Splitting at the Plastic Stage

Pottery clays can split at the plastic stage, sometimes ware can fall down right in front of your eyes. Understanding why is the key to avoiding the issue.

Details

When clay bodies contain significant percentages of sand, grog or unground clay particles coupled with inadequate amounts of fine plastic ingredients to bond the matrix, soft clays can form tiny splits. These can grow (over a period of minutes) and cause pieces to collapse. Most clay bodies used by potters and industry are made from 200 mesh air-floated clays. These can tolerate considerable aggregate particles and still maintain plastic strength. But when bodies are processed at coarser sizes (e.g. 42 mesh), they have a lower tolerance for this problem of splitting. Potters used to such bodies quickly learn what is needed to avoid it.

In susceptible bodies, each grain of larger material exposed at the surface during throwing or forming is an opportunity for water to enter and begin a tiny split in the soft clay. Splits grow if the clay is soft enough. As noted, this problem is exacerbated when the plastic clays being employed are not fine-ground and thus themselves contain a range of particle sizes above 200 mesh. And when significant percentages of less plastic silty clays are in the recipe. Most often, simply understanding the character of the clay bodies being used and the measures necessary to prevent this issue are enough to avoid any problems.

Ways a potter can contribute to this issue:

Create shapes that flare enough to stress the clay's ability to support itself at the stiffness being used and leave water or watery slip on the stress points. Flared bowls or bellied vases are examples of shapes that can put surface areas under tensile stress.
Cutting the rim off with a needle tool and leaving the flat surface uncompressed and with water on it.
Applying slip just after throwing the piece (rather than waiting until leather-hard).
Not wedging the clay well enough to taking too long to make the piece on the wheel.

Assuming the body is working as the manufacturer intended, a potter can handle this issue by simply doing the opposite of the above: Make more stable shapes, throw them more quickly, finish rims by throwing rather than cutting, apply slip to pieces later, wedge well and use a stiffness matching the shape being made.

Of course, manufacturers can do things that make this problem worse. Those who employ screening rather than air-separation equipment are more susceptible to equipment-caused homogeneity issues. This happens because oversize particles (those that do not pass the vibrating screens) are recycled to the grinding mill as a matter of process. However, if the recycle material builds up on the conveyor and the crude feed is slowed or stopped to wait for it to reduce, the material being delivered to the holding hopper or pugmill lacks the fine plastic portion needed to bond particles. This not only happens during a run, but also at the end when the equipment is allowed to run to recycle oversize until all of it passes the screens.

Related Information

Why do some clays split like this on throwing or forming?

This picture has its own page with more detail, click here to see it.

An example of splitting, where a clay develops a split minutes after throwing. This often happens at stretch-points with sandy or groggy or minimally processed natural bodies that have a wide range of particle sizes (e.g. native bodies). When larger particles are poorly bonded by fine plastic clays, they can form networks into which water can penetrate. This issue is referred to as "splitting" because it is progressive, a crack starts, and it grows over a period of minutes. If you use bodies susceptible to this issue, various habits can be formed to minimize this problem. Perhaps the most important is not to leave water or high-water-content slip on any surfaces experiencing tensile stress (e.g. flared bowls, bellied vases).

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By Tony Hansen
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