PPP: Why Use Cones?

September 1, 2006
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To understand why pyrometric cones were developed, we should go back to the time before they were in use to see how kilns were fired. Before the invention of the cone, determining when kiln firings were complete was done purely by eye. No accurate device was available that could indicate when to shut off the kiln.

To fill that need, Edward Orton Jr., a professor of ceramic engineering at The Ohio State University, developed a set of cones that he began to sell into the ceramic manufacturing industry. Originally designed to indicate when a firing was complete, the pyrometric cone is now used as a quality control tool to help determine temperature uniformity within the kiln, atmospheric differences, repeatability between firings, and accuracy of the thermocouple.

What is a Cone?

Cones are slender pyramids made from about 100 carefully controlled ceramic compositions. They are manufactured to bend in a repeatable manner over a relatively small temperature range (less than 40°F). The final bending position is a measure of the amount of heat absorbed during firing.

Throughout the ceramic manufacturing field, when we talk about what temperature to fire our product, we refer to the cone that bends over when that product is properly fired. Each cone is identified with a number stamped into its side. The numbering system has evolved since the first set of cones was introduced. The original set was numbered from 1 to 42, with cone 1 being the lowest temperature cone used at that time and 42 the highest.

Since then, cones have been developed that indicate temperatures below cone 1. Thus, cones 01 through 022 were introduced, and cone 022 is now the lowest temperature cone.

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 if you 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 shorter 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.

A cone measuring template. In this example, 72° is the end point of the cone's deformation.

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 (see the Temperature Equivalent Charts at the end of this article), an equivalent temperature can be determined if the heating rate is known by measuring the final bending position of the cone. Self-supporting cones duplicate their bending behavior with a standard deviation of 2.4 angular degrees, or less than + 2°C.

Clock position vs. angular deformation.

How are Cones Used?

Cones are placed throughout the kiln to act as a witness to the amount of heatwork performed in that area of the kiln. The cone bends when glass forms within the cone and it starts to become soft. The composition of the cone and the amount of heat determine when and how much glass is formed.

A witness cone bends due to the affect of gravity acting on it. Therefore, mounting height and angle are important variables that the user must control to achieve accuracy. The higher the cone is mounted or the more it leans over at the start, the greater the affect of gravity.

For this reason, Orton developed self-supporting witness cones, where mounting height and angle are fixed. It typically takes 15 to 25 minutes for a cone to bend, depending on the cone number. The cone bends slowly at first, but picks up speed once it reaches the halfway point. 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.

Large cones properly mounted in a cone plaque.

Three-Cone System

Many products used today, such as porcelain and lead-free glazes, need to be fired within a two-cone range. The three-cone system can be used to determine temperature uniformity and to check the performance of the KilnSitter(r) or electronic controller. The three-cone system consists of three consecutively numbered cones (e.g., cones O17, O18, O19, or cones 5, 6, 7):
  • guide cone, one cone number cooler than the firing cone
  • firing cone, the cone number recommended by the manufacturer of the glaze, slip, etc.
  • guard cone, one cone number hotter than the firing cone

Kiln Evaluation

Most kilns have temperature differences from top to bottom. The amount of difference depends on the design of the kiln, the age of the heating elements, the 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 (e.g., decal, bisque/glaze, porcelain/stoneware).

After firing, observe the three cones. If, on the bottom shelf, the guide cone has only bent half way, then ware is fired 11/2 cones lower. A guard cone on the top shelf bent halfway indicates that ware is a half-cone hotter, and a two-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.

A series of self-supporting cones at various levels of deformation on a kiln shelf.

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 degree (6 o'clock) position.

Controllers depend on the 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 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.

Selt-Supporting Witness Cones

Orton recommends self-supporting witness cones because they are easier to use and not as susceptible to bending variation. Many people use witness cones for every firing and the three-cone system periodically to check for changes in the kiln. Cones should be saved to compare different firings. When more than a half-cone difference occurs, it usually indicates that 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.

Editor's note: This information has been reprinted and updated with permission from the Orton Ceramic Foundation. It is not intended to be representative of all pyrometric products. For additional details regarding the products included in this article, visit http://www.ortonceramic.com .

* These large cones have different compositions and different temperature equivalents.

Temperature Equivalent Chart for Orton Pyrometric Cones (°F)

Cone Numbers 022-14

* These larger cones have different compositions and different temperature equivalents.

Temperature Equivalent Chart for Orton Pyrometric Cones (°C)

Cone Numbers 022-14

These tables provide a guide for the selection of cones. The actual bending temperature depends on firing conditions. Temperatures shown are for specific mounted height above base. For self supporting, 1 ¾ in.; for large, 2 in.; for small, 15/16 in. For large cones mounted at 1 ¾ in. height, use self-supporting temperatures.

Editor's note: Reprinted with permission from the Orton Ceramic Foundation. Visit http://www.ortonceramic.com for additional information.


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