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Morells Barn • Park Lane • Lower Bognor Road • Lagness • Chichester • West Sussex • PO20 1LR
Telephone: 01243 265845 • Fax: 01243 267582 |
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| Choosing the right kiln Choosing a kiln should not be a difficult task; the bricks used by most manufacturers will be the same. However the analogy “you get what you pay for” could not be more applicable than when buying a kiln. In comparing one make to another is not necessarily comparing like with like and it is essential to research adequate before commiting to what is probably the most important purchase the potter is ever going to make. Bricks will always be 9 x 4.5 x 3”; thus the sizes, in particular top-loading kilns, will be standardized. Some manufacturers will limit their range to the most popular sizes. Gas or Electric? The first consideration when choosing a kiln is the type of firing you want; oxidation (electric) or reduction (gas). Gas means with propane or natural gas. The only difference between the two will be the jet on the burner, which is relatively easily changed from one to the other. Generally the firing cost between similarly sized electric and gas kilns is not significantly different. Gas is often the more expensive because gas kilns do not burn the fuel efficiently and because a large amount of heat is lost through venting. The main advantage of gas is that a reduction firing is possible. However there are many disadvantages, especially for the relative novice. Obvious examples are the bisque firing which is best done in the relatively clean atmosphere of an electric kiln. The initial heating of an electric kiln can be easy to control; gas kilns need specialised venting partucularly if sited inside. Also, gas kilns require need much more attention and care to fire properly. Generally, when larger kilns are contemplated (15 cubic feet or more) gas is chosen. It is easier and less expensive to install a gas service than the electricity supply required by such a large electric kiln. Top v Front Loading An argument can be made for either and it is often what the individual prefers. It is suggested that a front-loading kiln is easier to load, but it should be remembered that you will be required to extend your back when reaching into a front loader. This strain is emphasized if the pieces are heavy. Generally front-loading kilns are more expensive. They are built with 4.5” thick walls. Alternatively top-loading kilns, with bricks set on edge, have 3” thick walls. Some imported brands will have 2.5” thick walls, reducing the number of bricks used in the build. As the bricks are one of the main material costs the price will always be lower. Round v Square On a per cubic foot comparison, the round and similar oval kilns are less expensive than square kilns because they are easier to build and use fewer bricks. Depending on the size and shape of the ware being made a square configuration may load more effectively or vice versa, but always balance this against the initial cost difference. Bricks v Fibre Insulating firebricks are used in the vast majority of kilns and they will always outlast fibre although fibre heats and cools faster, which can be desirable in certain instances. However, it is generally considered a disadvantage. Elements are much easier to replace in a brick kiln. In fibre lined kilns they are normally supported on ceramic rods; changing is therefore more complex and can be considerably more expensive. Temperature Most electric kilns produced in the UK will offer a maximum firing temperature of 1300°C (Cone 10). It is essential to appreciate that although probably all manufacturers refer to temperature because of the traditions of industry they are in fact referring to heat work (the effect of temperature over a period of time). Therefore if the kiln has a maximum operating temperature of 1300° and a kiln controller is fitted this should never be set to 1300°C or it will probably irreparably damage the fabric of the kiln. The damage will be greater in a kiln where the bricks are cemented (front-loading). Experiments with pyrometric cones to make initial comparisons of temperature to heat work are essential. Kiln Size Too small a kiln means too many firings. Too large means either wasting energy or the frustration of waiting too long for a full load. The larger the kiln the less expensive it is per cubic foot to buy. Also it is far less expensive to fire one large kiln rather than two firings in a kiln half the size due to the differences in heat loss and the time/energy required to heat up both the kiln and the load. One way to decide what size kiln is best is to draw a circle (top-loader) or square (front or top-loader) the size you think you want on newspaper. Try placing pieces you make on the paper to see if the size is adequate and the pieces pack efficiently. Gauging the height can be assessed in a similar way. Electricity supply Once you have selected the model or size of kiln you would like, you need to check your electricity supply is capable of taking such an appliance. If it is a 13amp model you should not have a problem as these kilns can be plugged into any 13amp socket. Remember that they should be plugged directly into the socket, never via a “gang” plug or extension lead. A kiln with a kW in excess of 13amp will always require a dedicated supply. This involves running a cable from your main fuse or distribution board to the room where the kiln is to be sited. An “isolator” or on/off switch