5. Kiln care
Firstly as, although I could share oodles of images of kilns, they are not very pretty things generally, and many potter’s survive without ever owning one as I did by attending courses and sharing the use of a friends. Ask schools if they have technician that have kilns if they’re willing to do firings to raise much needed funds, ask pottery suppliers themselves, some used to I know.
So aside from the image of my new kiln, (the old once still needs to be, not just repaired but re calibrated), all other images will be of examples of pottery and glazes, partly to highlight some of the things you can go onto next, if you haven’t tried them regardless of if you’re new to pottery or not, but also to keep you inspired while highlighting a few key dangers to be aware of.
When you know all of them… tell someone because I doubt any potter really knows them all given the wide range of effects and uses clay can be used for. Please don’t be put off as by following these basic rules. I have kept safe without any accidents at all requiring medical attention, except for a shard of glass that affected my left thumb this year, but that was away from the kiln anyway. I will be dividing pottery blogs up by number and title for ease of reference going forward.
Kilns come in all shapes and sizes. They can be as simple as a hole in the ground that allows air to escape when firing a pot and covered over; made of bricks, e.g. the bottle kilns Stoke-on-Trent is famous for among other towns in the region in their heyday, to those you can buy for use at home or in a studio. Among them you can buy electric, gas or combustible fuel types. If you know what you are doing you can build your own. Don’t otherwise, even if you think you understand, as I did while glass fusing. If you choose DIY I recommend to you do so with some who is very experienced. I was safe enough at all times (see link under the titles of all my pottery blogs now), the kiln though was less happy in that regard. I simply didn’t understand what glass would do when fired though. Hence this blog is an introduction only to what happens with clay. Glass is like water it finds its own level while being fired and my shed floor is not level.
Why two firings?
You can get away with one firing but these days that’s the exception rather than the rule for gas or electric kilns. The first firing is called a bisque firing and it’s main aim is to stabilise the clay by heating it to between 1000-1100F degrees before allowing it to slowly cool down again to room temperature before you ever think about opening the kiln. If you open any kiln while hot you risk thermal shock which is an extremely bad thing indeed as not only can your pot explode as cold air from the outside suddenly hits the hot air of the inside of the kiln but you yourself can get burned regardless and you run the risk of getting hit by a flying piece of red hot clay besides getting burned directly yourself. Plastic surgery isn’t cheap.
The same thing applies when the kiln is being fired. There’s no point in putting your clay in unless you want to destroy it by whacking it up to full temperature straight away and you must never open the kiln while it is firing even at the initial settings of as little as 50F. There are exceptions regarding pottery but I won’t be covering them in this blog. So clay is fired and left to cool down very slowly.
As a rough guide/example it takes 12 hours of firing by gradually increasing the temperature and 12 hours for the kiln to cool afterwards. So from cold for the first 7 hours the temperature is set to 65-70F degrees before going up again for another hour to around 160F. Be aware that’s it’s still hot enough to burn you badly at even 65 degrees. The next hour 250F and again after an hour 400F, then again 540F until finally going for full temperature 1100F then maybe left for a bit (called a soak time) before gradually being cooled down for another 12 hours minimum. I just turn mine off and unplug it.
Most manufactured kilns these days come with a temperature gauge to tell you went it’s at 20 degrees when it’s safe but still warm to open the door a crack to help it cool down further but it’s still very hot to touch anything without thick fire retardants gloves. All kilns are different so timings vary enormously. And all clays are different too so some need even higher firing temperatures, times and all glazes react to both differently.
Matching kiln, clay and glazes
- Step 1: Match your clay to your kiln for it’s maximum temperate.
- Step 2: Match your glazes to the firing temperature your kiln is capable of in the hope it will match roughly why you chose it
- Step 3: Make sure no glaze is left on the bottom of anything you make (for beads you need specialist stands and wires that don’t melt) before loading a kiln. Wipe their bottoms. No one wants a sticky one… in their kiln.