| Orton Firing Tip: Cracking and Warping Caused by
Drying and Casting |
Reprinted with the permission
of the Edward Orton Jr. Foundation
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?
Read more about Solving Cracking and Warping Problems 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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