Teresa on sat 31 jul 10
I am working very large. The piece you see is using 100lbs of clay. I ha=
=3D
ve
successfully complete smaller pieces using 25 lbs of clay for many years.=
=3D
I
use a 3 phase drying process with two damp cabinets and a final hot box w=
=3D
ith
heat lamps.=3D20
I use a pinch pot method to hand build the piece. The problem is when I=
=3D
go
large the pots are always cracking at the seam where I join them together=
=3D
.=3D20
I can not wedge 100lbs of clay. I am slamming the blocks together and th=
=3D
en
using a sculptors hammer to form the clay. Is there a better method for
joining the blocks of clay? Any suggestions would be appreciated.
Teresa
Out in Vegas where the heat is dry and the pool is the perfect temperatur=
=3D
e
all day and all night:-)
www.windancerstudio.com
email me off list if you want to: teresa@windancerstudio.com
Snail Scott on sat 31 jul 10
On Jul 31, 2010, at 10:50 AM, Teresa wrote:
> I use a pinch pot method to hand build the piece. The problem is
> when I go
> large the pots are always cracking at the seam where I join them
> together.
> I can not wedge 100lbs of clay. I am slamming the blocks together
> and then
> using a sculptors hammer to form the clay. Is there a better method
> for
> joining the blocks of clay?
My work is large coil-built (mostly) sculpture, but
with quite thick chunky coils that get pinched
into near-final form as I go, then scraped and
refined to finish.
Whether joining coils or pieces of clay in any
other form, I find that having a substantial overlap
is the key to avoiding cracking, and this becomes
even more important with finicky clay bodies.
Form each side of the joint into a sloping edge to
make an overlap: the more overlap, the stronger
the joint. Just as a lap joint in carpentry is stronger
than a butt joint because of the larger contact
surface, a substantial overlap in clay is helpful too.
Score lightly and shallowly but thoroughly (a
toothbrush is dandy, or a serrated rib), use a bare
minimum on slip (a wet toothbrush makes its own),
and wiggle the overlap together.
I've never actually hammered a joint, but any
process must integrate the particles from one side
of the joint with those on the other side, integrating
them like Velcro. Digging in deeply with fingers or
a blunt tool can accomplish this as well if clay is
soft, but both soft and stiffer clay can be joined quite
well by scoring, since it's the microscopically thin
interface between the two sides of the joint that
matters, not any other part of the clay. (Those parts
are each already solidly joined to themselves, right?)
It's only the contact surface that needs attention. You
can hammer or stretch or squish the clay to form it
and align the particles for strength, but it's all about
linking those tiny particles together at the contact
surface, nothing more.
Scoring gets the particles sitting up instead of lying
flat and parallel to the surface, and since only the
surface matters, scoring should never be deep, but
should disrupt the smoothness of as much of the
contact area as possible. Slip is just there to fill any
slight air gaps where the clay might not fit perfectly in
spite of wiggling it together, so the minimum amount
should be used. The wiggle (or shimmy) may seem
trivial, but it's critical. It forces out excess slip and air,
and helps mix the particles, and generally it takes
just a shimmy or two before you can feel the joint
really start to grip firmly.
I find that scoring and slip (or one or the other) can
often be dispensed with if the clay is gritty, as it seems
to sort of score itself, and clay bodies with a high
percentage of actual clay (as opposed to lots of
silica, feldspar, etc), seem to join more readily with
less fuss. Porcelain, however, needs attention to
detail in joining, and just squeezing the halves of a
joint together, even with a fair amount of squishing
(or hammering) may not always suffice.
Go ahead and use the hammer if you like it, but be
sure to prep the overlap before you begin.
-Snail
Dana & Chris Trabka on sun 1 aug 10
Teresa & Snail,
The practice that has worked for me is one side of the joint has a "V" shap=
e
and the
other side of the joint has a "U" shape. Importantly, after they have been
"joined" it is
important to make sure that the "joint" is not thicker than the wall above
the joint or
below the joint. If the "join" is thicker than the walls, it will show afte=
r
the glaze firing.
If using "slip" to join the two parts, consider adding vinegar to the mix
(or using "magic water")
Chris
>
> Form each side of the joint into a sloping edge to
> make an overlap: the more overlap, the stronger
> the joint. Just as a lap joint in carpentry is stronger
> than a butt joint because of the larger contact
> surface, a substantial overlap in clay is helpful too.
> Score lightly and shallowly but thoroughly (a
> toothbrush is dandy, or a serrated rib), use a bare
> minimum on slip (a wet toothbrush makes its own),
> and wiggle the overlap together.
>
paul gerhold on sun 1 aug 10
Teresa,
The "V and U" type join described by Dan and Chris is the best you can do!
I fill the U with slip made with magic water and then press the top down
tightly. Wipe off extra slip . Yes you waste slip but it is cheap compare=
d
to your time. Make sure both thicknesses are the same.
You don't say how long you take to dry a major piece but should probably be
a couple of weeks. Also can't imagine why you would need heat lamps in ari=
d
Vegas. It doesn't seem like they would contribute to even drying. Maybe
just add a couple of adjustable vents to the damp boxes.
Paul
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