Snail Scott on wed 3 nov 10
On Nov 3, 2010, at 2:17 PM, Fran Schwartz wrote:
> I have been building boxes of various sizes.After glaze firings I
> have, at
> times, been finding a crack in the top corner...
If the slabs are soft enough, make sure to
really join them well by digging into the
joint area and moving the clay across the
joint until the joint is completely worked
through. If the slabs are assembled when
too stiff for this, make sure you score and
slip well. this means not deep scoring, but
rather, maximum disruption of just the very
top level of the clay slab. A toothbrush does
a very good job of shallow-but-total scoring,
and if used wet, will make enough slip in
the process to need no extra slip added.
Put the edges together and give them a
small shimmy into position. This will give
good contact between the slabs but also
force any excess slip out of the joint. Extra
slip is as bad as too little! Then, leave the
joint alone for a few minutes until the moisture
from the slip has entered the surrounding clay
and is no longer a wetter layer in between
the slabs. Adding a reinforcing coil is good,
like belt-and-suspenders, but getting a good
joint in the first place is essential. If you do
without the coil, round the inner corner of the
joint just a bit so that it is no longer a very
sharp corner and there is no visible joint
remaining inside or out.
Lastly, if the rim shape allows for it, dry them
upside down to that the vulnerable rim will
not dry out ahead of the rest of the box. The
stresses of unequal drying are the greatest
threat to joints in clay. (Joints in California
are still illegal...) ;)
-Snail
Fran Schwartz on wed 3 nov 10
I have been building boxes of various sizes.After glaze firings I have, at
times, been finding a crack in the top corner.
What am I doing incorrectly, or not doing?
I have been adding extra coils at the corner- top, but this doesn't seem to
help.
Your suggestions will be greatly appreciated. Thanks, Fran Schwartz
Vince Pitelka on wed 3 nov 10
Fran Schwartz wrote:
I have been building boxes of various sizes.After glaze firings I have, at
times, been finding a crack in the top corner. What am I doing incorrectly,
or not doing? I have been adding extra coils at the corner- top, but this
doesn't seem to help.
Hi Fran -
How are you rolling your slabs? Do you cross-roll to eliminate the
unidirectional grain structure? If not, that could be the whole problem.
You can cross-roll either by rolling the slab once, turning it 90 degrees,
lowering the setting slightly and rolling it again. You can also cross rol=
l
with a rolling pin after you remove the slab from the slab roller. In
either case, it will help eliminate the unidirectional grain structure so
that the slab will shrink equally in all directions. This can make a huge
difference.
I always join rectilinear slab boxes with 45-degree bevel joints at the
corners. I started doing that because I was building from slabs that were
either laminated with colored clay patterns, or were textured with bisque
stamps, and I did not want the interior core of the slab showing anywhere.
When you cut the pieces of a form using templates, with 45-degree undercuts
along all edges, then the pattern ends up completely covering the form.
I join the component parts when they are stiff leather-hard. I score the
contact surfaces very thoroughly, add plenty of slurry, press the parts
together firmly, remove excess slurry with a short, stiff brush (like you
would use with oils or acrylics - works like a soft spatula). I never add
coils to reinforce the insides of the joints, because in doing so, I think
that sometimes people actually stretch the joint slightly, encouraging
cracking. Also, adding a soft coil to a stiff-leather-hard slab joint
introduces a differential shrinkage rate than in itself can cause a crack.
The only time I have ever added a coil is in very large-scale slab
construction. For average-size slab boxes, vases, teapots, pitchers, etc.,
I never add a reinforcing coil, and I never have my slab joints crack (knoc=
k
on wood, knock on wood). I often work with slabs less than 1/4" thick.
Please feel free to email any questions -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
paul gerhold on thu 4 nov 10
Fran,
After joining I like to use a wood tool to pull deep grooves across the joi=
n
so that the grooves crosse the two pieces of clay. Kind of looks like
stitches joining two pieces of fabric . Then fill in the grooves with
more clay and smooth. Makes a very strong join but does take some time to
do it right.
Paul
Randall Moody on thu 4 nov 10
I find that, as with most things, timing is important. I join when the slab=
s
are just shy of leather hard. I don't score per se but rather use a stiff
bristle brush and some vinegar water to scrub the area to be joined on both
sides. I find that using a serrated rib or one of those wire brushes to be
too aggressive or they introduce little furrows of air pockets. I also
recommend paddling the joint to further refine the edge and meld the pieces
together.
--Randall in Atlanta--
http://wrandallmoody.com
From: Clayart [mailto:Clayart@LSV.CERAMICS.ORG] On Behalf Of Fran Schwartz
Sent: Wednesday, November 03, 2010 3:17 PM
To: Clayart@LSV.CERAMICS.ORG
Subject: How to stop cracks in the corners of handbuilt boxes?
I have been building boxes of various sizes.After glaze firings I have, at
times, been finding a crack in the top corner.
What am I doing incorrectly, or not doing?
I have been adding extra coils at the corner- top, but this doesn't seem to
help.
Your suggestions will be greatly appreciated. Thanks, Fran Schwartz
Rob Addonizio on thu 4 nov 10
Fran,
Since I don't have a true idea of what the crack looks like, I can only g=
=3D
ive you=3D20
a guesstimate. But, since I have made quite a few rectangular box forms f=
=3D
or=3D20
bonsai, I can tell you it is probably from lack of compression. Try using=
=3D
a small=3D20
block of wood that fits the angle you are striving for and push on both s=
=3D
ides=3D20
with another block of the same kind. That may help.
Now, if this is a crack that is developing on the inside down the seam, y=
=3D
ou=3D20
may have another issue, that of warping during drying. If it is a tall fo=
=3D
rm, you=3D20
can try using a hairdryer on the inside so that it dries first. You can =
=3D
also use=3D20
plastic wrap on the outside as it dries, thus keeping the outside wetter =
=3D
than=3D20
the inside. If it dries good, inside then outside, you will have less war=
=3D
ping.
Now I must ask about your claybody. If it has a lot of temper (grog, sand=
=3D
) it=3D20
may also help to relieve this issue.=3D20
It would help if I had a pic. Could you send me one?
Hope it helps!
Rob Addonizio
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