How to Avoid Cracked Handmade Ceramic
Boxes
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Hint #6
Hint #1: Potter's Wheel
Speed
Limit
Hint #2: Firing
Stacks of Greenware Plates
Hint #3: Setting
the cone packs in the kiln.
Hint #4: Preventing
Stuck Lids with Lid Wax
Hint #5: Preventing
Warped & Cracked Tile
Hint #6: Preventing
Cracked Slab Boxes
Marvin Bartel
Boxes made from flat pieces of clay are more prone to
crack than most other clay forms.
WHY DO BOXES CRACK?
Moist clay shrinks a lot when it dries.
If a box is on the shelf to dry, the bottom will dry slower than the
top. This or other uneven shrinkage will often crack the box.
THE GENERAL PRINCIPLES
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Construct the whole box from pieces of clay that are all exactly the same
hardness at the time of assembly. Generally, this means that some
planning is required.
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When pre drying a slabs to be used for pieces of a box, wait to trim them
to size until they have firmed up a bit for assembly. Trim away about
an inch or more from all sides. Otherwise, some of the edges are
dryer than the centers of the slabs. Cracks may show up later.
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Assemble the slabs before the slabs get leatherhard. Use clay that
has firmed up, but while you can still bend it a bit without cracking.
Otherwise, the slip added in joining can cause too much variation in hardness
and subsequent drying shrinkage.
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Slip, water, and/or soft clay is added at the joints. This causes
softer areas. Therefore, wrap the whole box in plastic several days
before allowing any drying. This allows for the stress to even out
before serious drying shrinkage occurs.
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Dry it in a way that allows the water to leave at the same rate from all
surfaces including the bottom.
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The interior of the any box tends to dry slower than the exterior.
This may have a tendency to develop concave surfaces. Counteract
this by using slightly convex slabs and by drying the box slowly enough
to avoid concave surfaces.
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Dry boxes on wire racks (like old refrigerator shelving) that have LOTS
of air space under them. Four inches of air space under each layer
is minimum. I have mounted some wire closet shelving in our ceramics
room where it is high enough to be "out of the the way".
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A word about the clay
Clay is not all the same. Some clays shrink much more than others.
Adding openers like grog or fine sand to the clay can reduce shrinkage.
Most tribal potters add some "tempering" materials to their clay to help
with drying, to prevent cracking, and to prevent firing pop outs. You may
want to try different clay bodies to see which ones are best for slab building.
I once watches a Mexican villager making brick. He blended in
significant qaunties of dry pulverized horse manure to the clay before
forming the bricks. This helps the the bricks dry flat and without
cracks. It allows steam to escape easier during firing. It
makes bricks that are easier to build with because mortar sticks better.
The bricks insulate better because they are more porous.
Some handbuilding is done with "paperclay" for similar reasons.
This is a clay which is made with a siginificant portion of paper fiber.
Paperclay is very forgiving. It is much less apt the crack when combining
pieces that are different hardnesses.
List of Hints for Potters
For more hints, come back soon. Your
comments are welcome.
© Marvin
Bartel, all rights reserved. Readers may print
one copy for their own use. Anybody wishing to publish or make additional
copies of any part of this page or the images must obtain permission.
e-mail: marvinpb@goshen.edu
Goshen College Art Department
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