Firing a kiln is a skill that most don’t get a chance to learn during ceramics and pottery classes as more often than not, the firing is done for you between classes. When you finish your pieces, you just pop them on a shelf, then a few weeks later they reappear after they’ve been fired ready for glazing or to go home. So when you start making at home it can feel really scary the first time you fire your own kiln but with modern, electric kilns with controllers and a little bit of knowledge, there’s nothing to be frightened of and you can find many example firing programmes that can be copied until you feel more confident.
It may seem really obvious but the first thing to bear in mind when you’re looking at doing a firing is why we fire things in a kiln in the first place. The purpose of a firing is to alter the molecular structure of clay to turn it into ceramic. It requires heat and time to complete this process and the combination of these two varies depending on the clay you are using and the result you want from the firing. The higher the temperature and/or longer the time, the more the clay becomes vitrified (no longer porous) and changes to ceramic.
In addition to this you need to know what clay you’re making with. Is it Earthenware, Stoneware or Porcelain? What is its firing range? Typical firing ranges are Earthenware 1000C – 1140C, Stoneware 1180C – 1260C and Porcelain 1280C – 1320C but each clay will have its own range that should be included it its description or properties. The top of this range will be the point where the clay is vitrified but we don’t always fire to this hence a range. When we apply a glaze it creates a vitrified protective layer on the ceramic so it’s not always necessary to fire to the top temperature. However, having said that, if you are making something that you want to hold a liquid, it’s definitely best to get as close to vitrified ceramic as possible.
Keeping all this in mind, if you are doing a bisque firing to get some pieces ready to glaze then you don’t want to go to the top temperature or fire for too long because you need it to still be porous so it’s easy to glaze. Whereas with a glaze firing you want to get to the top temperature for the right amount of time for the glaze to fully vitrify and the clay to become ceramic. To do this we use firing programmes or schedules. These are comprised of different segments that are altered depending on the outcome you want to achieve but there are some ‘standard’ programmes you can use. Sometimes these are even pre-programmed into a controller.
Firing a kiln – the programme
A firing programme can include the following segments:
- 1st Ramp
- 2nd Ramp
- Top temperature
- Hold or Soak
The candling segment is when greenware (unfired clay pieces) is put in the kiln for a long hold at a low temperature (80-100C) to ensure that all the pieces of clay in the kiln have dried out completely. If you dry your pieces well before firing then you shouldn’t need this stage though. It’s best only to use this if you have to or as a precaution when firing important work.
The Ramps are the number of degrees per hour that the temperature is increased inside the kiln. The 1st Ramp usually runs up to 600C. It’s when most of the remaining moisture is driven off and is usually slower than the 2nd ramp. The 2nd Ramp runs from 600C to the top temperature and can be faster than the first ramp as the ‘water’ has all gone. The faster you raise the temperature per hour the more stress you are putting on the clay as it is undergoing the molecular changes that the heat is causing. The more stress you put clay under the more you are going to see cracks and other problems occurring so don’t be tempted to speed it up too much.
The top temperature is exactly that, the top temperature you’re firing to. This temperature is determined by the clay and glazes you are using.
The Hold/Soak is the final segment and is a period of time at which the kiln is held at the top temperature to allow for more heatwork to occur. Heatwork is the effect of the combination of heat and time on a piece as both affect the final outcome. It is possible to fire at a lower temperature for a longer period to get the same looking result as a higher but shorter firing.
The diagram above is the bisque firing programme I use with my small kiln at the studio. It has no candle segment; 1st ramp is 120C rise per hour up to 600C; 2nd ramp is 160C rise per hour up to the top temperature of 1000C; and it finishes with a 15 minute soak. For my glaze firing I have a 1st ramp of 140C rise per hour up to 600C; 2nd ramp is full power up to the top temperature of 1205C; and I finish with a 15 minute soak. You can raise the temperature quicker with the glaze firing because the clay has already been through a bisque firing so is completely dry except for the water absorbed from the glaze.
The other aspect to consider when firing a kiln is the venting. All kilns have vents/peepholes and when a firing begins they should be open to allow the water and impurities to burn off. This is why a kiln needs to be housed in a well ventilated area and why it’s not a good idea to be working in the same space as your kiln when it’s firing. I close my vent/peephole at 400C but many wait until 500C, anything around this point is good.
Please also make sure you are following all the safety instructions that come with your kiln to be sure of your safety and never put anything flammable near your kiln.
You can see more about what happens during a firing with this Ceramic Arts Network diagram.
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