Ward Burner Systems

Customized Combustion Equipment

Heat Work: What Cones Really Measure



by Marc Ward
Clay Times March 2001


    Every once in a while, I get a call from a customer who has bought a pyrometer....our conversation starts something like this,

    " I just bought a pyrometer from you guys and it's off".

    "Are you sure it's off?....How fast do you fire?"

    "Yea, it's off...We fire to cone 10 in about 18 hours and it reads about 50 or more degrees low."

    Then I say, "It sounds like it's right on".

    After I say this, there is usually some silence on the other end of the line. The misconception occurs because many potters think that cones measure temperature. They don't. Cones measure a thing called heat work. Pyrometers measure temperature. Heat work is a measure of time and temperature. If you look at your cone charts, you'll notice that the corresponding temperature for various cones is different depending on the rate of heat increase. The slower the heat rise, the lower the actual temperature will be when that cone falls. The faster the heat rise, the higher the actual temperature. This may sound like a sloppy way to measure things, but it is actually very precise. The beauty of this is that clay and glazes react just as the cone does. They behave the same to specific time and temperature. I've got a friend who manages production at a nearby aerospace firm that makes turbine blade molds. Talk about high tech! The bells and whistles have computerized bells and whistles. These kilns are monitored and controlled by computers on very tight schedules and cost BIG BUCKS!!. Look down the tunnel kiln line past all the sensors and controls and you know what you see in there?.... Orton cones.

    So, why use a pyrometer? Well once you set up your firing schedule and know that in X hours, when cone Y falls, you have temperature Z. If you stay to that schedule on subsequent firings, then your pyrometer will give you an accurate idea of where you are in relation to your cones. So, some of you are thinking, "Big deal, I've got cones... what good is a pyrometer?" A pyrometer (digital variety), can also give you a real good idea of how your burners are working.

    A digital pyrometer (the kind with a battery-driven numerical display) responds instantly to changes in temperature. You can change you burner settings and know instantly if the kiln is going up slower, faster or not at all. We've all had the experience of repeatedly checking cones and deciding after too many hours that the kiln is stalled. With a digital pyrometer you would know instantly.

    An analog pyrometer (the kind that doesn't use any power and has a needle and a dial) can also help, it's just not as responsive. You can stare at these for quite some time before you can discern any movement. The digital variety can actually save you money by informing you of your burners efficiency. If you turn up your burners, and the temperature remains the same (you'll know immediately), then you have too much air/not enough gas and are just wasting gas. You'll be able to judge your most efficient use of fuel by your rate of temperature climb. This can be particularly helpful when firing a wood kiln. You stoke, the temperature drops, then starts rising, when it stops rising, you stoke again..... I know, I know, the purists out there are rolling their eyes. But, I believe, you should embrace those things that can help you be a better potter, not to mention spending more time with your family. I've got another buddy who did the humble Japanese apprentice thing for several years. He has a digital pyrometer on his wood kiln and tells a story from his days in Japan as an apprentice. One of his jobs, of course, was to help fire a very old family kiln. Very honored position. The masters used a digital pyrometer on this very old kiln...that is unless tourists or customers were snooping around, then it disappeared. One must keep up appearances!