FAQs for Pyrometric Cones

How do you use cones?

Orton manufactures four different types of cones (Small cones, Bar cones, Large cones and Self- supporting cones).

The Large and Self-supporting cones are used to measure temperature uniformity and/or if there was sufficient heatwork done to mature the ware. Cones indicate whether the ware received an adequate amount of heatwork to properly mature the clay or glaze. A properly fired cone will bend over with the tip of the cone almost touching the shelf. After firing a kiln, a simple glance at the cone can tell if the firing was successful.

The small and bar cones are mainly used in the kiln shut-off device called the Kilnsitter. This is a mechanical device that shuts off the kiln when the temperature inside is sufficient to cause the cone to deform under the weight of the Kilnsitter rod.

The only true measurement of heatwork is from a Large or Self- supporting cone placed on the shelf next to the ware.

What do the terms heating rate and soak mean?

The heating rate refers to the rise in temperature of your kiln for every hour it is firing. If a kiln takes three hours to get to a temperature of 900 F, then it would have a heating rate of 300 F per hour. It is important to know the heating rate since it determines the temperature at which a cone will deform. We post the endpoint temperatures for all cones fired at several different heating rates in the Reference Library ( °F Wall Chart and °C Wall Chart )

The term Soak means that you hold the kiln at a specific temperature for a period of time. The practice of soaking a kiln is done to help equalize the temperature throughout the kiln or produce a desired effect in the firing.

On the Orton Cone chart, what do the numbers 27, 108, and 270 found at the top of the cone chart mean?

The numbers 27, 108, and 270 refer to the Heating Rate. The heating rate has an impact on the final temperature at which a cone will bend. Try thinking of those numbers in a different way. Replace the 27 with the word slow, 108 with medium, and 270 with fast. If you are firing fast, look only at the 270 column, for medium firing rates, look at the 108 column, and for long slow firings, refer to the 27 column.

Notice also that there are several headings above the heating rate numbers. The columns are broken down to the type of cone you are using, either Self-supporting, Large or small cones. The pink columns are for regular cone compositions and the white columns are for our special iron-free cones. Depending on the type of cone and the heating rate, the final temperature will be slightly different. For example, the temperature for a self-supporting cone 06 fired fast reads 1855 °F and the corresponding Large cone 06 reads 1852 °F.

Why are the cones numbered from 022 to 42?

When cones were invented over 100+ years ago, ceramics were fired at temperatures starting around 2000 degrees. No one envisioned firing ceramics at lower temperatures, so the first cone number they made was given a 1 and they went up from there to 42. As the industry evolved, we figured out how to fire ceramics at lower temperatures. The cones were formulated for these lower temperatures but a numbering system needed to be created. Edward Orton Jr. did not want to use a minus sign for numbering the new series of cones so instead he used a zero. The new cones were designated 01 to 022, 01 being the hottest cone in the new system and 022 the lowest.

Is the cone affected by a change in the kiln atmosphere?

Yes, if a cone is exposed to a reducing atmosphere, it will look considerably different then one fired in an oxidizing atmosphere. This is a valuable tool for monitoring the air quality in your kiln. Cones are more sensitive to atmosphere changes than most fired ware, therefore, they can serve as early warning signs that something has changed in the kiln and needs attention. For example, if you are used to seeing a cone that fires white and then suddenly looks gray, it is a clear indication that the atmosphere in the kiln had changed. Cones can be used in reduction environments to measure temperature uniformity and reproducibility. However, the ability to convert the bending angle to a temperature will change some since the charts are based on oxidizing atmospheres.

Is there any difference between the small cone and the Bar cone? Is one better than the other?

No, both the small cone and the bar are made from the same standardized powder and are tested to insure that they perform the same when placed into a Kiln's shut off device. For some users, the small cone is prefered since the final shut off temperature can be changed by shifting the cone slightly so the cross sectional thickness is greater or smaller. For those who do not want to be concerned with allignment of the cone, the bar is the product of choice.

What temperature did my cone fire to?

To find out the equivalent temperature, you must know the heating rate of your firing as well as the bending angle of the cone. The bending angle can be determined by using the Orton measuring template. Then, based on the heating rate and bending angle, you can look up the equivalent temperature on the Orton Cone Chart ( °F Wall Chart and °C Wall Chart ).

I have never used cones before, what cone should I use?

Most clay or glaze manufacturers have in the label of their product what cone value to fire their product. They have already determined, through testing, what cone will deform and bend over when the glaze or clay is at maturity. An example could be a stoneware glaze rated to mature at cone 6. For this product we would recommend either the large or self-supporting number 6 cone.

What is Heatwork?

Heatwork is a term used to describe the combined effect of time and temperature on the ware you are firing. Firing ceramics is very similar to cooking a cake. In many recipes, we must choose a temperature setting and also a time. For example, a cake recipe will say cook at 375 for 45 minutes for a sheet cake. However, if you were making cupcakes, the time is reduced to 30 minutes. It will take less time to cook the cupcakes since the heat is absorbed faster into smaller items. In ceramics, you must also allow the ware to absorb heat for a period of time. You cannot fire porcelain by placing it into a kiln set to reach 2232 F in one hour. It would be grossly underfired. An additional 19 hours would be required to mature the porcelain.

I have cones that are really old, can I still use them?

Yes, cones do not have a shelf life. If the cones are in good condition, were stored in a dry area, and do not break apart easily, they can still be used. Orton engineers regularly test fire cones that were made over 50 years ago without seeing any change in the fired results.

How do you measure a cone after it has been fired?

There are two methods used to "measure a cone. The first method requires the use of the Orton measuring template. The Orton measuring template is a plastic card used to measure the bending angle of large or self-supporting cones. The card will read bending angles of between 10 and 90 degrees. The fired cone is placed next to the card and the location of the tip of the cone indicates a bending angle. A cone bent to 90 degrees is considered to be properly fired.

The second method used to measure cones is the "clock method. This method requires that the indiviual translate the position of the tip of the cone to a clock face. The one o'clock position is for cones that have not bent at all, the six o'clock position is for a cone whose tip is touching the shelf. A cone in either the 5 or 6 o'clock position is considered properly fired.


Cone Angle Reported

What does "salting do to the accuracy of cones?

Visually, you'll notice the salt solution will make the cones shiny and they will look glazed. They are still able to bend as expected, the salt does not normally make them do strange things.

However, there is one concern to be aware of. Depending on the way the salt is added, it can cause the temperature of the kiln to suddenly decrease. (It is like throwing water onto a hot fire.) This sudden decrease in temperature can create a hard shell casing on the cone. This casing can freeze the cone in place and delay the bending action of the cone. This "hard shelling will become more prominent if the temperature decrease is followed by a rise in temperature that is not uniform. If the kiln cycles on and off and fluctuates the temperature instead of gradually rising, it will create a thicker shell on the cone.


Salted Cone

So at the end of the day, as long as the salt is added all at once, the temperature decrease is not too drastic, and the temperature recovers gradually, then the cones will work just fine.

Also, the iron bearing cones (cone 010 through cone 3) are more susceptible to hard shelling because of the presence of iron oxide.


Hot Disk Instruments added to Orton's Instrument Line

Orton is pleased to announce its partnership with Hot Disk of Gothenburg, Sweden representing Hot...

Jessica Putnam-Phillips on ClayShare

Jessica explains how Pyrometric cones help the firing process.  Click here to Watch her on ClayShare...

Sue McLeod and Witness Cones

We ran across this recent post from Sue McLeod that explains why she uses Witness Cones in her firings. ...