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close this book Village-Level Brickmaking
close this folder Appendices
View the document Appendix 1 - Clay Testing
View the document Appendix 2 - Making A Hinge Mould And Moulding Table
View the document Appendix 3 - The 3: 4: 5 Method
View the document Appendix 4 - Pyrometric Cones
View the document Appendix 5 - Woodlots
View the document Appendix 6 - References

Appendix 4 - Pyrometric Cones

A pyrometric cone is a cone made of a ceramic material that will deform or bend when a predetermined temperature is reached. It is used to measure the temperature in a back kiln. Each numbered cone corresponds to a known temperature at which it will deform. The two most common names or manufacturers of pyrometric cones are Orton and Seger. They both use a similar numbering system but the temperatures that correspond to those numbers vary slightly. The table on pyrometric cones listed here gives only some of the available cone numbers and corresponding temperatures.


Figure

PYROMETRIC CONES

CONE NUMBER

TEMPERATURE DEFORMS

 

SEGER

ORTON

014

815C

830C

013

835C

860C

012

855C

875C

011

880C

895C

010

900C

905C

09

920C

930C

08

940C

950C

07

960C

990C

06

980C

1015C

05

1000C

1040C

04

1020C

1060C

03

1040C

1080C

02

1060C

1125C

01

1080C

1145C

1

1100C

1160C

When building the field kiln, spaces (called spy holes or peep holes) are left, about 1 metre in length, 80 mm wide and 120 mm high. The cones are placed at the end of these holes and a brick "plug" covers the outside. The brick plug can then be removed during the kiln firing for observing the cones and estimating the temperature of the kiln at that particular place.

Normally three different cones are placed in each of the spy holes. These cones are usually embedded in a fireclay base to prevent them from falling over and then the whole unit is placed at the end of the spy hole. One of the cones is a number that will deform when the correct temperature (usually 950C) is reached. Another cone is a number that will deform at a slightly lower temperature; this will warn you that the firing is almost complete. The third cone is a number that will deform at a temperature higher than 950C; this will inform you that the kiln has been heated beyond the desired temperature and the bricks are likely to be overfired.

For example, using Orton Cones, cones N 010 08 and 06 are placed together inside the spy holes of the field kiln (each spy hole will get one set of these cones). Cone N 010 will deform at a temperature of 905 C. Cone N 08 will deform at 950 C (usually the correct temperature for firing bricks) and cone N 06 will deform at a temperature of 1015 C.


Figure

The cones are set at a slight angle in a special clay base. When the temperature of the kiln in the area of this set of cones reaches approximately 900C, the first cone (no. 010) will begin to deform. When the second cone (no. 08) begins to deform, the kiln in that area has reached the correct temperature for firing bricks and this temperature should then be maintained for 12 to 24 hours. If the third cone (no. 06) begins to deform, the kiln has passed the desired temperature and there is a risk of overfiring the bricks.


Figure

Pyrometric cones come in two sizes; either 30 mm or 50 mm tall and are sold in boxes of 50. When buying the cones, always specify the type of cone (Segar or Orton), the size (30 or 50 mm) and the number of the cone with its corresponding temperature.


Figure

Pyrometric cones are ideal, especially when firing a field kiln for the first time. They provide valuable accurate information and do not require the brickmaker to have experience in estimating the temperature of the kiln by observing the colours of the bricks inside the fire tunnels. They are also excellent training tools because the fret-time brickmaker can learn by relating the colours seen in the kiln to the information gained from the cones and then use this experience in the next kiln firing.