The so-called Process Temperature Control Rings (PTCR) enable accurate measurement of the distribution of heat inside the furnace. The size of the ring shrinks during firing in proportion to the amount of calorific energy absorbed – conclusions regarding the furnace temperature can be drawn from the extent of shrinking with the help of a table. It should be noted that PCTRs are not thermometers – their size changes in connection with the amount of calorific energy they absorb.