At any time we can measure the temperature in the kiln using pyrometers (the digital devices mounted outside the gas kilns). But temperature doesn't give us the whole story regarding how the glazes melt. A more accurate measure is given by using pyrometric cones. Clays and glazes are complex mixtures of minerals. Unlike a simple substance like ice, they don't melt suddenly at a particular temperature. The process of glaze melting requires time for the chemicals to combine and react as well as increasing temperature, so we need a way of measuring both. Pyrometric cones (most often just called cones) are a standardized way of measuring how the heat of the kiln has cumulatively effected the glazes. Cones are composed of much the same stuff as clay and glazes, and each cone number is formulated to deform after being exposed to a particular degree of heat treatment.. The cones are placed in the front of the kiln where they can be seen through the peep-holes in the door while the kiln is firing. When the desired cone bends, the kiln is shut down and allowed to cool. Some cone numbers with approximate corresponding temperatures and firing ranges for different types of work...
The amount of kiln furniture you use in loading a kiln will effect the firing time too.
Vitrify means that an item changes into glass or glassy state by heat and fusion.
It is difficult to predict when your kiln will shut off but you can narrow the time down to an approximate hour. The number of pieces you are firing will vary this a great deal.
Kilns with less than three inch side-walls may require one cone hotter temperatures than those with thicker walls to do the same job.
If you look in your kiln through a peep hole, pieces that are near the same temperature will be the same colour. Brighter objects are hotter than darker objects.