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Cracking and Warping
Orton Firing Tip -
CRACKING & WARPING
CAUSED BY DRYING & CRACKING
Reprinted with the permission of the Edward Orton Jr. Foundation
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In
some instances cracking and warping problems share a common source: the
casting and drying of the piece. In other cases, cracking may be
related to how the piece is fired. This Tip looks at problems related
to casting and drying.
Drying Ceramics
Ceramics contain clay which can absorb and hold water. Before firing,
it is important to remove all of the physical water so that the piece
will not crack or explode when heated. This is often accomplished in
steps with firing being the final stage. During firing, the chemical
water is removed from the piece and it gains strength while developing
physical surface characteristics.
Understanding Drying
Simplified, drying is the removal of water from body by evaporation. As
the ware is dried, the film of water separating the clay particles gets
thinner and thinner, the solid particles get closer together and the
piece shrinks. Shrinkage stops when the particles finally contact each
other.
Drying Faults
Cracking, distorting and warping are problems that may not become
evident until after firing. They are usually caused by drying too fast
or unevenly.
If ware is heated too fast, the pressure from water vapor inside the
piece can cause cracking. Ware dried only on one side can shrink more
on that side causing warping or bending of the somewhat plastic
(flexible) piece. When one surface finishes drying, the piece is now
too stiff to recover and the warping becomes permanent. This can lead
to cracking.
Bodies made of very plastic clays or compositions having a high clay
content require attention to uniform, slow drying.
Thicker walled pieces will often have a greater tendency to warp or distort.
Care needs to be taken to allow for uniform air movement around all
sides of a piece to avoid drying problems. Sometimes drying must be
slowed down to avoid cracking.
Handles on cups can have a tendency to pull away from the mug. Doll heads and chest cavities may deform inward.
Reducing Warping and Cracking
To reduce warping and cracking, take steps to dry more slowly and more evenly from all sides.
Don’t dry a flat object on a wet or cool surface like a formica or
plastic table top or damp newspaper. The piece can only dry on one side.
Instead, dry objects on something porous like wood or plaster or set
them so air can circulate around them. If necessary, turn pieces over
during drying for more even result.
Slow the drying of thick walled pieces and hand built ware.
Support areas during drying that might cause stresses to build up.
Drying Techniques
slip cast ware - may warp or crack if stressed (deformed) when removed
from the mold. Even if the ware is gently returned to the original
shape, the created stress will ultimately cause the piece to warp or
crack.
wheel thrown ware - should not distort during drying unless subjected
to further mechanical forces - let the ware dry naturally on a bat or
shelf and it should be fine.
thick handbuilt ware - needs to be dried for a very long time before it
can be fired or it may explode during firing. Several days may be
required or a low heat drying in an oven may be necessary to remove all
the water.
plates - even drying is particularly important with plates. Warping can
cause the center of plate to fall or arch up. Rims and centers must dry
evenly to prevent warps, humps and cracks.
drying tiles - drying tiles can present a particular challenge because
it can be difficult for the piece to dry evenly. Usually air is passed
over the top of the tile. This results in warping because the bottom of
the tile remains wet. Drying tiles in tile racks can help air movement
for more even drying.
Want To Learn More?
Order Orton’s 80 minute video, Key Principles f Successful Firing, $39.95 is also an excellent resource on firing.
Read more about Firing in the Orton Firing Line and Technical Tips
publications. Each issue is packed full of articles to help you learn
more about firing. Members of the Orton Firing Institute receive these
publications at no charge. Single copies are available to non-members
at a per issue rate. Orton's 80 minute video, Key Principles of
Successful Firing, is also an excellent resource on firing.
For information on Orton products, see your Orton dealer or
distributor. For information on the Firing Institute or publications,
contact:
Orton Firing Institute
PO Box 460
Westerville, OH 43086
(614) 895-2663 |
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