How to Fire Pottery and Ceramics – How to Choose A Kiln
A Kiln Has To Be Reliable and Economical
Having covered how to hand-build and glaze pottery and ceramics, its now time to look at how to fire pottery and ceramic using a kiln. A kiln in pottery is an oven for hardening by fire ceramic figures. A good kiln should be able to fire ceramics up to maximum temperatures in the range of 1400˚C and have the ability to show temperature to the accuracy of 1˚C. A temperature of 1400˚C is a very high temperature and very few metals and materials can withstand that. A good kiln has to be reliable and economical in terms of fuel cost.
Types of Kilns
There are electric fired kilns, gas fired kiln, wood fired kiln, oil fired kilns and coal fired kilns. Today, firing kilns with wood, coal and oil is like being an old alchemist, whilst firing kilns with electricity and gas is like being a modern day chemist who has a great degree of control over the end results.
Cost of Kiln Fuel
When you think of firing pottery and ceramics, you have to think about the cost of energy. Firing ceramics consumes a lot of fuel and the cost you pay for energy to fire your ceramics will be very crucial in determining the price of your pieces. In this article we consider only electricity and gas as source of energy to fire your kiln. Gas is usually propane gas which is bought by 48 kg cylinders which you link together into the kiln. You can also buy the gas in bulk tank, and you can also be living in those areas where gas is linked to users by a gas line.
Calculate the KW in 1Kg of Propane
Some Mathematics: 0.141 litres of Propane gas has 1 KW of energy. 1 litre of liquid propane is 0.512 Kg. 1 Kg of liquid propane has ((0.512/0.141) x 1) equals 13.85 KW. This means that 48 Kg cylinder of propane is capable of giving you 664.8 KW of energy. If you pay $75 for a 48 Kg cylinder of propane, then you are paying 11.3 cents for KW of Propane energy. In buying electricity you are charged energy per KW. If you are paying 10 cents per KW for electricity, then electricity as a source of power will be cheaper and you should look at the possibility of using electricity as the source of power to fire your kiln. The prices of fuel will vary from one type to the other in different countries.
As soon as your kiln reaches the temperature of 1100˚C, every 1˚C rise in temperature requires your kiln to draw energy at a very high rate. Depending on the size of your kiln, you can build temperatures very fast up to 1100˚C using a single phase source of electricity but after 1100˚C you may need three phase source of electricity (today, electricity is no longer distributed using two phase alternators). The logic is that the energy needed to raise the temperature from says 1200˚C to 1210˚C is far much more than the energy needed to raise temperature from 30˚C to 40˚C (this is especially serious in gas fired kilns as the more gas you add means more cooling) – really? – yes, the rate of energy loss to the atmosphere increases more at that point and you can only control the heat loss rate only if you can increase the atmospheric pressure, which you can’t. If your power source can not allow you to draw that extra power surge, then forget about reaching stoneware and porcelain temperatures. If you must use a medium to large size kilns, then you should have electricity power supply in 3-phase, otherwise go for small sized kilns. The other big drawback with kiln fired with electricity is that it is a bit difficulty to create a reducing firing atmosphere – you do it by adding moth balls which can strike on electric filaments. A reducing atmosphere is the creation of an atmosphere inside the kiln that has reduced oxygen so that the metal oxides can fuse in clay in beautiful colours that you like and that you can not get in an oxidizing atmosphere.
Reliable Source of Power Supply
You need a reliable source of power supply. And what happens when you have spent 8 hours firing your kiln up to 1200˚C and you have 80˚C more to go to 1280˚C and then all of a sudden the power company interrupt the supply of electricity for the next four hours or so? Another good thing with electric fired kilns is that they can be installed with computer programmes and controllers for monitoring the entire firing curve of your kiln from start to end, otherwise how do you feel if you were to stand next to a firing kiln for 10 hours to maximum temperature and another 7 hours for cooling? But be warned, kiln firing involve high temperatures that can cause great damages and therefore all kiln firings should be monitored closely by a real person.
