Tom at Hutchtel.net on sun 4 nov 07
We've found that most of the dunting happens on cooling, not heating.
That's one reason Ron's suggestion of putting it up top helps. Another is
to get a kiln shelf big enough to hold the rim, and fire it upside down.
Don't try to go over a center shelf crack if you're using half shelves, you
can't get them even enough to avoid warping the piece.
Tom Wirt
Hutchinson, MN
twirt@hutchtel.net
www.claycoyote.com
----- Original Message -----
From: "Eleanora Eden"
>I had given up trying to fire such platters, cracking was just about
>guaranteed.
> Then RR mentioned making sure to fire such items in the top half of the
> kiln.
> The other thing I do that I think nobody else mentioned is I put a moat of
> tiles or
> kiln posts around the flat item so that the heat isn't going directly onto
> the rim and
> creating that difference in heating.
> I glaze fire with stilts so the piece is never touching the shelf during
> the glaze fire.
Ann Brink on sun 4 nov 07
I have no choice but to use the half shelves. To maintain the shape of the
platter, I spread semi-circles of grog on each shelf, where the rim will be,
taking care not to get too close to the crack. I set the platter down and
sort of nestle it into the grog until I feel it is stable and supported. I
do this for the glaze firing as well.
Ann Brink in Lompoc CA, where we have ground fog for a change, but are
expecting 77F today.
(mostly about pottery) ps. I've posted
pictures of our "new" dog Walter
----- Original Message -----
From: "Tom at Hutchtel.net"
> We've found that most of the dunting happens on cooling, not heating.
> That's one reason Ron's suggestion of putting it up top helps. Another is
> to get a kiln shelf big enough to hold the rim, and fire it upside down.
> Don't try to go over a center shelf crack if you're using half shelves,
> you
> can't get them even enough to avoid warping the piece.
>
> Tom Wirt
> Hutchinson, MN
> twirt@hutchtel.net
> www.claycoyote.com
>
>
Craig Martell on sun 4 nov 07
Ann was saying:
>I have no choice but to use the half shelves.
Hello Ann:
Sure you have a choice or at least I hope you do.
I have round shelves that vary from 13 to 16 inches in diameter and I use
them to jump the cracks between half shelves in the electric and I also use
them for glaze fire in the gas kiln. I choose the diameters of the shelves
to be just a bit larger than the foot ring of the platter. I set the
shelves on six to eight wads for support and just plop the platters down on
them and fire.
I really don't think you have to go to much trouble with firing platters as
long as they are bisqued hot enough. I fire 16", 20" and 24" porcelain
platters all the time and I haven't had a dunt or crack in many years. I
raised my bisque to cone 03 (1988F) to give the piece some fired strength
so it will survive the quartz inversion on the way down. In my experience
pieces rarely crack on heating up. The clay is more open until the
hydroxyls are collapsed and the bound water is driven off at around 1300F
or so. Then the clays become metakaolin and are tighter and more compact
but this is after the quartz inversion so no problem on heat up. If you
bisque to cone 04 you will probably be ok.
I don't know what clays others use and what the formula of the body is etc,
ad nauseum, but some clays are too high in free silica so there's more of a
jolt during the quartz inversion.
regards, Craig Martell Hopewell, Oregon
Ann Brink on sun 4 nov 07
Thanks for your comments, Craig. Maybe I will get a 16" diam. round shelf
for the infrequent largest platters. The foot ring would just fit.
Actually...same idea, I sometimes elevate a square shelf over other low
pieces, and fire a platter on it., for instance a 15" one.
Ann Brink in Lompoc CA
----- Original Message -----
From: "Craig Martell"
> Ann was saying:
>>I have no choice but to use the half shelves.
>
> Hello Ann:
>
> Sure you have a choice or at least I hope you do.
>
> I have round shelves that vary from 13 to 16 inches in diameter and I use
> them to jump the cracks between half shelves in the electric and I also
> use
> them for glaze fire in the gas kiln. I choose the diameters of the
> shelves
> to be just a bit larger than the foot ring of the platter. I set the
> shelves on six to eight wads for support and just plop the platters down
> on
> them and fire.
>
> I really don't think you have to go to much trouble with firing platters
> as
> long as they are bisqued hot enough. I fire 16", 20" and 24" porcelain
> platters all the time and I haven't had a dunt or crack in many years. I
> raised my bisque to cone 03 (1988F) to give the piece some fired strength
> so it will survive the quartz inversion on the way down. In my experience
> pieces rarely crack on heating up. The clay is more open until the
> hydroxyls are collapsed and the bound water is driven off at around 1300F
> or so. Then the clays become metakaolin and are tighter and more compact
> but this is after the quartz inversion so no problem on heat up. If you
> bisque to cone 04 you will probably be ok.
>
> I don't know what clays others use and what the formula of the body is
> etc,
> ad nauseum, but some clays are too high in free silica so there's more of
> a
> jolt during the quartz inversion.
>
> regards, Craig Martell Hopewell, Oregon
>
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