What is a cone in pottery? What is meant by cone 6?

kiln cone
Cones are little triangular or pyramid-shaped pieces of clay (witness cones) that are placed in the kiln before it is fired. Each melts at a certain rate of temperate "cone" and is observed (witnessed) through the kiln's peep holes in order to assess what temperature has been reached. (heat-work) Potters say their piece, clay or glaze is "cone 10". Which means for it to fully mature (or melted in the case of glaze) it needs to reach a temperature of 2381 F over a measure of time and the combi...