Cones

For today I chose more or less solely ceramic topic. There were several questions lately: To what temperature do you fire your pieces? How many hours do you fire? Every time, when someone new sees us around the kiln during firing there is a question: How do you know, when to stop?  And I every time start to replay with opening one of our peepholes...

What you can see in such peephole depends on temperature and your "ability" to see. What you should see (among others) is holder or stand with what is called cones or more precisely pyrometric cones. I talk about small pieces of ceramic, which potters use for measuring of heatwork (combined effect of temperature and time). Although it can seems to be kind of not easy to get for "first lookers" the principle is simple and quite natural.

First cone melted down, second one start to soften...

Cones are made from mixture of ceramic materials  (the number of cone determines temperature -time value which can show during firing). More precise and wider definitions you can find reading Wiki. What maybe you can miss there is written between lines: for firing of ceramic is temperature as important as time. You can fire with thermometer, pyrometric cones or with other equipment, but than you will need to watch your watch. Basic benefit of cones lies in ist ceramic nature - it measure heatwork not temperature. For example: when thermometer shows actual temperature 1260°C for let's say one hour then cone 10 (1320°C when rise-up of temperature is 150°C/hour) starts to show us that its time is coming. It shows us how pots (clays and glazes) are feeling. And that is what I am interested in. How my pieces inside the kiln feel not what temperature is there.

Cone 6a in the midle of the kiln has just touch base...

In most cases we use five cones in three peepholes. There are three cones in hottest part of the kiln (06a, 9 and 11), one 6a in the middle of the kiln and one 6a. in coldest part of the kiln. First cone which start to react to rising temperature is 06a which should refer to 960°C. In this time we know that we should start first stronger reduction period. This 06a cone is usually down after around four hours of the firing (not counting several hours of pre-heating). Then after another eight-nine hours, the cone 9 starts slowly go down. Here more slowly means better - it is period, when pots need time to ripen. This cone nine should refer to temperature around 1300°C. After two aditional hours or so cone eleven goes down. Here it is already very difficult to see those cones - it is "white heat" and you need some experiences to recognize what is going on in there. This cone eleven refers to 1350°C. When this final front cone is down we start to watch cones thru peepholes in the midlle and in the back. Depending mostly on how tight the kiln is loaded after one up to five hours also the last 6a (1250°C) cone is down. This "firing curve" is not from school handbook but based on experiences with several hundred firing of different kilns. Each single firing is then our teacher for work in the future. And the best time to learn is during unloading a kiln we have fired. We can see mistakes we have made as well as ways to improve.

Thank you for reading and Happy New Year!