Pyrometric cones are useful in
determining when a firing is complete, if the kiln provided enough heat, if
there was a temperature difference in the kiln or if a problem occurred during
firing. Cones are made from carefully controlled compositions and they bend in a
repeatable manner (over a relatively small temperature range- less than 40ºF).
Small (junior) cones are used in a Kiln-Sitter™.
SSB Self-Supporting Cones "Witness Cones"
2.5" (6.35cm) tall Self Supporting cones DO NOT require mounting
in cone plaques , holders or pats. These are the easiest cones to use.
(The middle self-supporting cone is
perfectly fired. The left is over-fired, the right is under-fired.)
Orton Self-Supporting cones are
$9.20 for a box of 25
SSB Mid-Range Assortment Pack
Assortment includes 5 of each - Cone 4, 5, and 6
$6.99
SSB Low Fire Assortment Pack
Assortment includes 5 of each - Cone 04, 05, and 06
$6.99
LRB Large Standard Pyrometric Cones 2 1/2 inch (6.35 cm) Tall
Needs to be supported by a
cone plaque or a clay pat. Support Plaque
Shown Below $.99 Each
Orton LRB large Standard cones are $12.80
for a box of 50
SRB Small Cones "Junior
Cones"
1 1/8" (2.86 CM) Tall. Small
cones are commonly used in the kiln sitter. The tapered shape allows
some flexibility to adjust the shut-off temperature by placing the
thinner or thicker portion of the cone under the sensing rod. Small
cones can also be used as miniature witness cones when space is limited.
Support Plaque Shown Below $.99 Each
Orton Junior cones are $9.40 for a box of 50
Pyrometric Bars
Pyrometric Bars are 1 inch
(2.54 cm) Long. The uniform shape allows for more consistent control
when used in Kiln-Sitter.
Orton PYROMETRIC BARS are $11.20 for a box of 50
Orton large standard bar cones
support plaque
$1.89 each
Orton Medium standard bar cones
support plaque
$1.49 each
Orton Small Regular bar cones
support plaque
$1.89 each
Pyrometric
Cones for Ceramic Arts
Pyrometric
cones measure heat-work (the effect of time and
temperature).
Pyrometric
cones are slender pyramids made from about 100 carefully
controlled compositions. Each cone number is unique in
measuring temperature within a small temperature range (less
than 30°C). Cones measure the amount of heat absorbed. As
the cone nears its maturing range, it softens and the tip
begins to bend, drawn down by the influence of gravity
3 cone method for monitoring heat-work
Pyrometric Bars are 1
inch (2.54 cm) Long. The uniform shape allows for more consistent
control when used in Kiln-Sitter.
Temperature Equivalents use the same numbering system as small cones.
View Temperature Equivalents in either Degrees C or Degrees F chart.
Cone/Temperature
Conversion Chart
Cone
Temp at 108ºF/hr
Temp at 270ºF/hr
Kiln Color
Ware and Glaze Types
10
2345
2381
White
Stoneware Glaze
9
2300
2336
8
2280
2320
7
2262
2295
6
2232
2269
Porcelain Bisque
5
2167
2205
4
2124
2161
Porcelain Glaze
3
2106
2138
2
2088
2127
1
2079
2109
01
2046
2080
Yellow
02
2016
2052
03
1987
2019
04
1945
1971
05
1888
1911
06
1828
1855
Bisque,Low Glaze
07
1789
1809
Orange
Red Family Glazes
08
1728
1753
09
1688
1706
010
1657
1679
011
1607
1641
012
1582
1620
013
1539
1582
Red
014
1485
1540
015
1456
1504
016
1422
1465
017
1360
1405
Glass Sagging
018
1319
1353
China Paints
019
1252
1283
Metallics, Lusters
020
1159
1180
Dull Red
Decals
021
1112
1143
Glass Paint
022
1087
1094
How to Separate Small Cones
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.
to O22. Cone Bending
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
fired. 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,
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. 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.