Firing Temperatures, Cones and Vitrification
I almost exclusively work in stoneware clay, fired at what is known as mid-range or mid-fire temperatures. Now temperature in ceramics is not an absolute number, but is measured in pyrometric cones. Bear with me, this will make sense I hope.
What are Cones?
A cone is a tall thin pyramid shape made of a precisely controlled blend of materials to measure heat work. Heat work is not just measured temperature, it’s also time. Imagine baking a cake - you put it in the oven at 150 degrees for 30 minutes and it bakes and rises - the temperature working on the mixture over time to cook it. You couldn’t just put the cake in the oven at 300 degrees for half the time and expect it to be cooked - the middle wouldn’t have enough time at high temperature to be done, and the outside would be burnt. But you could probably put the cake in at a lower temperature and bake it for longer, and it would cook all the way through. That’s heat work. Pyrometric Cones are made to melt with a specific amount of heat work, and there are lots of different ones. the actual temperature needed to melt a cone will change based on how long you fire for.
Here’s a chart showing the cone numbers and temperatures. You will notice that the actual final temperature that will melt the cone changes based on whether the firing rate was slow, medium or fast.
I use mid-range clay, that I fire to around cone 7. The temperature on my small kiln is actually set to 1210 degrees, but because I fire quite slowly, it melts the number 7 cone perfectly. In my larger kiln the temperature needs to be 1220 degrees because it’s not quite as well-insulated, even though the firing schedule is nearly identical. I could fire to a lower temperature and it would still make it ceramic, but the clay might not be fully vitrified.
What is vitrification?
Vitrification, from vitreum, Latin for “glass” is the most important, and perhaps the least understood process in ceramics.
The clay we use is always some combination of silica, alumina, and flux, with silica forming the glass, alumina making the glass more durable, and flux lowering the temperature at which the glass forms. The glass formed in the process of vitrification, even in tiny amounts, is what holds ceramic materials together. A fully vitrified clay (i.e. fired to the correct temperature) will be the strongest, and the least porous. If your clay is not vitrified it can still absorb water and will not be as strong.
Some clays do not vitrify at all, these are classed as Earthenware clays and must be fully encased in glaze to avoid moisture absorption. Stoneware clays when vitrified have an absorption rate of 1-2% on average, and high fired Porcelain is really more like glass than ceramic, and has zero absorption when vitrified.
Firing temperatures and clay type
I use mid-range stoneware clay because to me it represents the best compromise between vitrification and firing cost. High firing clay wears out your kiln elements a lot faster, and uses quite a bit more energy - as previously noted in the section on cones, you may not need to raise your top temperature to reach a higher cone number, but you will need to hold a temperature for a longer time. Check with your clay manufacturer to see the firing range - and then aim for the higher side of that range. You’ll also need to make sure that the firing range of your glazes matches the range of your clay, or you might have some glaze disasters!
I did quite a lot of testing to find the best temperature to fire my clay that results in vitrification and does not ruin my glazes (or kiln shelves). Unfortunately there are no short cuts to testing as all kilns are slightly different, but a really good place to start is to put cones in every firing, on every shelf, so that you can see exactly how much heat work each part of your kiln produces. From there you can adjust your firing schedule and move things around in the kiln to take advantage of the different ‘zones’. I know in my larger kiln that it’s hotter at the bottom, so any glazes that need a slightly higher temperature go there - it saves me having to do more than one firing.
I’m happy to give advice, or expand on these subjects to the best of my knowledge, so please do leave a comment, or contact me if I can help in any way.
Happy firing!