KILNS & FURNACES
Electric in air to 1800°C • Research • Heat treatment
Controlled atmosphere • Melting • Gas, Natural/LPG to 2300°C
Tetlow Kiln Pty. Ltd. ABN: 26 166 832 300 54 Howleys Road, Notting Hill, 3168 Victoria Australia
T: +61 3 8545 8296 F: +61 3 9544 2723 E: info@tetlow.com.au www.tetlow.com.au
12.3 CONES (PYROSCOPES)
Cones are an important and useful way of controlling your kiln’s firing. Buller rings and
Holdcroft bars fall into the same category.
Staffordshire cones come generally in two sizes, standard cones are 2½ inches (63.5 mm)
tall and miniature cones are 1 inch (25.4 mm) tall. The cones are three-sided conical
shape and made of a carefully controlled mixture of ceramic materials, which is designed
to give a graduated scale of fusing temperatures at approximately 20°C intervals.
The cones which melt at the lower temperature contain a higher proportion of fluxes than
those melting at the higher temperatures, which contain increasingly larger proportions of
refractory oxides. This melting or fusing temperature is denoted by a number which is
stamped into the back of the cone and by reference to the Staffordshire cone chart.
Although it is commonly assumed that the Staffordshire cones melt at indicated
temperatures on the chart, the cones can also melt and collapse when they have been
subjected to a certain temperature, or rate of temperature increase, for a certain length of
time.
Time factor is important because it is usually assumed that when a kiln fires to 1000°C
over a period of 8 to 10 hours, the ware has been fired to its recommended temperature
and therefore fired correctly. However, if firing is controlled solely by a pyrometer, then the
kiln would have switched off when the temperature reached 1000°C and the ware fired in
3 to 4 hours would not have been properly fired and would probably be underfired when
withdrawn from the kiln. This is where the time factor of Staffordshire cone becomes
useful.
Staffordshire cones will only collapse when subjected to heat for a certain length of time
and will not collapse if they are fired too rapidly to a temperature considerably higher than
the cone number would indicate. Similarly, if the cones are fired too slowly, they will
probably collapse at a temperature lower than that indicated by the cone number. In this
way, Staffordshire cones give an indication of the amount of heat work applied to the ware
and not merely the temperature to which the ware is subjected.
One of the most important considerations in the use of pyroscopes is the way in which
they are mounted. This is generally done by inserting the base of the cones either into
special cone holders or into a pad of plastic clay, but regardless of the type of mounting it
is important that all the cones be embedded to the same depth. It is necessary for the
cones to be placed at an angle of about 15˚ to the vertical, and to ensure this is the case,
the manufacturer slants the base of the cone so that this inclination is automatically
achieved when the cone is stood upright on its base.
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