Diane Echlin on wed 7 mar 01
> Cindy wrote:
> If you're re-firing previously matured work or firing raw glazed pieces,
> you'll want to start up slowly as you would in a bisque firing.
Can you elaborate on this a little? I'm contemplating reglazing some mugs that
didn't come out as well as I would like, with more of the same glaze over the
original glaze. Why go slower?
Thanks!
Diane in CT
Cindy Strnad on wed 7 mar 01
Hi, Sara.
There's really no need to be self-conscious on clayart, and I hope you won't
be. The discussion going on now is really pretty much academic. There are a
very few people who ask too much, and I'm convinced they don't stay around
long anyhow.
It's sometimes difficult to find things in the archives, because subject
headers don't always describe the content of the message. I'm going to send
this answer to Clayart as well as to you, in the hope that I've given it a
good header and it will be easy to find next time some intrepid explorer
goes looking for it.
As for ramps, you're right. There's precious little information available.
Most of what I've learned has been trial and error, and yes, it does make a
difference what clays and glazes you're using, and even the elevation at
which you have your kiln.
I can only tell you what I do. This may or may not be the best way, but
it'll give you something to get started on at least. It's all in Fahrenheit.
Bisque
100/hr to 180. Hold for 1-10 hours.
(I do this because it allows free water to escape before it reaches the
point where it could explode into steam. As I'm at a mile high altitude, the
boiling point is lower than it would be for most people. The ten hours is
extreme, and I only use it for large sculptural pots that fill the entire
kiln and took me days to carve. Usually it's more like 2-4 hours.)
150/hr to 500, no hold
(This allows at least most of the chemically bound water to escape gently.)
300/hr to 1830.
(Once bisque has passed the delicate stage of water release, you can move it
along fairly quickly. I bisque to ^06, but people have different
preferences. If you find your ware dries slowly after glazing, you may want
to try a cooler bisque.)
Cool naturally.
(I don't hold my bisque at temperature because I haven't had much trouble
with pinholing. As you do, you may want to try a 30-60 minute hold before
cooling.)
Glaze
For a kiln in which there are no previously glazed and fired pieces, and no
raw glazed pieces:
300/hr to 2100
50/hr to 2200, hold 45 minutes. ^6 has barely begun to bend.
Cool at 100/hr to 1000 degrees, then cool naturally.
I usually fire to just under ^6. If you want a full ^6, you can hold longer,
until ^6 is properly bent.
This one could use some experimenting--especially the cooling cycle. It
works great, but is probably over-conservative. I think you could cool
faster at least to 1800, then slow down until you reach, maybe 1400, then
cool naturally.
If you're not firing any crystalline mattes or iron reds, you can dispense
with the whole cooling cycle and allow the kiln to cool naturally.
If you're re-firing previously matured work or firing raw glazed pieces,
you'll want to start up slowly as you would in a bisque firing.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
John Jensen on thu 8 mar 01
My kilns are in a covered shed and the temperature inside tends to reflect
the weather. I was surprised some years back to notice on very cold nights
that so much steam is still escaping at 1400 degrees F that that the kiln
seemed to be on fire. The condensing steam looked like smoke and there was
a lot of it. I had always thought all molecular water was gone by around
1000 Deg F..or around red heat, so once I got red heat I let it rip.
Actually has worked out ok that way; but, still, it's alarming to realize
how much water is still there at that temperature.
John Jensen, Mudbug Pottery, Toadhouse.com, Annapolis.
Gavin Stairs on sat 10 mar 01
At 09:37 AM 3/8/01, John Jensen wrote:
>My kilns are in a covered shed and the temperature inside tends to reflect
>the weather. I was surprised some years back to notice on very cold nights
>that so much steam is still escaping at 1400 degrees F that that the kiln
>seemed to be on fire. The condensing steam looked like smoke and there was
>a lot of it. I had always thought all molecular water was gone by around
>1000 Deg F..or around red heat, so once I got red heat I let it rip.
>Actually has worked out ok that way; but, still, it's alarming to realize
>how much water is still there at that temperature.
Well, John, there are two possibilities. The first is that if you are fuel
firing, especially with wood or natural gas, there is a lot of water and/or
hydrogen in the fuel, which all turns into water vapor and then condensed
steam. So you will be emitting water as long as you are firing.
The second possibility is true of all firing methods. That is, as you
convert water/hydrogen to steam, it will work its way into any spaces in
the kiln wall, including fibre and IFB voids, also cracks in the bricks,
etc. This water will condense at the first cool layer it comes to, which
is probably only an inch or two into the wall to begin with. As the firing
progresses, the wall heats up, and the condensate zone progresses toward
the outside of the kiln. If you have generated a fair amount of water in
the early firing, it will escape over this whole time, during which the
kiln interior can become quite hot, well above the temperature at which
water is expressed.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Cindy Strnad on sat 10 mar 01
Hello, John.
Do you think all that steam is really just water escaping from your ware? I
wonder if it could be atmospheric water, also.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Cindy Strnad on sat 10 mar 01
I'm re-sending this because I don't think it made the list last time. If it
did, my apologies:
Hi, Diane.
You need to start up your glaze firing slowly when you're re-firing
previously glaze-fired work because these pieces, being vitrified, are much
more prone to minor explosions than porous bisque ware.
(Obviously, the raw-glazed pieces, like raw pieces in a bisque load, need
plenty of time to release residual moisture--that's why they need to go
slow.)
Even though a piece has been through the entire firing cycle once, it is
still likely to contain volatile gasses for release. This possibility
increases with the thickness of the walls of the piece. You may get away
with a relatively fast cycle for thin-walled cups, but don't try it with
that 16" tall sculptural piece you worked all day on. Because the piece is
vitrified, the gasses cannot escape quickly, as they would from a bisque
piece. They need plenty of time to find their way out.
The chance of the piece blowing up in the kiln is directly proportionate to
the time you spent working on it. This is what Murphy would have
said, had he been a potter. And he would have been right, too, imo. No point
in tempting fate.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
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