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Q&A on Pyrometric Cones

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Pyrometric cones are very important for successful, repeatable firings. But we get many questions about cones and how they work. So here is a summary of almost everything I know about cones

Q. How do cones work?
A. 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 or the weight of the sensing rod for cones used in the Kiln-Sitter. It typically takes about 20 minutes for the cone to bend fully. Each higher cone number requires more heat to bend. Faster heating rates require the cone to be heated to a higher temperature.

Q. When is my cone considered "bent"?
A. Cone bending is measured in degrees. Straight up is 0 degrees. When the tip is completely bent over so that the tip just barely touches the kiln shelf, that is 90 degrees. In general, a cone is considered "bent" when it is at about 90 degrees. But different potters have different thoughts about this. The most important thing is for you to be consistent with your own firings, in order to maintain predictable results. Cones bend faster the further along they are, so watch closely toward the end!

Q. What temperature does a particular Cone correspond to?
A. This is a common question, with no real answer. It depends on the rate of temperature increase. Remember that cones measure total heat over the whole firing. So if you fire quickly, your maximum temperature will have to be hotter than if you fire slowly (to reach the same Cone). The following link contains a chart of temperatures assuming a certain temperature rise. The rate of increase during the last 300 degrees is the most important. In other words, during the first 1500 degrees of firing (Fahrenheit), how fast you fire won't affect the end temperature very much. It is primarily the higher temperatures that will affect the end temperature. See Chart Below

Q. I have an old box of cones. Is it still good?
A. Yes. Cones do not change with age. According to Orton who manufactures the cones: "We have Pyrometric cones over 25 years old that we use for comparison testing and standardization. We do keep improving cones, though. Today’s cones are more uniform and perform better and more consistently than those we made 5, 10, or 50 years ago. Also, some changes occur when the supplier of our raw materials goes out of business.

Q. My cones got wet. Can they still be used?
A. Cones are not normally affected by usual amounts of moisture such as humidity in the air. If the cones are damp it is best to dry them out before use. If the cones have been extremely wet they should not be used. An example of this might be cones that were in a flooded basement or studio. These cones may have retained their shape but they may have lost their strength and can possibly break off in the Kiln-Sitter if they are small cones. The large cones that have been wet may not deform with the proper accuracy.

Q. Why should I use Cones?
A. If you have a manual kiln, the only way to determine accurately when it is time to turn the kiln off is with Cones. If you have a Kiln-Sitter, the Cone will trip the Kiln-Sitter when the Cone bends, turning off the kiln. If you don't have a Kiln-Sitter you will manually watch the Cone through a spy hole or peep hole, and turn the kiln off when the Cone is bent.

If you have an electronic kiln, you don't need Cones. But it is still a very good idea to put Cones in. You can ensure that your kiln temperature is calibrated correctly. By putting Cones on every shelf, you can determine if there are hot and cold spots in the kiln. If the power goes out, you will be able to continue firing if you have Cones in. And finally, if you have any problems with your firing such as glazes not coming out the right color, or bubbling, you will know for sure what temperature was reached in that part of the kiln, which will be useful for troubleshooting. At the very least, with an electronic kiln, you should put Cones in every few firings to make sure the kiln doesn't drift and continues to fire at the right temperature as the elements age.

Q. What Cone number should I use?
A. It is best to use the Cone number your glaze matures at, plus 1 Cone above and 1 Cone below. When the lowest Cone bends, you can start getting ready to turn off the kiln, or start slowing it down (especially with gas kilns). You will be at the right temperature to turn off when the Cone below is very slightly bent, the Cone above is extremely bent over (almost melted looking), and the middle Cone is bent to 90 degrees.

Q. Which type of Cone do I want?
A. - Orton Jr Cones are used in Kiln-Sitters.
- Orton Standard Cones are used in Cone packs. A Cone pack is usually made by taking 3 cones, imbedding them in some clay, and allowing the clay to dry before firing. The 3 Cones include one at the firing temperature called the Firing Cone, 1 above called the Guard Cone and 1 below called the Guide Cone. For example, if you are firing at Cone 6, you use a #5, #6 and #7 Cone.
- Orton Self Supporting Cones are used for placement in the kiln when you aren't using a clay pack. They stand up by themselves.
- The last two are often called "witness cones" because you are "witnessing" or "watching" to see how far they bend.
- It is possible to use Orton Jr Cones as "witness cones" but these cones require slightly higher temperatures to bend, so you won't be as accurate.

