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Firing Log

ancient kiln | 21st century logbook

December 28, 2006

Podcast: Eighth Firing

Filed under: anagama,anagama, 8th,Firing,Kiln,sound & video — odin @ 5:23 am

Download the Eighth Firing Podcast directly or through iTunes (mp3, 56:45, 42mb).

Note: The iTunes link requires iTunes to be installed on your computer. If you do not have iTunes, use the “direct “link — it is probably most convenient to right click the link, choose “save as” from the context menu, and then listen to the file in your preferred player after it has downloaded.

I wrapped up my eighth anagama firing in the early hours of Dec. 23rd. The kiln is still sealed and I am (im)patiently waiting for Saturday to open it up. During the firing, I made some recordings of what I was doing or thinking. The purpose was twofold: 1) In the future, I will be able to listen to all my wrong thoughts — perhaps with a sense of nostalgia. 2) I knew that post-firing, I would need some kind of project to help me keep my grubby mitts off the kiln door. So far, the editing task has worked quite well, although I also built a light tent in the meantime for taking better pictures of pottery.

Some basic data points: I lit the gas burner around 9:00 pm on Dec. 17, started with the wood about 24 hours later, stopped firing about 10:30 pm on Dec. 22, and had the kiln sealed up by 1:50 am on Dec. 23. The kiln consumed about 3.5 cords of wood during the firing. From 5:00 am, Dec. 18 through 2:00 am, Dec 23, I slept 18 out of 117 hours.

I took only a few pictures during the firing, but they can be viewed in the photo album for the eighth firing. As for the pieces — I hope to open the kiln on Saturday. Till then, I’ll spend the time worrying and second guessing.

1 Comment »

  1. I think this was the podcast where you ranted about type K thermocouples. Anyway, here are some comments.

    First off, keep in mind that many authorities, such as Omega, state that standard composition type K thermocouples are not meant to measure temperatures higher than 1250 C.

    Omega thermocouple info page: http://www.omega.com/thermocouples.html

    Potters are getting readings using type K at temperatures higher than this but the thermocouples wear out faster than most industrial users would tolerate, and the accuracy is not great. Using thick wire for the thermocouple and putting it into a protection tube are two methods for extending the life and range of the type K sensor.

    L&L kilns have a web page describing their research into the life of various thermocouples. Basically, they soak an electric kiln with 23 different thermocouples in it at Cone 10 for an hour and call that a firing. They say: “One of our standard Type K 8 gauge thermocouples in the industrial protection tube achieved 139 firings”.

    Thats 139 hours at cone 10. Without the protection tube they only got 100 firings, or 100 hours.

    L&L page in question: http://www.hotkilns.com/shielded-thermocouple.html

    Now consider your typical anagama firing where temperatures above what Omega states is the limit for type K last for days. 139 hours before losing accuracy, which L&L thought was good, suddenly doesn’t seem like such a long time. Things get worse if you aren’t using a protection tube to keep the ash and kiln gases away. You can’t expect a type K to last for very many anagama firings.

    However, after listening to you rave about your perception that there is something fishy about type K pyrometers on your podcast, I’d say that it is actually another factor that is troubling you.

    A thermocouple can only indicate a temperature difference, rather than any absolute temperature. A pyrometer may contain enough circuitry to change that measurement of temperature difference into a meaningful reading of what is happening at the hot end of the probe, but it may not. A millivolt meter such as what you are using doesn’t have any of this circuitry.

    The tip of the thermocouple that is furthest into the kiln is called the “hot junction”, and the other end of those wires, in your case probably where they attach to your millivoltmeter, is called the “cold junction”. The temperature difference between these two spots is what your meter is measuring.

    Most potters assume that if the reading on their pyrometer changes, it can only mean the temperature inside the kiln has changed, but this is not always the case. A change in the reading can also mean that the temperature OUTSIDE the kiln has changed. The exact spot that matters is where the thermocouple wires attach to the meter.

    Potters who don’t understand that the cold junction temperature can matter may find themselves mystified by what is going on with their pyrometer readings. Some potters have even gone as far as to rave about pyrometer limitations on their podcasts, revealing their until then secret doubts about their own judgement, or, possibly, even their own sanity.

    More modern pyrometers, especially digital types, are likely to have circuitry inside them which measures the cold junction temperature, automatically compensates, then shows you what the reading is, but a lot of old style pyrometers do not. Followers of ancient ritual, such as millivoltmeter wielding types like yourself, need to understand what is going on.

    Consider this: If you take a pyrometer reading at dawn, then take another one a few hours later as the sun warms the “cold junction” temperature outside the kiln by 10 C or so, your interpretation of those two readings will indicate that your kiln has declined in temperature by 10 C when it hasn’t. Or in your case you may detect a 0.2 millivolt decline and think that means your kiln temperature is falling. What has happened is that the difference between the temperature of the outside of the kiln and the inside has changed by that amount. If you take a reading in the afternoon, then another one after nightfall when the outside kiln temperature where your pyrometer is may have fallen by 10 C, you may end up thinking that your internal kiln temperature has risen by 10 C when it hasn’t. Again, unless your pyrometer has compensating circuitry, you may think it is telling you the kiln has increased in temperature when in fact it is only that the outside of the kiln has decreased in temperature.

    No wonder one of your interview subjects stated he now fires without a pyrometer.

    What does all this mean, really? It depends what you are relying on the pyrometer reading for. If you are some future potter running an anagama using robot stokers controlled by a computer relying on a pyrometer, that computer will have to know about and compensate for every error so that it will know for certain what is going on. In the meantime, just learn what your pyrometer reading means, and you’ll be able to use this useful tool and get more out of it.

    Type K can be economical to use and give good results if you know what you are doing. Use a mullite protection tube. Renew the thermocouple wire periodically, when you think it has been exposed to temperatures in excess of 1260 C for more than 150 hours or so. Buy 8 guage type K wires on eBay and make your own replacement probes. Spring for a cold junction compensated digital pyrometer to use as your measuring instrument. I got a Fuji PXR3 8 stage ramp controller that by the way is an excellent digital cold junction compensated pyrometer for all common thermocouple types for $40 on eBay. These things are installed in a lot of espresso machines all over the place and consequently they appear on the used market. Get to know and trust your perception of the kiln color at the high temperature range and rely on that instead of the millivolt reading. Don’t be so ready to question your gut feelings at peak temperature, now that you understand more about the sources of error type K has.
    Or move to platinum/platinum rhodium (type R or S).

    Thanks again for taking the time to put these podcasts up. May you find the energy to do many more.

    Comment by David Lewis — March 7, 2008 @ 1:39 pm

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