We refer to this absorption of heat as 'heatwork'. When the amount of heatwork for two firings is the same, the pieces will look identical, even if one is fired to a higher temperature for a short time and another is fired at a lower temperature for a longer time. Since cones measure heatwork, all manufacturers recommend the cone number to which their product should be fired. Guide Cone - one cone number cooler than firing cone. Guard Cone - one cone number hotter than firing cone.
Most kilns have temperature differences from top to bottom.
The amount of difference depends on the design of the kiln, age of the heating elements, load distribution in the kiln, and the cone number to which the kiln is fired. Usually, kilns have a greater temperature difference at cooler cone numbers. Use cones on the lower, middle and top shelves to determine how much difference exists during firing. After firing, observe the 3 cones. If, on the bottom shelf, the Guide cone has only bent half way, then ware is fired one and half cones lower.