Weight loss (of clay during bisque-firing): Part 1

By Hideo Mabuchi / Stanford University

Today I learned how to use a new analytic instrument that we have in the Nano Shared Facilities at Stanford:  something called a Thermal Gravimetric Analyzer (TGA).  This is basically a super-sensitive scale that can hold a sample of material inside a small electric oven, and that records changes in the weight of the sample as you ramp up the temperature.  As a quick test, I and my trainer set up a measurement with a tiny sample of stoneware clay and recorded the weight changes from room temperature up to 1000C.  That corresponds to about 1832F, which is a standard sort of bisque temperature, although we were increasing the temperature at quite a high rate, so I’m not sure it makes sense to associate this with any particular cone.

The clay sample was a pinch (about 15 mg) of bone dry dark stoneware that I ground roughly into a coarse powder.  The oven on the TGA is basically a soda can-sized electric kiln, and the sample sits inside of it in a little platinum pan (these pans are quite expensive you can imagine, but you have to use a special metal such as platinum in order for the pan not to melt at 1000C).  I had actually meant to set up the TGA to ramp at what I thought might be a reasonable bisque schedule for powdered bone-dry clay, going from room temperature to 1000C in six hours.  But we made a mistake and actually set it running at a ramp rate of 30C/minute, so the oven hit 1000C after just half-an-hour!

Here is a plot of the weight of the sample versus temperature (in red), as well as the rate of weight loss versus temperature (in blue):



We see that there is an initial period of weight loss at low temperatures (below 100C), then a major episode in the vicinity of 500C, and some additional loss of weight from about 600C-800C.  At this point it would be sensible to question whether the temperature ramp was too fast to allow all of the “boiling off” of chemical species from the clay to occur completely at any given temperature.  After this quick half-hour experiment finished, I did re-run the same sample to ramp from room temperature back up to 1000C more slowly, over the course of about four hours.  The weight of the sample changed negligibly during the entire second run, suggesting that in fact all the weight loss that was going to happen up to 1000C in my sample happened within that first half-hour experiment.  Perhaps it’s okay to increase the temperature that quickly with a small amount of powdered bone-dry clay.  However, it’s possible that if we ran a fresh sample of green (bone dry) clay from room temperature up to 1000C over four or more hours, we would get plots of weight and weight loss versus temperature that look somewhat different than the ones above—since if we ramp too quickly, chemical processes that could happen at some given temperature, given enough time, won’t actually get a chance to happen until the temperature is already raised much higher.  I will give the slower ramp with green clay a try next time I use the TGA, hopefully also under both neutral and reducing atmospheric conditions (see below).

It seems reasonable to assume that the weight loss below 100C should simply be the boiling off of “water of plasticity” and “pore water”; according to John Britt (The Complete Guide to High-Fire Glazes) one expects that organic carbon and “chemically combined” water should burn out from 300C-700C; and any remaining carbon (and sulfur) should burn out from 700C-900C provided there is sufficient oxygen present in the oven atmosphere.  Steve Davis has written a great little article about the latter issue and its importance for preventing bloating in high-iron stoneware.

Unfortunately at this point, I am not entirely sure what the oxidation conditions of this quick experiment were.  My trainer initially said that he thought the TGA worked with ambient atmospheric conditions (that is, with room air just like an electric kiln), but when we looked more closely at the configuration of the TGA instrument after the measurements were done, it actually looked to me like the TGA ran the experiments in a dry nitrogen atmosphere.  That would suggest that the oven atmosphere was oxygen-free and that perhaps the burnout of carbon was not complete, meaning we would have had more weight loss in an oxygen-containing atmosphere, or alternatively that the burnout of carbon proceeded at the expense of heavy reduction of iron oxide in the clay body (see the previous references to Britt and Davis), meaning that we might have had less weight loss in an oxygen-containing atmosphere.  Anyway, we think it should be possible next time to set up the TGA to run both with dry nitrogen and normal air in the oven, and then to compare the results.

Hopefully soon we will also be able to connect the TGA to an instrument called an FT-IR spectrometer, which will allow us to do some characterization of what exactly is boiling off from the sample.  According to the info cited from Britt above, we should expect that below 300C we are mainly just boiling off water while above 700C we should see mainly carbon monoxide and carbon dioxide.  The TGA and FT-IR spectrometer models that we have are designed to make these sorts of measurements, but no one has yet tried to do this in our facility.  So the people running the facility are happy to give it a try, but we need to consult first with some engineers from the equipment manufacturers to make sure we make all the connections correctly and get the set-up right.

Stay tuned…


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4 Responses to Weight loss (of clay during bisque-firing): Part 1

  1. James Meade says:

    Interesting experiment. How might I conduct a similar test with very basic technology? I am thinking it would be good to link up with one of the physics teachers I work with to do some cross curricular projects with our classes here at the high school.

    James Meade

    • Steve Davis says:

      I would take the articles to your fellow teachers and see what they can do. I’sure they wold be excited by the collaboration.

      • Hideo says:

        Hi James, Steve-

        I’ve been thinking about how to reproduce this sort of data inexpensively and here’s a simple idea. Make a bunch of small draw-rings like you would use in a soda firing, as similar to one another as can reasonably be managed, and make sure to scribe a number in the bottom of each so that you can keep track of them. Also try to dry them to a uniform degree; maybe it would be best to let them all go super completely bone dry. Weigh all of the rings as accurately as possible and make a list of the ring #’s and their weights. Put the rings in a kiln set to slow bisque and place them either near a peep-hole (if you have large peep holes and small rings) or at the easiest place to pull them out of the top. During the bisque firing, pull out rings at regular temperature intervals (for example every 20 or 50 degrees) and let them cool. Be sure to make a list of which ring came out at what temperature. After all the rings are cool, re-weigh them and divide the new weight of each by its starting weight. Make a plot of the results versus the temperatures they were drawn at, and hopefully it will resemble the one I got with the fancy TGA!


  2. James Meade says:

    Sounds like a plan. I could load them at the top of my electric kiln and pull them out that way during the bisque sort of Raku style.

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