If your home is connected to a gas line in the street, to fire a gas kiln, the gas pressure from the gas line should be at least 20 cm (8”) of water column pressure, otherwise you will need to ask your gas company to provide you with a free gas meter that can provide 20 cm of water column pressure. If you decide to use propane gas in 48 Kg cylinders, then you will need at least two cylinders for a medium sized kiln. If smaller cylinders are used, the propane gas will freeze into solid inside the cylinder and the kiln will not fire. To prevent gas from freezing, you will need to continuously run cold water on the cylinders. If you use the 48 Kg cylinders full of gas, the firing is fast and beautiful with easy manoeuvrings of all firing techniques you can think of. Going past the temperature of 1100˚C in kiln firing requires your cylinders to provide enough gas pressure. If the cylinders are below half full of gas, they may not provide sufficient pressure to raise the temperatures past 1100˚C – you may need to have standby propane cylinders full of gas so that once you approach that temperature, you can switch to the cylinders full of gas that can provide sufficient pressure.
Pyrometer and Thermocouple
A pyrometer is a device that is placed outside the kiln to measure temperatures inside the kiln. To do that, a pyrometer is connected to a thermocouple which is the metal rod that goes inside the kiln. The best material for a thermocouple is platinum-rhodium because it can withstand temperatures of 1400˚C and above. This expensive platinum-rhodium rod should be encased in inconel tube for protection. When you buy a kiln, let the kiln manufacturer provide you with a digital pyrometer with platinum-rhodium rod thermocouple encased in inconel tube. Other makes of thermocouples are cheap but will not last for long if they encounter stoneware/porcelain temperatures.
Insulating Bricks and Ceramic Fibre
Kilns are made of light insulating bricks and ceramic fibre which are heat energy efficient. Bricks are bricks and have a lot of weight regardless of being called light bricks. A small top loader kiln with internal capacity of 4 cubic feet may weigh 200 Kg. A medium sized front load kiln made of insulating bricks and ceramic fibre may weigh 500 Kg whilst the large sized kilns may weigh well over 1000 Kg. Such weights may not be easy for you to keep moving your kiln around and need to be taken into consideration when you purchase your kiln.
In stacking your clay pieces in a kiln, you will need kiln furniture which is a set of shelves supported by props/posts inside the kiln. Use assorted sizes of shelves and length of props so that different arrangements can be accommodated for different sizes of pottery. Shelves and props should be made of material that can withstand high temperatures. A good material should be high alumina or silicon carbide. Let the manufacturer of the kiln provide you with kiln furniture made of high alumina or silicon carbide. Once you become experienced enough in firing pottery and ceramic, you will be able to make your own kiln furniture using fire clays and grogs.
Front loading kilns
A big kiln is economic in the sense that the unit cost of firing a piece of ceramic is lower if the kiln is fired when it is full of pottery. If you believe you can hand build enough pottery pieces to fill a large front load kiln, then its better you go for a bigger kiln. Front loading kilns are easier to load. A beginner in pottery may not go for a big kiln because he/she is unlikely to make enough pottery pieces to fill such a kiln. It is also true that you can fill that kiln with your pieces and they all go into waste upon firing. Big kilns are ideal for professionals who know their jobs well such that they can say with certainty that more than 99% of their pottery will fire well and can be sold to people who will happily buy them. A beginner in pottery should start with the small top load kilns so that if anything goes wrong in firing, the loss is small. However, small sized kilns would restrict a beginner from firing large sized pottery figures.
Spend $1500 for a Kiln
Be prepared to spend no less than $1500 for a good medium sized kiln including kiln furniture and pyrometer / thermocouple. Look for different kilns suppliers in different stores, and in different countries too (including South Africa). You can as well add a little bit more money to get a kiln installed with computer programmes and controllers that will monitor the entire firing curve of your kiln from start to end.
The next article is on How to Fire A Kiln and Kiln Firing Techniques
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