Q. How exactly does a Kiln Sitter work?
A. The Kiln Sitter shuts off the kiln when a small cone placed under the sensing rod receives enough heat for it to fully bend. Bending is caused by the weight of the sensing rod. Note that 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 may be necessary. You can use a witness cone (or the 3-cone pack) to determine whether your kiln is shutting off early. Put the witness cones on a shelf near the Kiln Sitter to determine if a difference exists between the shelf and Kiln Sitter cones.

Cone Equivalent Temperatures (F)

Self Supporting Cones

1 3/4" mounting height

Large Cones

Small

15/16" Height

Regular

Iron Free

Regular

Iron Free

Regular

PCE

Heating Rate (F/hour)

Heating Rate (F/hour)

Cone

27

108

270

27

108

270

108

270

108

270

540

270

022

1049

1087

1094

N/A

N/A

1166

021

1076

1112

1143

N/A

N/A

1189

020

1125

1159

1180

N/A

N/A

1231

019

1213

1252

1283

1249

1279

1333

018

1267

1319

1353

1314

1350

1386

017

1301

1360

1405

1357

1402

1443

016

1368

1422

1465

1416

1461

1517

015

1382

1456

1504

1450

1501

1549

014

1395

1485

1540

1485

1537

1598

013

1485

1539

1582

1539

1578

1616

012

1549

1582

1620

1576

1616

1652

011

1575

1607

1641

1603

1638

1679

010

1636

1657

1679

1600

1627

1639

1648

1675

1623

1636

1686

09

1665

1688

1706

1650

1686

1702

1683

1702

1683

1699

1751

08

1692

1728

1753

1695

1735

1755

1728

1749

1733

1751

1801

07

1764

1789

1809

1747

1780

1800

1783

1805

1778

1796

1846

06

1798

1828

1855

1776

1816

1828

1823

1852

1816

1825

1873

05½

1839

1859

1877

1814

1854

1870

1854

1873

1852

1868

1909

05

1870

1888

1911

1855

1899

1915

1886

1915

1890

1911

1944

04

1915

1945

1971

1909

1942

1956

1940

1958

1940

1953

2008

03

1960

1987

2019

1951

1990

1999

1987

2014

1989

1996

2068

02

1972

2016

2052

1983

2021

2039

2014

2048

2016

2035

2098

01

1999

2046

2080

2014

2053

2073

2043

2079

2052

2070

2152

1

2028

2079

2109

2046

2082

2098

2077

2109

2079

2095

2163

2

2034

2088

2127

2088

2124

2174

3

2039

2106

2138

2066

2109

2124

2106

2134

2104

2120

2185

4

2086

2124

2161

2120

2158

2208

5

2118

2167

2205

2163

2201

2230

5½

2133

2197

2237

N/A

N/A

N/A

6

2165

2232

2269

2228

2266

2291

7

2194

2262

2295

2259

2291

2307

8

2212

2280

2320

2277

2316

2372

9

2235

2300

2336

2295

2332

2403

10

2284

2345

2381

2340

2377

2426

11

2322

2361

2399

2359

2394

2437

12

2345

2383

2419

2379

2419

2471

2439

13

2428

2458

2410

2455

2460

13½

2466

2493

14

2489

2523

2530

2491

2548

14½

2527

2568

15

2583

2602

2595

2608

2606

15½

2617

2633

16

2655

2687

2651

2683

2716

17

2694

2709

2691

2705

2754

18

2736

2746

2732

2743

2772

19

2772

2786

2768

2782

2806

20

2811

2824

2808

2820

2847

21

2851

2860

2847

2856

2883

23

2890

2898

2887

2894

2921

26

2892

2921

2950

27

2937

2961

2984

28

2946

2971

2995

29

2955

2993

3018

30

2977

3009

3029

31

3022

3054

3061

31½

3063

3088

3090

32

3103

3123

3123

32½

3124

3146

3135

33

3150

3166

3169

34

3195

3198

3205

35

3243

3243

3245

36

3268

3265

3279

37

3308

38

3362

39

3389

40

3425

41

3578

42

3659


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source: BigCeramicStore.com and Orton


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