Maybe you heard ceramicists talk about the temperature of a ceramic kiln. But instead of degrees Celsius, they spoke of cone 10. What are these cones, and what are they used for?
What are cones?
Cones are small pyrometric coneshaped things that are composed of a mixture of specific ceramic materials. They are so composed that they bend and melt at a certain temperature and duration of time, depending on the number on the cone.
Cones date back to the days when there were no digital temperature controllers. Back then, they were the only way to tell when the kiln was hot enough and you could stop firing. Today, they are still used, albeit rather occasionally,
- to check if your kiln is still reaching its temperature,
- to check what the actual ‘heatwork’ of the ceramics is,
- or to find out if you have hotter or colder spots in your kiln.
Typically, three cones with consecutive numbers are used and placed in a holder in front of the peephole in your kiln. If the first cone bends over, pay attention because the final temperature you want is not far off.
If the middle cone bends over, then you have reached your temperature, and if the last one does too, then you have gone too far. In a perfect firing, the cone of your desired temperature is bent over to a 90°C angle.
Time and temperature
Cones give more information than temperature alone, they indicate what is happening to the ceramics in the kiln. They tell you something about the heating rate. Compare it to an ordinary kitchen oven: just because the oven has reached 200°C doesn’t mean the chicken in that oven is cooked. So too with your ceramics in the ceramics kiln.
For example, soaking, holding the end temperature in our kiln, can make your glazes melt out more, and a cone with a higher number may have bended, even though the temperature remained the same. Cones thus measure the combination of time and temperature.
How do you 'read' cones?
There are several series of cones on the market. The best known are the Orton cones. Cones are numbered on a scale from 022 to 14, from low temperature to high temperature.
Note: cone 6 is not the same as cone 06. If the cone number starts with a 0, you can consider it a negative number. So a cone numbered 014 is higher than a cone numbered 020.
To find out what temperature you need for the glaze you bought, look up the cone number on the label in this table.
