially fluorine and lead vapour and also if reducing atmosphere forms in the kiln. To avoid this corrosion and give elements a longer life the kiln should be well vented in the first part of the firing.
Elements are shiny and metallic when first supplied, the appearance of the metal may sometimes differ as they are coloured for batch identification. When fired the shine is soon replaced by a dull matt finish as the oxide layer becomes established on the element surface. Traditionally new elements were first fired to around 1100°C in an empty kiln to help the oxide layer to form more quickly. In practice most people do not bother with this as it is not seen to make a noticeable difference, however if clays or glazes are being used that produce significant volumes of fluorine or lead fume then it would be wise to pre-fire the elements.
As elements wear they become more brittle, thinner and greyer, they also distort and slump; in extreme cases they will bunch up and slump so that many of the loops are clustered together. The heat output diminishes with wear causing the kiln to fire more slowly, it may be necessary to reduce kiln temperatures to take account of increased heatwork generated by a longer firing cycle.
Elements can sometimes come out of the element grooves, often this occurs in top loaders because they have not been sufficiently stretched to create tension forcing the element to back of the wall. It can also happen if elements are too long. It is possible to correct this but you cannot just bend the element as it is likely to snap. If you first heat the element to red heat with a blow lamp you will then be able to bend it more safely using long nosed pliers.
We can supply elements for all kilns made by Potterycrafts providing they are 20 years old or less from time of manufacture, including those now discontinued, if enquiring about elements for your kiln please email us stating the kiln model and serial number, its power rating in KW, voltage and the date of manufactuer (if known). This information should be on your kiln's data plate