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Important Facts About Cones
A pyrometer measures temperature only; Cones measure the effects of time and temperature (called “Heat Work”). A cone number is a measurement of this Heat Work. The temperature listed on a cone chart is the temperature at which that cone deforms when heated at a specific rate. If the rate is changed, the temperature will change. This is why a pyrometer reading may not always appear to match your cones. If a discrepancy occurs during a firing between a pyrometer readout and a cone deformation, always follow the cone. Clays and glazes are formulated to mature at specific cone numbers. When a manufacturer labels a glaze to fire at cone 05, it means that in order to mature, the heat work required to deform an Orton Large Cone 05 must be done. Witness Cones are the only way to measure this heat work. When a manufacturer refers to a witness cone number, they are referring to a large or self-supporting Orton Cone. (Junior Cones are not used as witness cones).

Using Witness Cones
To use witness cones, select 3 cones as follows:

• A Guide Cone - One cone number below your goal.

• A Fire Cone - The cone number you wish to achieve.

• A Guard Cone - One cone higher than your goal.

Place standard large cones in a holder or cone plaque designed to hold them at the proper eight degree angle. If using self-support cones, their special built-in base will place them at the correct angle. Place cones in order: Guide\Fire\Guard. (For example, to fire to cone 05, place the cones facing left 060504). This allows each cone to melt without interfering with the others. Cone plaques should be visible through your kiln’s spy holes during firing so that you can monitor your progress and manually shut down if automatic devices should fail. Additional cone plaques can be placed in other areas of the kiln for analysis after firing is completed. They will tell you if your kiln fires evenly throughout or where your hot and cold spots are.

Cones and Kiln Sitters or Controllers
A Kiln Sitter is a valuable shut-off device, but should not be considered the determining factor with reference to proper heat work. The cone in the sitter has additional variables that affect its melt: proximity to the kiln wall and elements and the weight of the sensing rod resting on it. When a Junior Cone is used, the tapered shape can cause variances if positioned too far right or left. (We recommend Bar Cones to eliminate this variable). Because of these other factors, a kiln sitter cone can deform prematurely, shutting down the firing before proper heat work is complete. Visual cones will tell you if this happens and adjustments to the sitter can be made if necessary.

Programmable Controllers are much more accurate than kiln sitters if you pay attention to your ramp rates - especially toward the end of the firing. By following the heating (ramp) rate on Orton’s Cone Chart for at least the last 90-120 minutes of firing, you can be fairly certain that the degree temperature equivalent on the chart will be the end point of your firing. Even so, only visual cones will confirm your heat work and allow you to adjust your firing profile if necessary.

Use witness cones in every firing: Both Kiln Sitters and Controllers can fail due to mechanical malfunction. In either case, properly using witness cones will insure that you don’t experience a serious melt-down. Cones are cheap; kilns are not.

Please note: We carry Orton Cones because only Orton maintains the highest standards of quality control in the testing and production of their cones. Orton is known industry-wide as the leader in temperature measurement and control. Cones are crucial to successful firings of your valuable work - don’t take chances with cheaper brands. Orton Cones are the best!

"Cones should always be used in groups of at least three in sequence in the desired heat range... "

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