A pyrometer measures temperature only; Cones measure the effects of time and temperature (called "Heat Work"). A cone number is a measurement of this Heat Work. The temperature listed on a cone chart is the temperature at which that cone deforms when heated at a specific rate. If the rate is changed, the temperature will change. This is why a pyrometer reading may not always appear to match your cones. If a discrepancy occurs during a firing between a pyrometer readout and a cone deformation, always follow the cone. Clays and glazes are formulated to mature at specific cone numbers. For example, when a manufacturer labels a glaze to fire at cone 05, it means that in order to mature, the heat work required to deform an Orton Large Cone 05 must be done. Witness Cones are the only way to measure this heat work. When a manufacturer refers to a witness cone number, they are referring to a Large or Self-Supporting Orton Cone. (Junior Cones are not used as witness cones)