The composition and structure of cones is such that they bend and deform when subjected to heat for a period of time, (i.e. their operation is dependent upon “heat work” and not upon temperature only. They are graded according to the amount of heat they will withstand and are placed upright in holders in the kiln where they can be viewed from the spyhole.
Since their deformation characteristics mirror the maturing characteristics of the ceramic products being fired, cones provide an outstanding method of indicating heat work carried out during a firing.
It is usual to use a set of three consecutive cones: one to bend below the optimum firing point, one at the optimum firing point and one above. Orton Pyro-bars are supplied for use in
kiln-sitters and mirror similar characteristics to that of cones.
Following recalibration in 1996 the cones marked * now bend 10-20°C later than previously.
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