This is especially important during the first several firings of glass so that you can assure your kiln is properly calibrated. If you do not get a good cone 021 firing with an 021 setting, please contact your distributor or kiln manufacturer for instructions in correcting the problem. After you have calibrated your kiln, you may wish to have 021 cones on every shelf in every firing, and to use 022 and 021 cones about once a week to make sure all is well. If you have pieces which bend, warp or appear to be under-fired, make sure to use all three cones. If you use a manual or semi-automatic kiln, place an 021 bar cone into kiln sitter.
Some kilns require different bar cones to reach temperature.
Close all peep holes. If using a venting system, close kiln just as a normal firing according to manufacturer's instructions. For an electronically controlled kiln, program kiln computer to 021, medium ramp, zero hold or soak time, then start kiln. Allow the kiln to cool down slowly. Other glassware may work well, but we cannot anticipate the results you might get.