Understanding Pyrometric Cones
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For more information about cones and how they work, click here for
The Edward Orton Jr. Ceramic Foundation

Pyrometric cones are slender pyramids made from about 100 carefully controlled compositions and have been used since the late 1800's to monitor kiln performance. Originally thought to be temperature indicators, studies demonstrated that time and atmosphere also affects cone bending behavior. Each cone number is unique in measuring temperature within a small temperature range (less than 30¡C)

Cones measure the amount of heat absorbed. As the cone nears its maturing range, it softens and the tip begins to bend, drawn down by the influence of gravity or the weight of the sensing rod for cones or bars used in the Kiln-Sitter¨. It typically takes about 20 minutes for the cone to bend fully. Each higher cone number requires more heat to bend. Faster heating rates require the cone to be heated to a higher temperature.
Cone diagram

Pyrometeric cones are designed to mature (bend) at precise moments during the firing, by either shutting the kiln off at the proper point (as in the case of the Kiln Sitter and sitter cones) or used to simply indicate that a certain point was reached, as in the case of the witness cones.  The exact moment the cone matures depends primarily upon two factors: time and temperature. The temperature present at the moment "cone" is achieved depends upon the rate of temperature increase. With this simple concept in mind it's easy to understand why "cone" and temperature are not the same thing. For a proper cone firing, time and temperature must always be considered.

Large and Self-supporting cones are used as witness cones to monitor the firing and often are used to decide when the firing is complete.Use Self-Supporting or Large Cones whenever possible. Self-supporting Cones give the most consistent, repeatable performance due to the fixed mounting height and angle.

Small cones and Pyrometric bars are used in a Kiln-Sitter, a mechanical switch that cuts off power in electric kilns. Small cones are occasionally used as witness cones, but caution is recommended, since these shorter cones require slightly higher temperatures to bend, as compared with Large or Self-supporting cones.

Cones are still needed when a kiln is equipped with an electronic controller. Electronic controllers allow the kiln to be heated to a temperature. Controllers use a thermocouple to sense temperature. These change with time, causing the kiln to fire hotter than desired. Witness cones will check whether the controller is firing accurately.

Heating rate is important in the cone selection process. The temperatures shown in the temperature equivalent chart for each cone number are based upon specific heating rates. For faster heating rates, the cone must be heated to higher temperatures before it will bend.

Height of the firing chamber or the space available in the furnace can also be a factor in deciding which cone to use.

Consider the temperature range. While cones deform through a combination of time and temperature, the expected temperature range can determine the cone number and even the type of cone to use. For very high temperatures, a Large Cone or a PCE Cone may be needed. Self-supporting Cones are available for all low or moderate temperature use. Only small cones or bars should be used in the Kiln Sitter

Orton is the only manufacturer in the United States that makes Pyrometric cones and to our knowledge is the only one that provides charts and tables to allow the quantitative evaluation of an equivalent temperature from measurements of the cone bending angle.
The Edward Orton Jr. Ceramic Foundation maintains an excellent website with an incredible amount of information. Click on the links here to read about these topics:
Reference Library for temperature equivalent tables and Charts for converting cone bending angle to temperature.
Why Use Cones
Selecting Cone Type
How to Use Cones
Troubleshooting Cones
Ask Orton
Orton Ceramic Foundation ©2000 All Rights Reserved

Pyrometric Cone Chart
The chart below attempts to do the impossible by assigning a temperature to a cone number. The values given are valid only when the rate of temperature increase is controlled as per the chart. Special Thanks to The Edward Orton Jr. Ceramic Foundation for the use of cone/temperature data.

Pyrometer Users:
Because of the time/temperature relationship of the cone, it's not recommended that you attempt to calibrate a pyrometer using a cone.

Please note that the Cone Chart gives 3 different temperatures for each cone number. As discussed above, the equivalent cone temperature depends upon how fast one gets to it.
Generally speaking, the temperatures in the 108¡F - 60¡C column are used when discussing ceramics, porcelain, china and pottery.

For a handy Chart of what Cone to use in a sitter and what cone to use as a witness cone with reference to firing situations, see Helpful Tips on Firing
Heating Rate ¡/hour During Last 90 to 120 Minutes of Firing
Cone
27¡F - 15¡C
108¡F - 60¡C
270¡F - 150¡C
022
  
1087¡F - 586¡C
1094¡F - 590¡C
021
  
1112¡F - 600¡C
1143¡F - 617¡C
020
  
1159¡F - 626¡C
1180¡F - 638¡C
019
1213¡F - 656¡C
1252¡F - 678¡C
1283¡F - 695¡C
018
1267¡F - 686¡C
1319¡F - 715¡C
1353¡F - 734¡C
017
1301¡F - 705¡C
1360¡F - 738¡C
1405¡F - 763¡C
016
1368¡F - 742¡C
1422¡F - 772¡C
1465¡F - 796¡C
015
1382¡F - 750¡C
1456¡F - 791¡C
1504¡F - 818¡C
014
1395¡F - 757¡C
1485¡F - 807¡C
1540¡F - 838¡C
013
1485¡F - 807¡C
1539¡F - 837¡C
1582¡F - 861¡C
012
1549¡F - 843¡C
1582¡F - 861¡C
1620¡F - 882¡C
011
1575¡F - 857¡C
1607¡F - 875¡C
1641¡F - 894¡C
010
1636¡F - 891¡C
1657¡F - 903¡C
1679¡F - 915¡C
09
1665¡F - 907¡C
1688¡F - 920¡C
1706¡F - 930¡C
08
1692¡F - 922¡C
1728¡F - 942¡C
1753¡F - 956¡C
07
1764¡F - 962¡C
1789¡F - 976¡C
1809¡F - 987¡C
06
1798¡F - 981¡C
1828¡F - 998¡C
1855¡F - 1013¡C
05-1/2
1839¡F - 1004¡C
1859¡F -1015¡C
1877¡F - 1025¡C
05
1870¡F - 1021¡C
1888¡F - 1031¡C
1911¡F - 1044¡C
04
1915¡F - 1046¡C
1945¡F - 1063¡C
1971¡F - 1077¡C
03
1960¡F - 1071¡C
1987¡F - 1086¡C
2019¡F - 1104¡C
02
1972¡F - 1078¡C
2016¡F - 1102¡C
2052¡F - 1122¡C
01
1999¡F - 1093¡C
2046¡F - 1119¡C
2080¡F - 1138¡C
1
2028¡F - 1109¡C
2079¡F - 1137¡C
2109¡F - 1154¡C
2
2034¡F - 1112¡C
2088¡F - 1142¡C
2127¡F - 1164¡C
3
2039¡F - 1115¡C
2106¡F - 1152¡C
2138¡F - 1170¡C
4
2086¡F - 1141¡C
2124¡F - 1162¡C
2161¡F - 1183¡C
5
2118¡F - 1159¡C
2167¡F - 1186¡C
2205¡F - 1207¡C
5-1/2
2133¡F - 1167¡C
2197¡F - 1203¡C
2237¡F - 1225¡C
6
2165¡F - 1185¡C
2232¡F - 1222¡C
2269¡F - 1243¡C
7
2194¡F - 1201¡C
2262¡F - 1239¡C
2295¡F - 1257¡C
8
2212¡F - 1211¡C
2280¡F - 1249¡C
2320¡F - 1271¡C
9
2235¡F - 1224¡C
2300¡F - 1260¡C
2336¡F - 1280¡C
10
2284¡F - 1251¡C
2345¡F - 1285¡C
2381¡F - 1305¡C