Pyrometric cone

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Self Supporting Cones prior to firing (top) and after (bottom)

Seger Cones after use

Pyrometric cones are pyrometric devices that are used to measure heatwork during the firing of ceramic materials. The cones, often used in sets of three as shown in the illustration, are positioned in a kiln with the wares to be fired and provide a visual indication of when the wares have reached a required state of maturity, a combination of time and temperature. Thus, pyrometric cones give a temperature equivalent, they are not simple temperature-measuring devices.

The pyrometric cone is described by Dodd and Murfin (1994) as 'A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base of a definitive temperature. Pyrometric cones are made in series, the temperature interval between the successive cones usually being 20 degrees Celsius. The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK)'.

1 Numbering Method
2 Usage
3 Control of variability
4 History
5 References
6 External links

[edit] Numbering Method

The coolest cone number is 022 and the hottest is 42. The first cones were numbered from 1 to 20. When cooler cones were developed, an '0' was placed before the number. So cones cooler than Cone 1 decrease in melting temperature from 01 to 02, etc. to 022. Both temperature and time and sometimes atmosphere affect the final bending position of a cone. Temperature is the predominant variable. We refer to the temperature as an equivalent temperature, since actual firing conditions may vary somewhat from those in which the cones were originally standardized. Observation of cone bending is used determine when a kiln has reached a desired state, Additionally, small cones or bars can be arranged to mechanically trigger kiln controls when the temperature rises enough for them to deform. Precise, consistent placement of large and small cones must be followed to ensure the proper temperature equivalent is being reached. Every effort needs to be made to always have the cone inclined at 8° from the vertical. Large Cones must be mounted 2 inches above the plaque and Small Cones mounted 15/16 inches. The invention of Self-Supporting Cones by the Orton Ceramic Foundation in the early 1970's eliminates the possibility of human error when mounting, by building a base directly onto the cone.

[edit] Usage

Many products used today, such as porcelain and lead-free glazes, need to be fired within a 2-cone range. The 3-cone system can be used to determine temperature uniformity and to check the performance of a electronic controller. The 3-cone system consists of three consecutively numbered cones:

Firing Cone - cone recommended by manufacturer of glaze, slip, etc.
Guide Cone - one cone number cooler than firing cone.
Guard Cone - one cone number hotter than firing cone.

For example: Cones O17, O18, O19 or Cones 5, 6, 7.

Additionally, most kilns have temperature differences from top to bottom. The amount of difference depends on the design of the kiln, age of the heating elements, load distribution in the kiln, and the cone number to which the kiln is fired. Usually, kilns have a greater temperature difference at cooler cone numbers. Cones should be used on the lower, middle and top shelves to determine how much difference exists during firing. This will aid in the way the kiln is loaded and fired to reduce the difference. Downdraft venting will also even out temperatures variance.

[edit] Control of variability

Pyrometric cones are sensitive measuring devices and it is important to users they should remain consistent in the way that they react to heating. Cone manufacturers operate procedures to control variability (within batches and between batches) to ensure that cones of a given grade remain consistent in their properties over long periods. Orton cones, for example, are made and tested in compliance with procedures agreed with the National Bureau of Standards for the control of variability.

Even though they share a similar system of numbering, Orton cones and Seger cones are not exactly alike in their characteristics and when a change is made from cones made by one manufacturer to those made by another allowances for the differences are sometimes necessary.

[edit] History

Archaeologists working at Northern Song period (960 to 1127 AD) kiln sites in the Chinese provinces of Shaanxi and Shanxi have reported that pyrometric cones about five centimetres tall and made from loess were used to help control the firing of the kilns.

In 1782, English potter and industrialist Josiah Wedgwood created more accurately scaled pyrometric beads. This led him to be elected a fellow of the Royal Society.

The modern form of the pyrometric cone was developed by the German ceramics technologist Hermann Seger and first used to control the firing of porcelain wares at the Königliche Porzellanmanufaktur (Royal Porcelain Works) in Berlin, in 1886. Seger cones are to this day made by a small number of companies and the term is often used in Europe as a synonym for pyrometric cones.

The Standard Pyrometric Cone Company was founded by Edward J. Orton, Jr. in 1896 to provide a calibrated, visual device to measure the amount of heat delivered to ceramic wares during firing. Following the death of Dr Orton in 1932, a charitable trust was established to ensure the continued operation of the company, for the benefit of the ceramic arts and industry.

Commercially produced pyrometric cones replaced glaze cones used by European and American potters in earlier times. Glaze cones were made by evaporating water from a liquid glaze until the resulting mass reached the consistency of a plastic clay. The plastic mixture was then formed into cones that were dried and set in a soft pad of clay in a kiln. When observed through the viewing port of a kiln the potter could see when a glaze cone had reached its melting point. Asian potters used draw rings, rings of clay dipped in glaze, for a similar purpose. The rings were removed from the kiln through special loopholes in the kiln walls using metal rods and examined for signs of melting in the glaze.

[edit] References

Dodd, A. and Murfin, D. (ed.) (1994) Dictionary Of Ceramics. 3rd edition. Institute of Materials. Woodhead Publishing Limited, Cambridge. ISBN 0-901716-56-1.

Hamer, Frank and Hamer, Janet (1991). The Potter's Dictionary of Materials and Techniques. Third edition. A & C Black Publishers, Limited, London, England. ISBN 0-8122-3112-0.