Kilns are rarely uniform in temperature, particularly the larger ones. This is because there is much more to heat in a larger kiln than in a smaller one. Using witness cones or shelf cones, one can determine the amount of heating variation within the kiln. To evaluate kiln uniformity, sets of three consecutively numbered cones can be placed on each shelf during firing. Afterwards, they can be examined for differences to determine how much variation occurred. Typically, you will see at least one cone difference from top to bottom of the kiln. This can be even larger depending on the kiln construction and the cone number being fired. How fast the kiln is heated and how heavily loaded it is will also affect temperature distribution in the kiln.
While a single cone may be used on the shelf to `witness' the firing, it is common practice to use three different cones. A group consists of a Firing Cone - the desired cone for the ware being produced, the Guide Cone - one cone number cooler, and the Guard Cone - one cone number hotter than the firing cone. These cones are set deep in the kiln. In larger kilns, using this three cone system on the bottom shelf, middle of the kiln, and top shelf will give you a good idea of over-all firing uniformity.
When the Guide Cone starts to bend, the ware is approaching maturity. The bending of the Firing Cone indicates firing is at the correct point. If the Guard Cone has bent, you have exceeded the best time-temperature relationship. If you want to view the cones through the peepholes, welders glasses should be used to avoid possible eye injury. Sunglasses are not recommended.
Using the three cone system can often be the first line of defense in examining firing problems.