Janet H Walker on wed 13 jan 99
Judy Musicant asked the other day how on earth to control the
cooling of an electric kiln when one does not have the luxury of a
controller or pyrometer. I thought some people might be interested
in some thoughts that John Post and I had on the subject. John and
I are getting iron reds that are red enough for us by slowing down
the cooling of our kilns.
Cheers,
Jan Walker
------- Forwarded Message
Date: Mon, 11 Jan 1999 15:41:03 +0000
To: Janet H Walker
From: John Post
Subject: Re: controlled cooling
[Jan to Judy]
>Judy -- When John Post and I do our controlled cooling with the
>controller, we have the luxury of being able to set it so that the
>temperature drops at a controlled rate. I use about 100 degrees F
>per hour but I don't remember whether John does it slower or faster
>than that. in any case, this is quite a lot slower than the kiln
>would be cooling by itself in the range where we do the controlling.
>The kiln temp drops fastest when the kiln is hottest. My Skutt
>(1027) drops about 400F in the first hour after I turn it off!
>Anyhow, I work in centigrade temps myself because I like small round
>numbers so I cool the kiln at 50C/hr between 900 and 700C. This
>means that I am still applying heat to the kiln for more than four
>hours after it reaches temperature. Actually, it works out to six
>hours of extra heat because I also control the drop from ^6 cone
>temps down to the really slow cooling range.
>
>So, you say, what to do if no pyrometer? I don't know really what
>to tell you. One thought -- you are supposed to be able to guess
>the ballpark temperature of the kiln by the color of the glow inside
>the peepholes. There are charts for this in the appendix to most
>texts. So with a few firings for practice, you can probably try to
>guess a color in the range of 800C and then use enough power to hold
>something like that color for a few hours. I think this would give
>you an approximation to what we are doing. Or, you could do what
>John did and buy a controller...
>
>Whaddya think John?
>
>Jan
>
Hi Jan,
I like it. I would add that a good way to determine the color/temperature
of your kiln by eye for this would be to look inside your kiln when the
kiln sitter has just dropped for a bisque firing. If you look at this
color, a nice orangy color, you could probably find a way to have your kiln
soak at this temp very easily. On my kiln without a controller I let the
sitter fall and then I turn the switches to medium for 4 hours. This keeps
a nice orange glow in the kiln, but doesn't cause the temp to rise.
Instead it is actually falling slowly. After 4 hours I drop the switches
down to low for 2 hours and then shut it off.
I would look at my fired results to make changes. If the pots are still
coming out brown and not red, I might set the switches between medium and
high for those first four hours of cooling. Or I might increase the amount
of time in that range up to 5 or 6 hours. And I for sure would test
varying amounts of iron in my base glaze to determine which amount works
best based on the rate of controlled cooling that can be achieved in the
kiln.
I'll say this, the money I spent on my controller was well worth it. I
wouldn't buy another electric kiln without one.
Jan feel free to forward this to the list.
John Post
Sterling Heights, Michigan USA
rp1mrvl@moa.net
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