Pyrometric cones provide valuable information in any firing, but they are critical in atmospheric kilns. Properly packed and positioned, they display in real time what’s going on in the kiln.
Pyrometric cones are great because they are made of clay and other ceramic materials and tell you how the clay is reacting to the combination of time and temperature in the kiln. For example, if I am stalling a kiln at cone 7 for a longer amount of time, let’s say 5 to 8 hours, during those eight hours, without increasing the temperature beyond what is listed as the range for cone 7, it is likely that cone 8, and even cone 9 could begin to get soft. If you are firing for a longer amount of time but at a lower temperature, due to heat work, the clay can still react like you reached that higher temperature.
Cones also help with knowing if you are reaching a similar temperature throughout the kiln. You can place a single target-temperature cone in several different spots throughout the kiln. Some cones can be seen during the firing, while others you get to see during unloading. By looking at the target-temperature cones placed throughout the kiln, you can see how the temperature stays the same or varies by location. This can be especially helpful with smaller electric and gas kilns where you are troubleshooting glaze problems, or with raku kilns when you want to pull a piece at a very specific temperature.
Most pyrometric cones are made by The Orton Ceramic Foundation and range from cone 022 to cone 14—cone 022 being the coolest or lowest temperature and cone 14 being the hottest or highest temperature. Each cone is designed to melt at a specific temperature. Depending on the type of firing you are doing and the temperature you are planning to reach, the cones you use in your cone pack will vary. Cone packs can be anything from a single cone to as many as you see fit to use to gather the information you need (1). The cones you use will likely change gradually as you learn which ones provide valuable information and then become standardized as you get comfortable firing. The important thing is that you place them in the kiln where you can easily see them through a peep hole (2) and that you put them in a clay-made carrier (also called a boat), so they don’t melt all over the shelf.
Making a Cone Pack
First, start by making the boats. Roll out a thin slab of clay. I like to make slabs as thin as I can while still being able to handle them without worry of breakage. After the slab is rolled out, cut off a piece that is a few inches (8–10 cm) bigger than a half brick. Take that slab piece and wrap and fold it around the brick, using the half brick like a hump mold (3). Then, cut the side wall of the boat about ½–¼ inch (1.3–0.6 cm) tall while it is still being supported on the hard brick (4).
After you make this cut on all four sides, gently remove the boat (5), then place it on a flat table to poke it with holes with a fettling knife or needle tool. Once the holes are poked, set it aside. I cut the walls at this height because it is low enough to still easily see the cones, but high enough to catch any of the melting cones. Poking small needle-tool holes in it helps it dry fully, making it less likely to explode in the kiln (6). This is important because even if you generally bisque fire your work for the kiln, cone packs are most often unfired when loaded. Having a cone pack explode in the kiln is the worst. It can literally ruin any work near it. Along with poking holes in the boat, I also highly recommend making cone packs several days in advance to ensure they are dry.
Now you are ready to set up the cones you are using. First, separate and organize the cones in order of coolest to hottest in one or two rows that are close to equal in number (7). For example, the cone packs I use in the wood kiln consist of two rows. The first row’s cones are 08, 1, 3, 5; the second row is cones 7, 8, 9, 10, 11. I don’t recommend making a row longer than five cones.
Start with more cones and as you get better at reading the kiln’s other cues, reduce the number of cones in your cone pack. To start, I suggest having a cone to indicate body reduction (cone 08), a side stoke cone (cone 1), an early cone to see where it is getting hot first to keep the kiln even (cones 3 and 5), a surface development cone (cone 7), cones to keep track of temperature control (cones 8, 9, and 10), and a safety cone (cone 11). I recommend including a safety cone that is one cone hotter than your desired peak temperature.
Then, set the cones you are using in a thin coil. The coil should be big enough so that the cones’ bases are just smaller than the coil. A good way to check that you are setting the cone correctly in the coil is to first hold the cone on a table, with the bottom of the cone flush with the table. You will notice a slight 7% slant in the cone; make sure you organize the cones so they are all slanting in the same direction and that the lowest number cone is last in the row. When you place the cone in the coil, gently pinch the coil around the base of the cone to secure it in the coil and then move on to the next cone. After you place your last cone for that row, use your fettling knife to cut the coil just behind the cone (8). After you have finished your one or two rows of cones, place them in the cone boat and let the whole unit dry together. Make sure there is enough room behind the rows in the boat for the cones to melt and that the two rows go in opposite directions (9). This makes it easier to differentiate the cone rows in the kiln, where visibility can be low. If you are using only one row of cones, I recommend making the cone boat narrower than the width of a half brick.
Other Cone Packs
There are other ways to make a cone pack as well. The thing to remember is that you want to be able to clearly see the cones and not have them melt on one another or the shelf. Other materials I have used, or seen used, as cone boats include fiber board and 1-inch (2.5-cm) ceramic fiber insulation leftover from a kiln build. In both cases, you can press the cones in the insulation or carve out a divot for the cones to use these materials as a boat. Then, cut it in a shape that leaves an appropriate amount to catch the melting cone. If you don’t have too much cone melt, you can reuse this boat several times. If you are making a cone pack for the raku kiln, this is what I would advise doing because it is much easier than making a boat in clay and bisque firing it. (A greenware cone boat is too dangerous to use in a raku kiln.)
Note: There are self-supporting cones. If you just need one cone as a sight cone, you could use the self-supporting kind, but I always advise using a boat because the cone could fall over. You just never know.
When placing your cone packs in the kiln, it is important to place them a few inches (7–10 cm) away from the port for a better reading. If they are too close to the port, the regular introduction of cold air when you read your cone packs will affect their ability to indicate the correct temperature. The number of cone packs in your kiln is also up to you. I suggest having at least one in each zone: front, back, middle, top, and bottom. Of these, it is also a good idea to ensure that there are a few on the left side of the kiln and a few on the right side of the kiln. This will help you know if your kiln is firing evenly top to bottom, front to back, and side to side.
Excerpted from Mastering Kilns and Firing by Lindsay Oesterritter, published by Quarto/Voyageur Press. To learn more, visit quartoknows.com/r/mastering-kilns, www.amazon.com, and www.barnesandnoble.com.