Kate Johnson on tue 4 jan 05
So, I need opinions again, alas. I went to the discussion by categories
page but didn't see anything that would fit, and the search function didn't
turn up anything useful...
I looked on http://www.potters.org/categories.htm
The sgraffito owl-and-owlets plate I was making that is inspired by a 1730s
piece was just working PERFECTLY...all systems go. Took the slip coating
evenly, came out of the mold well, I did the sgraffito at the perfec time,
got the shapes right, no cracking or warping in drying. It looked GOOD.
Till we fired it. It was a plate with a raised rim, and the rim just neatly
collapsed all the way around, very evenly, to touch the kiln shelf. ACK. I
know we've talked about the reasons plate rims collapse, and I'm sorry I
couldn't find that, or remember, but...help, please?
The rim wasn't thick or overly thin, but it seemed sturdy enough...perhaps
not. It was handbuilt, slab work, so reasonably uniform thickness
throughout--that usually works fine for me. Might have gotten a WEE bit
thinner where I fit it carefully into the break between the flat bottom and
the raised rim, but tried to be careful there...I use a sponge, gently, to
push the clay down into the form.
Another plate on that shelf (top shelf) also collapsed...it had been a slab
allowed to dry almost leather hard before putting into a gently curved mold,
so it had a very subtle curve to begin with...now the curve would have to be
measured in millimeters. It's almost flat. Make a nice coaster, if a
bit large...
It was an electric kiln, recently repaired. The plates on the bottom shelf
did NOT collapse (unfortunately they were the ones I mentioned earlier that
cracked all around the bottom, so they're unusable for anything but demos,
but...they look fine.)
I'm assuming the good plates on the top shelf got too hot, even at cone 06,
and partially melted. They thought they were witness cones, I guess...(and
no, he didn't use witness cones in the kiln, so we're flying a bit blind
here...)
Any advice or opinions more than welcome...
Best--Kate Johnson
Graphics/Fine Arts
http://www.cathyjohnson.info/
http://www.epsi.net/graphic/
http://www.ebsqart.com/Artists/cmd_1494_profile.htm
Ama Menec on wed 5 jan 05
Hi Kate,
I'm assuming you are using red earthenware clay for the body? If you are,
and the red clay has turned dark brown and is shiny where unglazed, and
maybe a bit blistered too, then yes it has been overfired. Did you fire this
the right side up or upside down with its' rim on the kiln shelf? If it was
upside down it may have stuck and cracked off while shrinking or maybe even
dunted if the firing was fast....Was it a wide rim? May it have needed more
support? I've only had students slipware plate rims come off as a result of
over enthusiastic turning...but it shows how strong red earthenware can be;
they only seem to drop off if under 3mm thick at the join between rim and
body!
Ama, Totnes, Devon, UK.
----- Original Message -----
From: "Kate Johnson"
To:
Sent: Tuesday, January 04, 2005 3:10 PM
Subject: Collapsed plates
> So, I need opinions again, alas. I went to the discussion by
categories
> page but didn't see anything that would fit, and the search function
didn't
> turn up anything useful...
>
> I looked on http://www.potters.org/categories.htm
>
> The sgraffito owl-and-owlets plate I was making that is inspired by a
1730s
> piece was just working PERFECTLY...all systems go. Took the slip coating
> evenly, came out of the mold well, I did the sgraffito at the perfec time,
> got the shapes right, no cracking or warping in drying. It looked GOOD.
>
> Till we fired it. It was a plate with a raised rim, and the rim just
neatly
> collapsed all the way around, very evenly, to touch the kiln shelf. ACK.
I
> know we've talked about the reasons plate rims collapse, and I'm sorry I
> couldn't find that, or remember, but...help, please?
>
> The rim wasn't thick or overly thin, but it seemed sturdy enough...perhaps
> not. It was handbuilt, slab work, so reasonably uniform thickness
> throughout--that usually works fine for me. Might have gotten a WEE bit
> thinner where I fit it carefully into the break between the flat bottom
and
> the raised rim, but tried to be careful there...I use a sponge, gently, to
> push the clay down into the form.
>
> Another plate on that shelf (top shelf) also collapsed...it had been a
slab
> allowed to dry almost leather hard before putting into a gently curved
mold,
> so it had a very subtle curve to begin with...now the curve would have to
be
> measured in millimeters. It's almost flat. Make a nice coaster, if a
> bit large...
>
> It was an electric kiln, recently repaired. The plates on the bottom
shelf
> did NOT collapse (unfortunately they were the ones I mentioned earlier
that
> cracked all around the bottom, so they're unusable for anything but demos,
> but...they look fine.)
>
> I'm assuming the good plates on the top shelf got too hot, even at cone
06,
> and partially melted. They thought they were witness cones, I
guess...(and
> no, he didn't use witness cones in the kiln, so we're flying a bit blind
> here...)
>
> Any advice or opinions more than welcome...
>
> Best--Kate Johnson
> Graphics/Fine Arts
> http://www.cathyjohnson.info/
> http://www.epsi.net/graphic/
> http://www.ebsqart.com/Artists/cmd_1494_profile.htm
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Anti-Virus.
> Version: 7.0.298 / Virus Database: 265.6.7 - Release Date: 30/12/04
>
>
--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.298 / Virus Database: 265.6.7 - Release Date: 30/12/04
Kate Johnson on wed 5 jan 05
Hi Ama....
>
> I'm assuming you are using red earthenware clay for the body?
Yes...I believe that one was grogged, somewhat. Sometimes I use straight
red earthenware, very smooth, sometimes with grog...
If you are,
> and the red clay has turned dark brown and is shiny where unglazed, and
> maybe a bit blistered too, then yes it has been overfired.
Nope, it looked perfect, just...collapsed. No blisters, no dark brown, no
shiny. Maybe it didn't get THAT hot, just enough to make it fall...
Did you fire this
> the right side up or upside down with its' rim on the kiln shelf? If it
> was
> upside down it may have stuck and cracked off while shrinking or maybe
> even
> dunted if the firing was fast....
Right side up...no sticking...
Was it a wide rim? May it have needed more
> support?
THAT may be...it was wider than I sometimes use. You can see it here,
http://pg.photos.yahoo.com/ph/graphicartepsi/album?.dir=bbb8 second row from
the bottom...BEFORE firing. When I get it back home (after the ice storm
we're having!) I'll shoot the "after" picture.
I've only had students slipware plate rims come off as a result of
> over enthusiastic turning...but it shows how strong red earthenware can
> be;
> they only seem to drop off if under 3mm thick at the join between rim and
> body!
Mine wasn't turned, but slab-built, since I don't have a wheel (and am
trying to approximate a particular style of the early stuff, using press
molds...) I do wonder if it was a bit too thin for its width, though...
Thank you!
Best--
Kate
| |
|