should be sited convenient to the kiln placement. The kiln is then wired to the isolator. This is a very similar installation to an electric cooker. A modern house has 100amps of power approximately supplied to it. All the appliances used within the house cannot exceed this total. If you are unsure consult your electrician to check if you have enough available amps to enable your kiln to be installed. The voltage in your property must also be checked. Colloquially we are said to use 230volts in the UK, but through local area demands and also internal demands the voltage can drop. If your kiln is “plated” for 230v it means it needs this amount of voltage all the time it is operating. It has been known that potters have been advised by ill-informed electrical people that if your tumble dryer or washing machine works your kiln should work. This unfortunately is not the case. Normal household appliances have parameters in which they can work (i.e. if the clothes in your tumble dryer are not dry you can put it back on for longer etc.) A kiln must have 230v all the time otherwise a drop off in performance will be noted especially at stoneware temperatures. Modern, as well as more and more older properties now have electricity supplies involving residual circuit devices (RCD’s). Kilns should not be wired in using an RCD as the moisture in a new kiln or from bisque firings will cause the supply to “trip”. If in doubt, consult a competent electrician. Electricity Phases It is standard for all domestic properties to have a single phase supply. Commercial premises or institutions can have three phase electricity supplies. Depending what appliances are already in use in a domestic property an 8 cubic foot kiln, which will be rated at about 60amps, can be installed on a domestic supply. The advantage of a three phase supply is that the amperage of a kiln can be divided into three; therefore, the 8 cubic foot at 60 amps would be divided to 20amps per phase. Any kiln over 13amps can be converted easily to operate on a three phase supply. Dividing the amps also means much larger cubic capacities can be installed. Wall Thickness Kilns imported from America can have walls only 2.5” thick or 3” thick and no secondary insulation (a 0.5” thickness ceramic fibre blanket sandwiched between the brick and the kiln cladding). British made kilns invariably have 3” thick walls with the ceramic fibre secondary insulation. Analysis of the energy used for heat storage in the brick (the energy absorbed by the brick itself) versus the energy lost through the bricks shows that unless you plan a soak or a long firing at very high temperatures (where energy loss becomes more pronounced) or need to cool very slowly such as in crystal glazes, then the advantage of the thicker bricks and walls is almost negligible. Again it is stated by American manufacturers that where the fibre insulation is used as secondary insulation the energy savings is minimal at low temperatures and the advantage can only be noticed at high stoneware temperature firing (cone 8-10). We strongly recommend kilns with 3.5” thick bricks and 0.5” fibre backup as they can be used for any temperature up to 1300°C (cone 10). Also, the user is not limited to only lower temperature within the earthenware range. Elements We are often asked how long elements last. Unfortunately there is not a simple answer. The variables are the kW of the kiln (the more power going into a kiln the longer the elements should last), the amount of ware in each firing, and perhaps most importantly the temperature the kiln is fired to. Kilns fired to stoneware temperatures will need elements replacing far more often than kilns only fired to earthenware or bisque temperatures. The difference can be surprising, six to ten times more firings at the lower temperatures is possible. Controllers Over the years kiln controls have varied from the Dawson Kiln-Sitter to computer based programmable controllers. The kiln-sitter was fitted to virtually all kilns imported from the States. They worked by using a pyrometric cone or bar, which bent from exposure to heat over a period of time. These were simple reasonably reliable devices. As electronic controllers have fallen in price the kiln-sitter has faded into the distance. The modern kiln controllers permit almost unlimited hands-off control over the entire process. They are relatively simple to use and allow controlled temperature rises over one or more ramps. Some also control the cooling cycle and the majority will also allow a soaking or dwell time during the heating up process and at the end of a firing. The latest controllers are also self diagnostic and can show error messages if something has failed. Zone Controls Most kilns on the market have each of the elements controlled at the same time as a single zone. In kilns with large cubic capacities, achieving an even temperature distribution within the firing chamber can be a problem. To help resolve this problem, some controllers split the kiln into zones, which are then controlled individually. Having this facility adds to the overall cost of the kiln. Kiln Positioning A kiln should be placed on a non-flammable surface (not wood or lino). Also allow at least 12” (30cm) gap around the kiln to allow heat to dissipate. Front-loading kilns should have at least 18” (45cm) space at the rear of the kiln for servicing or element changing. If this is not possible castors should always be fitted to allow the kiln to be moved easily for access. The floor should be checked before siting to ensure