Firing with a controller to a cone number or a temperature may not be exact. If shivering occurs, fire one cone cooler. You may have to select another body or a different glaze. Firing too cool is not a good idea since the strength is reduced and porosity increased, both of which may cause problems during use of the final piece. If the temperature in your kiln varies by more than 1 to 2 cones, then glazed ware in one part of your kiln may fire okay, while ware in another part will have a problem.
Most kilns vary in temperature from top to bottom.
To determine how much your kiln varies, place witness cones on each shelf when making firings. Usually, there is less difference top to bottom for hotter firings. Each kiln has its own personality and the solution for improving temperature uniformity may vary. If you have glaze firing problems because of too much variation, then we recommend the following. Make sure cracks and holes are repaired to keep heat in your kiln.