Temperature and Heatwork

What are cones?

Firing ceramics in a kiln is a bit like cooking a pizza in the oven. As with Italian wood-fired stoves, you could set your oven on very high heat and cook the pizza in just four minutes. Or you could cook the pizza for a longer amount of time at lower heat. Either way, you'll end up with edible pizza.

Pyrometric cones are a measurement of the total amount of heatwork. Heatwork is the combination of time and temperature. We can say that two pizzas received a similar amount of heatwork if they are both fully cooked, even if they were cooked at different temperatures for different durations of time. Similarly, we can say that two ceramic vases were fired to a certain cone (e.g. similar amount of heatwork) even if the firings were different in temperature & time.

It's important to note that firings are often uneven: a kiln may have cool and hot spots, and the difference in temperature may be one or more cones. Just because a cone has fallen doesn't mean the entire kiln (including the ceramics and kiln furniture) has also reached that cone temperature. So it's a good idea to sometimes place extra cones throughout the kiln to determine if the firing is even.

For more information on cones, see the excellent Digitalfire article Interpreting Orton Conesopen in new window as well as Orton's Pyrometric Cones Resourcesopen in new window.

Setting & Reading Cones

The quantity of cones and cone packs as well as their arrangement and placement vary from potter to potter, kiln to kiln. The following is only one approach.

conetypes

There are a number of different cone manufacturers. Each brand has unique cone formulations and firing characteristics. Cone numbers between brands may be quite different, for instance the Chinese "Seger" cone 9 is approximately Orton cone 12. Even when cone numbers between brands are rated for the same temperature, actual firing behaviour may be quite different. Glazy recommends using Orton cones as they seem to be the most reliable and accurate.

breakconesBreaking apart pairs of Orton cones using a cylindrical support.

conelengthLength of large cones above cone plaque.

According to Orton:

"Large Cones are normally mounted so exactly 2" of the cone is exposed above the top of the cone plaque or clay pat. Small Cones are mounted so that 15/16" of the cone is exposed above the top of the plaque or pat."

Measure your cones and memorize how deeply they should be mounted.

conepackmakebAfter flattening a coil of clay, push cones downwards so that the angle is identical with a cone placed directly on the flat surface.

conepack1bWet cone packs can explode. Make your packs in advance or poke holes into the pack to let steam to escape.

This cone pack is for an Orton cone 10 firing.

The guide cone 9 will alert us when we are close to temperature. The guard cone 11 will let us know if we are beginning to overfire.

While more expensive, pre-made cone plaques and self-supporting cones eliminate height and angle inconsistencies.

conepacklowFor mid and high fire, a "boat" is made to catch low-temperature cones. This boat is a folded slab.

conesinkilnViewing low-temperature cones in the kiln at about 900°C. Cone 010 is bending, while 08, 1, and 3 have not moved. It's advisable to place cone packs at the top and bottom of your kiln in order to understand and control temperature differences while firing.

conepack2A cone pack after firing. In this firing, once cone 9 fell to 5 o'clock the kiln was intentionally stalled. Even though the temperature did not rise, cone 10 completely fell after about one hour. Cones measure heatwork, the effect of both temperature and time.

template The Orton Cone Templateopen in new window can be used to determine the degree of deformation for Orton cones.

chineseconesInexpensive Chinese cones. Cones 7 and 9 are down, but cone 8 is still standing. The entire box of cones should be thrown away. But which cone is defective, 8 or 9? Compared to the cost of a firing and the work inside, the cost of cones is negligible. Buy the most reliable cones you can find.

ConeDiagram2_1200|690x219

Download PDF chart of Orton Cone positions: Color (566.2 KB) A4 Black & White (78.2 KB)

Viewing Cones

Warning: Prolonged or cumulative long-term exposure to infrared and ultraviolet light can seriously damage your eyes.

ANSI-compliant infrared and ultraviolet eye-protection glasses are a must when viewing inside the kiln at high temperatures.

Sunglasses that only filter ultraviolet light are NOT sufficient.

For more information, see Eye Protection in the Pottery Studioopen in new window

There are many tricks for seeing cones more clearly inside the kiln.

The simplest method is using a high-intensity LED flashlight with an output of at least 400 lumens and a focused beam.

gogglesflashlight.jpg#5 welding goggles and 2000 lumens LED flashlight.

Temperature Equivalent Charts for Cones

References:

Orton Pyrometric Cones Celsius (°C)

Self Supporting ConesLarge ConesSmall Cones
RegularIron FreeRegularIron FreeRegular
Heating Rate ̊C/hour (last 100 ̊C of firing)
Cone156015015601506015060150300
022586590   N/AN/A  630
021600617   N/AN/A  643
020626638   N/AN/A  666
019656678695   676693  723
018686715734   712732  752
017705738763   736761  784
016742772796   769794  825
015750791818   788816  843
014757807838   807836  870
013807837861   837859  880
012843861882   858880  900
011857875894   873892  915
010891903915871886893898913884891919
09907920930899919928917928917926955
08922942956924946957942954945955983
079629769879539719829739859709801008
06981998101396999199899510119919961023
05 1/21004101510259901012102110121023101110201043
0510211031104410131037104610301046103210441062
0410461063107710431061106910601070106010671098
0310711086110410661088109310861101108710911131
0210781102112210841105111511011120110211131148
0110931119113811011123113411171137112211321178
111091137115411191139114811361154113711461184
2111211421164   11421162  1190
311151152117011301154116211521168115111601196
4114111621183   11601181  1209
5115911861207   11841205  1221
5 1/2116712031225   N/AN/A  N/A
6118512221243   12201241  1255
7120112391257   12371255  1264
8121112491271   12471269  1300
9122412601280   12571278  1317
10125112851305   12821303  1330
11127212941315   12931312  1336
12128513061326   13041324  1355
13131013311348   13211346  N/A
14135113651384   13881366  N/A

Orton Pyrometric Cones Fahrenheit (°F)

Self Supporting ConesLarge ConesSmall Cones
RegularIron FreeRegularIron FreeRegular
Heating Rate ̊F/hour (last 200 ̊F of firing)
Cone2710827027108270108270108270540
02210871094   N/AN/A  1166
02111121143   N/AN/A  1189
02011591180   N/AN/A  1231
019121312521283   12491279  1333
018126713191353   13141350  1386
017130113601405   13571402  1443
016136814221465   14161461  1517
015138214561504   14501501  1549
014139514851540   14851537  1598
013148515391582   15391578  1616
012154915821620   15761616  1652
011157516071641   16031638  1679
01016361657167916001627163916481675162316361686
0916651688170616501686170216831702168316991751
0816921728175316951735175517281749173317511801
0717641789180917471780180017831805177817961846
0617981828185517761816182818231852181618251873
05 1/218391859187718141854187018541873185218681909
0518701888191118551899191518861915189019111944
0419151945197119091942195619401958194019532008
0319601987201919511990199919872014198919962068
0219722016205219832021203920142048201620352098
0119992046208020142053207320432079205220702152
120282079210920462082209820772109207920952163
2203420882127   20882124  2174
320392106213820662109212421062134210421202185
4208621242161   21202158  2208
5211821672205   21632201  2230
5 1/2213321972237   21942233  N/A
6216522322269   22282266  2291
7219422622295   22592291  2307
8221222802320   22772316  2372
9223523002336   22952332  2403
10228423452381   23402377  2426
11232223612399   23592394  2437
12234523832419   23792415  2471
13238924282458   24102455  N/A
14246424892523   25302491  N/A
Last Updated: 7/17/2021, 10:57:20 PM