Thursday, December 20, 2012

What cones are and what they do

            In essence, cones are a way of labeling a certain amount of "melted-ness" of ceramic materials. Pyrometric cones are specially formulated from glassy materials to melt at a certain point based on time and temperature - thus they melt similarly to the way that other ceramic materials do in the kiln. 

            The cones themselves are constructed like little pyramids, and they bend over when they are "done." Thus we can use them to measure whether a kiln has fired appropriately for the clay and glazes we are using. The thermocouples in the kiln do measure temperature, but since both time and temperature combine to create what is called "heat work," the temperature doesn't necessarily tell you all you need to know about the firing. 
            These are cones that have not been fired yet. From left to right, they are cone 5, cone 6, and cone 7. These are the cones I use in a glaze firing. 

            Cones are numbered from cone 022 (coolest) to cone 15 (hottest). (Actually they do go hotter, but no studio potter would use the upper limits beyond cone 13 or so.) Cone 6 and Cone 10 (most often used) are around 2232 F and 2350 F respectively. So anytime I say something is fired to cone 6, I'm saying I fired it to around 2232 F, but with pyrometric cones to measure the heat work. Saying "cone 6" is more accurate than the temperature for the purpose of knowing how the clay and glazes will react. 
            Here is an example. In the picture below, you see two cone packs, both with cones 5, 6, and 7 (from left to right). The one on the left was in the first test firing of my new kiln, and the one on the right was in the kiln during the latest firing. 

            The cone pack on the left shows a kiln that has fired to cone 7, and the one on the right shows a kiln that has fired to about cone 5 1/2. You can tell because the cone 7 on the left cone pack has curled over to about halfway, and the cone 6 on the right cone pack is nearly, but not quite, to halfway while the cone 5 in that cone pack is very floppy. 
            Now, what was the difference in temperature here? Cone 6 is "supposed" to be 2232 degrees Fahrenheit. The kiln fired to 2236 F in the firing represented on the left, yet it shows as cone 7.  This is because of the heat work I mentioned above - the firing schedule must have been slow enough near the peak temperature to produce more heat work, and thus the cone 7 began to bend despite the actual temperature. According to the kiln, this was a perfect firing. According to the cones, the kiln overfired. The cones are right. If the test tiles from this post had been in that firing, the glazes would have been even runnier.
            So what did I change for the next firing? I fired to a peak temperature of 2180 F rather than 2236 F. 2180 F is within the range of cone 5. I also programmed the kiln with a slower rate of climb near the end of the firing, and a fifteen minute hold at peak temperature. These adjustments resulted in the cones that you see on the right. As far as I'm concerned, this was a perfect firing. The cones are right where I want them, because the glazes and clays I use are formulated for cones 5 - 6.
            Pyrometers are super devices, and they can be helpful during firings. But cones are the best way to judge the heatwork during and after a firing. You can fire a kiln without a pyrometer, but it is very hard to do without cones. I have the luxury of a programmable kiln, so I can theoretically just tell it what to do and leave it alone. But the cones offer me an extra diagnostic tool, not just to know when something goes wrong, but also to know when things are going well, and to give me the option to observe the end of the firing and shut off the kiln at just the right time if I so choose. 


  1. Does increasing the ceramic you fire at one time in the kiln impact the heat work which gets done at a particular temperature?

  2. Yes, probably. Kilns tend to fire more evenly when they are evenly packed with pots than if they are unevenly packed. This is one reason we try to fire kilns with as much work as we can fit in them at a time, rather than firing them partially empty.