it is level. Always ensure the floor can take the weight of the kiln, this will normally only apply to front-loading kilns when they are not sited on the ground floor. The kiln controller (when connected to the kiln via a cable) should be mounted at a convenient height for access and to one side of the kiln. Access to the controller should be able to be done with ease and without being necessary to touch the fabric of the kiln. A kiln will always get hot in use; therefore nothing should be sited within touching distance of the kiln or placed on top of the kiln. It is also unwise to store any materials around a kiln. Venting Venting has come to the fore as an important health and safety concern. When firing, a kiln can emit gases from glazes that are being fired and carbon monoxide from the clay. If a kiln is being sited in a room such as a garage and air can flow freely in and out it may be decided that ventilation is adequate, especially if the room is not occupied at times when the kiln is fired. If a kiln is for a school, college or public institution it is almost certain that mechanical ventilation is necessary. The individual establishment should carry out a risk assessment and/or contact their local authority for advice. It should be noted that for a mechanical vent to work efficiently there must be an air inlet into the room and this should be sited on a wall opposite to the wall where the vent is sited. The air inlet is essential if a fire door is fitted to the kiln room. If a kiln is sited in a classroom an adequate barrier or cage should surround the kiln to prevent unauthorised or accidental contact with the kiln. Other Expenses When estimating the cost of a kiln, include the cost of kiln furniture (shelves and props to stack ware on different levels within the kiln). Generally one shelf will complete one layer, but as the kiln gets bigger a layer may be made up of two or more shelves. A few points to remember are: • A shelf should always be placed on the base of the kiln to protect the bricks. • The largest pieces can be placed on the top shelf to avoid unstable stacking. • At least 1” of space should be left between ware and the shelf above to allow for air circulation. • Large shelves can be heavy and unwieldy. • Large bottomed ware, such as platters should not be placed spanning two half shelves. Estimating Electric Kiln Firing Costs Costing your firing A= Kilowatt rating of kiln (taken from the kiln’s data plate) B= Firing times in hours x 0.6 (since the kiln is only on full power towards the end of the firing cycle.) C= Cost per kilowatt/unit (taken from your electricity bill) Always use a qualifed competent electrican or engineer. |
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| KILN & EQUIPMENT DELIVERY Delivery of kilns or equipment is often the aspect that is neglected the most. You must be aware how your kiln will arrive and what you may be responsible for. If your front-loading kiln arrives on a pallet weighing over 250kg and the delivery company leaves it on your drive, how do you get it on the ground and into position? We would like to make all customers aware of what is included in the three services we offer. PALLET DELIVERY The service can be applied to the delivery of top-loading kilns and wheels or any piece of equipment with external dimensions no greater than a metre square. The equipment is strapped to a pallet and delivered by means of a lorry with a tail lift. These vehicles normally have a pallet truck that can be used to manoeuvre the pallet on flat ground, but it must be appreciated that the price given is to deliver to site and off-load. It is the customer’s responsibility to remove the kiln or equipment from the pallet and move into position. Nabertherm Kilns are delivered by DHL pallet delivery service. The drivers will only off-load to the nearest convenient point. (Note: If a delivery driver offers to help further they are not covered by insurance and you are accepting responsibility for any damage that may occur to property or the kiln.) DELIVER, OFF-LOAD INTO POSITION This is more of a tailored service and covers all sizes and shapes of equipment. Again, it is delivered by vehicle with a tail-lift. The vehicles will always have a pallet truck and the driver will off-load and move the equipment into position. A survey is requested to give the carrier information on the surfaces and site to be encountered. This service can negotiate one step of no more than 4”, but cannot easily deliver heavy equipment on uneven or soft terrain. For large or relatively unstable equipment assistance is requested from the customer. A completed kiln delivery questionnaire is required before delivery prices can be confirmed. This service is not available to all UK locations. NORTHERN KILNS (Chieftain, Champion & Rocket Kilns) SPECIAL DELIVERY This service is unique to Northern Kilns (Chieftain Round and Oval, Champion and Rocket Kilns) and is only provided for this manufacturer’s kilns. They deliver and off-load into situ negotiating no more than one step. They will install the kiln to a suitable isolator (switch) if applicable and if it is situated within 2 metres of the kiln. They will also instruct on use of the controllers supplied with their kilns. A completed kiln delivery questionnaire is required before delivery prices can be confirmed. (Note: This service is unrivalled, but the price reflects the comprehensive package offered and the removal of stress and responsibility to the customer.) |
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