Thursday, 24 December 2009

Temperature Equivalents of Orton Pyrometric Cones

The pyrometric cones used by ceramicists can be very useful for checking the temperatures within your kiln. Bullseye have a test described on their website. The Orton cones can provide an alternate means. This process will also test the accuracy of the temperature readings of you controller/output.

You need to place the cones on supports all around the kiln. The cones are placed horizontally on kiln furniture with as much of the thin point extending beyond the edge of the support as possible. Although this is not the traditional recommendation, it is the practice of ceramicists all over. The behaviour of the cones will indicate both the temperature achieved - if you fire them according to instructions - and where the hotter and cooler parts of your kiln are located.

You do need to make visual observations to determine when the cone has matured. So you begin checking about 20C - 15C below the indicated maturing temperature. What you will see is the point of the cone bending down. When the point of the cone is pointing directly down, the maturing temperature has been achieved.

You can now check the temperature that is recorded by your read out. Write that down some where. Switch the kiln off now, if you want to see what temperature differences there are within your kiln. You do not need to do any controlled cooling. When cool enough, you can open the kiln and observe where the temperature has differed, by the extent to which the cones are pointing down. If the cone has completely conformed to the edge of its support, it has been over fired. Those that do not point directly down, have not reached the maturing temperature.

The cone numbers that are useful for kiln forming are 022 - 011. Remember that to achieve the temperatures, the cones must be fired at the indicated rate. Any other firing rates will not give accurate temperatures, as the cones are measuring heat work.

Large Orton Cones fired at the rate of 60C/hr over the last 100C will give the following temperature equivalents:
019: 676
018: 712
017: 736
016: 769
015: 788
014: 807
013: 837
012: 858
011: 873

However if you fire large cones at 150C/hr over the last 100C, you will get the following temperature equivalents:
019: 693
018: 732
017: 761
016: 794
015: 816
014: 836
013: 859
012: 880
011: 892

You of course, get different temperatures for the small cones of the same numbers. The small cones must be fired at 300C/hr over the last 100C.
022: 630
021: 643
020: 666
019: 723
018: 752
017: 784
016: 825
015: 843
014: 870
013: 880
012: 900
011: 915

If you decide to use self supporting cones, the evidence you are looking for is slightly different. In this case, the cone has achieved the heat work when the point is level with the base. If you fire the self supporting cones at 60C/hr for the last 100C you will get the following temperature equivalents:
022: 586
021: 600
020: 626
019: 678
018: 715
017: 738
016: 772
015: 791
014: 807
013: 837
012: 861
011: 875

A wall chart is available from the manufacturer