iandol on tue 21 aug 01
Dear Curtis,
I must confess that the fact that no one responded to that had escaped
my attention.
A few years ago I did have some catastrophes with a series of plates
where there was widespread cracking across the base of each plate.
These were fired flat on their foot rings but I was well aware of the
cause, the decoration was underglaze colour and to limit the spread of
the area I had incised each colour field. During drying this incision
acted as a stress raiser and during bisque firing the incision
propagated right through the clay.
The other events were in a salt kiln where I side stacked several
plates to maximise the kiln space. Fired to cone *8 down these curled
up on themselves. Superb pyroplastic deformation.
My current technique is to cut 3 notches out of the footring and fire
on a thin bed of alumina sand. My shelves are not all that thick,
about 5/8, 18*18 so raw plates can be up to 22in dia. I have no
problems with these either in stoneware or porcelain. Since I fire
slowly, less than 100 C/hr no trouble is anticipated.
I think I posed the question because it was not being made clear what
was being proposed. People were talking about firing on the rim
without saying it was vertical. Just wanted to clear up the ambiguity
so I made a seemingly stupid comment about resting a plate on the
curcumference of it's rim.
I think the question of heat in in the air or air temperature is a bit
of a red herring. In an electric kiln convection should soon mix cold
with hot and in a gas or wood kiln the natural draght soon gets rid of
cool air. Unless someone comes up with pyrometer measurements taken
inside and outside we just do not know. The lie to cool air inside a
pot having an adverse influence is given by all those tea pots,
casseroles, tureens and canisters which are fired with their lids in
place as well as traditional saggar firing. Have you ever heard of any
problems with these? I think people would have fewer cracking problems
with plates if they used shielding around the rim and fired flat. If a
thin rim heats up fast in comparison to the base then there will be
circumfrencial stress set up as one expands before the other. The
conformation of the cracks will depend on on the patterns of residual
wedging and throwing strains and the degree to which these relax as
the clay anneals.
My worry about firing a plate resting vertical on its edge is that
this becomes a point of maximum stress, very local with the whole
weight on a tangential point. Any imperfection could act as a stress
raiser and initiate a fracture.
Thanks for bring this to my attantion.
Best regards,
Ivor Lewis.
-----Original Message-----
From: Curtis G. Nelson
To: iandol@tell.net.au
Date: Tuesday, 21 August 2001 2:35
Subject: bisque firing plates on rims
>Ivor -
>
>A short while ago you posted a question about whether bisque firing
plates on
>their rims meant vertically on edge or upside down resting on the
entire
>circumference of the rim while in a horizontal position.
>
>I don't recall seeing any replies on Clayart. Did you receive any
directly?
>
>I would think the upside down and horizontal method would exacerbate
the
>problem of poor kiln heat balance and uniformity due to the trapped
air under
>the plate. This condition sometimes leads to cracks. But then maybe
this is
>not relevant in the relatively low temperature of a bisque firing.
Any comment
>to this speculation?
>
>If firing is done while the plates are in a vertical position,
perhaps there's
>the risk of warping the plate. But then maybe this is not relevant in
the
>relatively low temperature of a bisque firing.
>Any comments to this speculation?
>
>Your post got me to thinking about the upside down and horizontal
position.
>Before reading it, I had always thought vertical firing was what
people were
>talking about. Now I wonder. What did you find out?
>
>I believe that the advantage of vertical positioning and firing would
be that
>a greater number of pieces could be included in the firing. Whereas,
>horizontal positioning would mean (if you stacked several on top of
one
>another) that only the bottom plate would actually be resting on it's
rim. And
>if, in another approach, each plate were given it's own setter shelf
so it
>could have complete rim contact, the kiln would be quickly
overburdened with
>shelves and furniture. That fills the kiln inefficiently and makes
for
>uneconomical firing.
>
>Cheers,
>
>Curt Nelson
>
>barnaby@enteract.com
>
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