General information About Cone Sitters

Most kilns come fashioned with some sort of control system. Some older kilns however are controlled by use of shelf cones and the operator keeping track of time and monitoring the status of cone degradation via the peep holes.There is also a pyrometer that can be inserted in a peephole, but still needs to be monitored by the operator.

Most of the older kilns come equipped with kiln sitters AKA cone sitters. These are usually a combination of timer switches and a set of prongs that hold cones and will disengage operation when the cone reaches maturity. This was the ‘primo’ in kilns for many years.

Newer kilns have electronic controls which operate primarily with computer chips in conjunction with the kiln sitter. This article is directed to those kilns with the older kiln sitter.

It is rare that a timer will malfunction, though it is possible. Herein we will be addressing the most common problem which is the cone sitter itself. The most frequent issue with a kiln misfiring, is the fault of the sitter. Often if is only a matter of recalibrating the shut off flange by adjusting the set screw and plate which holds the kiln sitter in place during operation.. This will be addressed in a further article on kiln repair – How to Calibrate a Kiln Sitter.

When you begin having misfires with your kiln, you need to examine your elements (see Kiln Repair-Elements) and then the kiln sitter. Most often an over fire is caused by kiln sitter operation and under fires are most often kiln elements. A basic maintenance of your kiln sitter will prevent many misfires. You can also get over fires and under fires if the flange plate is out of calibration.

A kiln sitter is a tube that extends through the kiln wall. Inside the kiln, the sitter will have three ‘prongs.’ Two (on the bottom side) will be stationary. The top prong is movable and lifts to allow you to insert the cone then it lowers and sits on top of the cone. As the cone softens the weight of this bar/prong bends the cone and thereby drops the outside control switch turning off the kiln. The lower two prongs are removable and should be coated with kiln wash periodically. DO NOT kiln wash the top prong. You should also examine them frequently and sand off any build up of kiln wash or cone debris. The two bottom ones can be easily removed and replaced and I recommend that you do so annually if your kiln has high use. If the top one goes bad, you will need to replace the entire tube assembly. You will notice that the moving prong functions with the exterior flange that automatically drops when the cone reaches temperature and shuts off the kiln.

Trouble shooting misfires is relatively easy. If you have checked your prongs and calibrated your flange plate as well as checked the elements and you still have issues, you will probably need to replace the sitter.

When using sitters, it is imperative that you use automatic timers as back up. NEVER ignore a misfire caused by timer shut-down. This is a warning that something may be amiss and needs attention.