Ever wonder if your kiln is actually reaching the targeted temperature? How do you really know your ceramic pieces are getting the proper heat-work? Whether you are using an electronic controller or a manual kiln (using a Dawson kiln sitter), miss-firings due to improper temperature readings can happen. Witness cones, also known as “visual cones”, monitor the kiln’s firing temperatures and can help reveal a potential problem.
A witness cone is a self-supporting pyrometric cone and will stand alone on the kiln shelf (not to be confused with the cone in the kiln sitter in a manual kiln.) It is recommended that witness cones be used in all kilns – even those equipped with an electronic controller. Electronic controllers, which use a thermocouple to measure temperature, can “drift” over time, meaning their readings become less accurate. And both types of controllers shut-off when a target temperature is reached but this is only half of a proper firing cycle. The other half of a complete firing is time. The right amount of heat, and the right duration will produce a proper firing.
Cones measure “heat-work” (time and temperature). When placed at various areas in the kiln, witness cones will measure the time and temperature within the kiln, uniformity of the firing and in some cases the accuracy of the kiln sitter. When a kiln is fired to the proper temperature over a period of time the witness cone will give you a “visual” of how the inside of the kiln chamber fired and how the heat was distributed.
Using witness cones is simply placing the cone/s onto various shelves in different areas of kiln. These are commonly used in unison sets of three such as cone 07, 06, and 05. (Keep the cones far enough from the ware so that when the cones bend, they will not touch the ware)
- Guide cone, one cone number cooler than the firing cone
- Firing cone, the cone number recommended by the manufacturer of the glaze, slip, etc.
- Guard cone, one cone number hotter than the firing cone
From the fired witness cones you can establish a firing pattern in your kiln; where a cool spot is; is it firing hotter in one area then another or if it is mechanical problem. Following the above procedure and using the instructions supplied by your kiln manufacturer will allow you to determine a number of possibilities. Does the kiln sitter need adjusting (manual kiln), is an element not working at its full capacity?
Know your kiln. Know your firing procedure. Know the witness cone is not a foolproof method of determining an inaccuracy in your kiln but it the first step to start making sure your firing program is a success!