Why do some clay makers say there is only one point of vitrification for each body – say, cone 5?
A. Probably because they want you to buy their matching glazes. Firing a kiln is an art not a science, even with a digital firing you can not count on duplicating a firing. My cone 5 may not be your cone 5. Kilns can have a 1 to 11/2 cone difference from top to bottom, just from the hot air rise alone. Due to the different ways a kiln can be loaded and fired you can have a tremendous difference in clay working temperature (the actual “felt” temperature of the clay and glaze itself), even in the same kiln load. A heavily loaded kiln with lots of clay mass will fire just as fast as a lightly loaded kiln. The difference being that the very full kiln load will actually feel a little cooler to the clay and glazes due to more clay weight being heated. The lightly loaded kiln may feel slightly hotter to the clay for the same reason. Clay mass also includes shelves and posts. We believe that you need a little leeway of usable cone ranges just to accommodate firing fluctuations. If you have any
