The temperature equivalents apply to large
2.5" pyrometric cones when heated at the indicated heating rate
in an air atmosphere. Small cones in the automatic shut-off have
approximately the same temperature equivalent.
When looking into the observation holes all objects that are near
the same temperature will be near the same color. Brighter
appearing objects are hotter than darker ones.
This table is for general information. Consult the manufacturer's
instructions for the correct cone number to use.
Why Use Pyrometric Cones
Cones have been used in firing ceramics for more than
100 years. They are useful because they can determine when a
firing is complete, or if the kiln was provided enough heat, or
if there is a temperature difference in the kiln, or if a
problem occurred during the firing.
What is a Cone?
Cones are slender pyramids made from about 100
carefully controlled compositions. They bend in a repeatable
manner over a relatively small temperature range (less than
50°F). The final bending position is a measure of the amount of
heat absorbed. We refer to one cone number as being hotter or
cooler than another. The coolest cone number is O22 and the
hottest is 42. The first cones were numbered from 1 to 20. When
cooler cones were developed, an 'O' was placed before the
number. So cones cooler than Cone 1 increase from O1 to O2, etc.
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.
Using charts available from Orton, an equivalent temperature can
be determined by measuring the final bending position of the
Cone, if the heating rate is known. Orton Self-Supporting Cones
duplicate their bending behavior with a standard deviation of
2.4 angular degrees or less than + 2°C.
How are Cones Used?
Cones are used as witness cones located on a kiln shelf near the
ware or in a kiln shut off device (e.g. Kiln-Sitter®). The cone
bends when glass forms and it becomes soft. The composition of
the cone and the amount of heat determine when and how much
glass if formed. It is important to note that it is the weight
of the sensing rod that causes the cone (or bar) in a
Kiln-Sitter® to bend. Changes in weight affect the bending of
the cone. A witness cone bends because of gravity acting on it.
Therefore, mounting height and angle is important. The higher
the cone or the more it leans over at the start, the more
gravity affects bending, causing early bending. It is for this
reason Orton developed Self-supporting witness cones some 20
years ago, where mounting height and angle is fixed. It
typically takes 15 to 25 minutes for a cone to bend, depending
upon the cone number. The cone bends slowly at first but once it
reaches the halfway point, it bends quickly. When the cone tip
reaches a point level with the base, it is considered properly
fired; however, the difference between a cone touching the shelf
and a cone at the 4 o'clock position is small and rarely affects
the fired results.
Why use Cones?
Firing ceramics is much like baking, except temperatures are
higher. Ceramics can be fired over a range of temperatures. Some
products have a wide firing range while others have a narrow
range. Firing to a slightly lower temperature requires the ware
to be held for a longer time, just as it would be done if one
were baking a turkey. This is because it takes time for the ware
to absorb all of the heat needed to properly "cook" the piece.
We refer to this absorption of heat as 'heatwork'. When the
amount of heatwork for two firings is the same, the pieces will
look identical, even if one is fired to a higher temperature for
a short time and another is fired at a lower temperature for a
longer time. Since cones measure heatwork, all manufacturers
recommend the cone number to which their product should be
Three Cone System
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 the Kiln-Sitter® or electronic controller. The
3-cone system consists of three consecutively numbered cones:
• Firing Cone - cone recommended by manufacturer of glaze, slip,
• 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.
Cones Help Evaluate Kilns
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. Use cones
on the lower, middle and top shelves to determine how much
difference exists during firing. It's best to do this for each
type of firing you do - i.e. decal, bisque/glaze,
porcelain/stoneware. After firing, observe the 3 cones. If, on
the bottom shelf, the Guide cone has only bent half way, then
ware is fired one and half cones lower. A Guard cone on the top
shelf bent halfway indicates that ware is a half cone hotter and
a 2-cone difference exists between the top and bottom of the
kiln. If you find a difference, make changes in the way the kiln
is loaded and fired to reduce this difference. Adding downdraft
venting will also even out temperatures. The kiln can also be
upgraded to Orton's UniTemp™ controller, which keeps the kiln
uniform in temperature (see Kiln Controllers).
Checking Kiln Sitter® Performance
The Kiln Sitter® shuts off the kiln when a small cone (or bar)
placed under the sensing rod receives enough heat for it to
fully bend. Bending is caused by the weight of the sensing rod.
Because the cone in the Kiln Sitter® is located at the kiln wall
(closer to the heating elements), it frequently receives more
heat than witness cones, causing the kiln to shut off early.
Using the next hotter cone/bar may be necessary. Use the 3-cone
system on a shelf near the Kiln Sitter® to determine if a
difference exists between the shelf and Kiln Sitter® cones.
Checking Controller Performance
Electronic controllers allow firing to a temperature (and even a
cone number). The controller uses a temperature reading measured
by one or more thermocouple(s) placed through the refractory
wall of the kiln. A Self-supporting witness cone will check
whether the controller is firing accurately. Place the cone in a
location near the thermocouple. After the firing, determine if
the Firing Cone has bent properly. There should be no more than
a half cone difference. Orton encourages the use of electronic
controllers; however, we strongly recommend the use of at least
one witness cone for every firing to assure that the kiln really
did fire to the right cone number. As mentioned earlier, bodies,
glazes and decoration products are all formulated to be fired to
a cone number, bent to the 90° (6 o'clock) position. Controllers
depend upon accurate measurement of temperature and proper
programming to fire properly. Most controllers use a Type K
thermocouple, which may not give an accurate temperature and
which will change after being used. It is not unusual for a Type
K thermocouple to have an error of more than 25°F when fired to
Cone 6 repeatedly. This is more than a full cone. By using
witness cones, you can determine whether the firing was to the
right cone number.
Self-supporting Witness Cones
Orton recommends Self-supporting witness cones because they are
easier to use and less subject to bending variation. Many people
use witness cones every firing and the 3-cone system
periodically to check for changes in the kiln. Cones are saved
to compare different firings. When more than a half cone
difference occurs, it usually indicates a problem exists. This
gives you an opportunity to fix the problem or change the way
the kiln is being fired to avoid future problems. Cones are the
best way to inexpensively monitor your kiln.