Snail Scott on thu 31 jan 02
At 06:56 PM 1/30/02 -0500, you wrote:
Has anyone tried making 4 tiles a
>little bigger than the feet to set them on? Wouldn't this actually be about
>the same process in shrinking/moving and much less clay? A small tile should
>shrink the same % as big tile if made of same clay as pot? Or, am I missing
>something.?
Well, having tiles under the feet would help distribute
the load, allowing the grog 'ball bearings' underneath
to have better effect, but four discrete tiles will
each shrink toward their own center points, or at least
toward the point under the foot of the pot, but the pot
itself is shrinking toward its own center point, and a
single slab would also shrink as a unit with the pot.
Whether individual tiles might work will probably depend
on the weight of the pot. If it's light enough, the tiles
would probably skate on the grog toward the pot's center
as it shrinks. If the pot is rather heavy, the tiles
might not move, and the tile (and feet) would try to stay
in place as the pot shrinks above it.
If you're whipping out these slab pots quickly, so that
the cost of materials is a significant component of your
cost of production, a full slab might indeed be a waste.
With my work, however, the cost of the clay is negligible
compared with the labor invested in a single piece, when
a $2000 sculpture might be ruined by the lack of $2 worth
of clay in a firing slab.
By the way, the thickness of the slab, and its flatness,
will matter. The heavier the work, the thicker it will
need to be to avoid cracking during firing, since the
feet of your pot will tend to hold those parts of the
slab against the kiln shelf. If it splits between the
feet, it might as well have been separate tiles. For a
2' piece of sculpture, (maybe 50# dry weight,) I will
generally make my firing slabs 3/4 of an inch thick.
Also, if the slab is slightly warped, the weight of the
piece may crack it. Dry it between pieces of drywall,
turning the drywall 'sandwich' if it is resting on a
nonporous surface so that each side can dry evenly.
(And weight it.) I stack my drywall on an old kiln
stand to give it good airflow all the way around.
Note: I only use firing slabs when the weight of the
work is borne on small, widely spaced supports. If
the points of support are fairly close together, the
warping due to shrinkage may be small enough to not
matter. (10% over 5 inches is only a 1/2 inch; 10%
over 2 feet is 2 1/2 inches!) And if the base of the
piece is large enough to distribute its load pretty
well without help, it may not need a firing slab at
all, just a little sand or grog underneath.
-Snail
Dale Cochoy on thu 31 jan 02
----- Original Message -----
From: "Snail Scott"
Subject: Re: Foot cracks problem / firing slabs
> At 06:56 PM 1/30/02 -0500, you wrote:
> Has anyone tried making 4 tiles a
> >little bigger than the feet to set them on? Wouldn't this actually be
about
> >the same process in shrinking/moving and much less clay? A small tile
should
> >shrink the same % as big tile if made of same clay as pot? Or, am I
missing
> >something.?
>
> Well, having tiles under the feet would help distribute
> the load, allowing the grog 'ball bearings' underneath
> to have better effect, but four discrete tiles will
> each shrink toward their own center points, or at least
> toward the point under the foot of the pot, but the pot
> itself is shrinking toward its own center point, and a
> single slab would also shrink as a unit with the pot.
>
> Whether individual tiles might work will probably depend
> on the weight of the pot. If it's light enough, the tiles
> would probably skate on the grog toward the pot's center
> as it shrinks. If the pot is rather heavy, the tiles
> might not move, and the tile (and feet) would try to stay
> in place as the pot shrinks above it.
Good logic there!
> By the way, the thickness of the slab, and its flatness,
> will matter. The heavier the work, the thicker it will
> need to be to avoid cracking during firing, since the
> feet of your pot will tend to hold those parts of the
> slab against the kiln shelf. If it splits between the
> feet, it might as well have been separate tiles. For a
> 2' piece of sculpture, (maybe 50# dry weight,) I will
> generally make my firing slabs 3/4 of an inch thick.
Most of my slab pots are 12 pounds to a max of about 30 pounds. The slabs
I y the out on are , I think it was, 5/16".
Dale
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