If you use cones for firing that need to be mounted in a pat of clay and placed in front of a peep hole you might find a cone holder designed for that purpose easier to use.
Use witness cones in the kiln even if you have an electronic controller or kiln sitter. This will show you the temperature that was actually reached inside the kiln (heat-work). See my article: '(Orton Standard)Temperature Chart ' at http://www.ceramics-tips.com/RscArticleV.asp?id=369
Always use the proper cone. Do not try to get by with one that is close if you are out of the proper one.
Cones are available in different numbers and the user must insure that items being fired are fired to the cone recommended by the manufacturer.
If the rate of temperature rise is slow during firing, a cone may bend at a lower temperature than if the temperature rise is rapid.
If you use a test cone when firing, it should be placed about three inches behind a peep hole so you can see it easily.
Cones come in two sizes. Small cones 1-1/8” long are used in kiln sitters and large cones 2-1'4”long are placed throughout the load, and at the level of the kiln sitter to determine temperature variations and the accuracy of the kiln sitter. The latter are called witness cones. The shorter length and higher density of the small cone makes them stand longer than larger cones when placed on ends. However, when used in the kiln sitter, the small cones bend at the same time as large cones standing.
A pyrometric cone is a small pyramid of clay and mineraloxide.
Since cones are made from similar material to the ware being fired in a kiln and are subjected to the same heat treatment as the ware being fired, it is the most accurate indicator of proper firing time.