Measuring Heatwork & Temperature

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I have been trying to be scientific about arranging my firing schedule in a small electric kiln with a controller. I understand the concept of heatwork and use cones on all shelves in my kiln to monitor results. I soak at top temperature and fire down so want to compensate for the additional heatwork these procedures involve when I set the top temperature on my controller. I understood that this could be done by looking at the area under a graph of total heatwork, but having compared theoretical models using the Orton cone firing tables, if this is indeed the case, I must be missing something.

The following example uses °C, as I am in the UK, and I am referring to the Orton theoretical temperature values.

  1. In the Orton table Cone 7 is reached at 1237°C when firing at 60°C /hr, if this is a straight ramp, this would take 20.3hrs, assuming a starting room temp of 17°C ((1237-17)/60). If the x-axis is time and the y-axis temperature, the area under the graph is a triangle and can be calculated as (1237 x 20.3)/2 = 12,555

  2. When firing at 150°C/hr cone 7 is reached at 1255°C. As a straight ramp this would take 8.25hrs ((1255-17)/150). Then the area under the graph is also a triangle calculated as (1255x8.25)/2 = 5179

SO, these areas which are supposed to represent total heatwork to achieve cone 7 are not the same.

What am I missing here?

I’m not familiar with that approach, and I’ve never heard that heatwork can be calculated as the area of a firing profile. It doesn’t seem correct, though. For example, say I fire a kiln 150°C/hr to 1000°C, then I hold the kiln for 10 hrs, then finish the firing 150°C/hr to 1255°C. The Orton cone charts only compare the rate of heating & temperature for the final hour of firing… I think that’s all you can really compare… Here is a visualization:

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Thanks Derek. I found an article on Ceramic Arts Network with a graph showing total heatwork as the area under the graph, and that made sense as an accumulated time/temperature concept. There must be a scientific way of calculating total heatwork on a material. Perhaps it is not applicable here though.

The term heatwork is not a proper scientific concept.

Above is an article that explains in easy terms the concept of specific heat capacity (a scientific concept) and its interesting relationship to temperature.

I have evolved an experiential approach to increasing or decreasing cone results by manipulating holding time at peak temperature, or, alternatively, adjusting peak temperature.

I make numerous test runs using a ramp program on my kiln increasing the hold time by 5 minutes at peak temperature each run (or whatever equal increments of time you want to use). I put three Orton cones in the empty kiln (e.g. 5, 6, 7) and measure the melt angle as precisely as possible at the end of each run. Make certain to note the starting temperature and open the kiln after the same amount of time has passed. You should start at the same start temp each time, but that is hard to control. Then I chart melt angle as a function of hold time for a given peak temperature. (this chart is specific to an empty kiln, so use it as a working tool, not a bible.) You will quickly learn how to use your chart to your advantage.

Then I load my kiln and run the program that the chart indicates will provide the desired cone. At the end of the run, if my Orton cone is melted to the desired angle, I assume that I have reached the desired temperature. My test procedure is not quite as simple as this, but this gives you the idea. Expect variations based on the materials you have in the kiln and how densely you have packed the kiln, etc. I had to go through this exercise last year because my kiln was not in sync with my Orton cones. I am now getting the results I want.

But to muddy the waters, There is a long running dispute about this between some kiln manufacturers and Orton. Check with your kiln manufacturer to see what they think about adding or reducing heat energy to achieve the cone you desire and then run tests and see for yourself how your kiln is responding.

As far as heat capacity is concerned, you can either alter the peak temperature or adjust holding time. However, your glazes may respond better to one method over the other. Have patience and be consistent. Best of luck to you.

There may be other simpler ways to learn the behavior of your kiln and how to calibrate it to your work style and needs. If anyone has information to share, it will be appreciated.

P.S. even if you calculate the area under a curve to arrive at some values, you will have to learn how to apply those values to your kiln and work style to get the results you want.

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Thanks very much for sharing your method, Marsha. I now have a new kiln and it seems exactly the right time to do the research you suggest here. I’ve learned a lot about this over the past few months and understand the concepts much better than before so am looking forward to some rigorous testing.