Splitting at the Plastic Stage

Pottery clays can split or tear 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

Splitting at the plastic stage is different from cracking during drying. It occurs during forming or within minutes afterward, usually at a location being stretched, such as the belly or rim of a thrown vessel. 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 them.

In susceptible bodies, grains of larger material exposed at the surface are potential stress concentrators and interruptions in the continuous plastic matrix. Water or watery slip on an already stretched area may then encourage an incipient separation to open and propagate. 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 or taking too long to make the piece on the wheel.
Bending and stretching the clay beyond its plastic limit (most common with non-plastic clays).
Using pure reclaim clay from which a fraction of the finer particles have been lost.

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.

Clay body manufacturers using screening-based grinding systems can be susceptible to equipment-caused variations in body composition and particle-size distribution. Material too coarse to pass the vibrating screens is normally returned to the grinding mill, creating a circulating load. Problems can arise when this recycle accumulates and the addition of fresh crude feed is slowed or stopped while the system clears it. Much of the naturally fine, plastic-clay fraction in that material has already passed through the screens, leaving the circulating load enriched in harder or more difficult-to-grind particles. Although further grinding produces particles fine enough to pass the screens, this tail-end material can differ from the normal product in both mineralogy and particle-size distribution. It may therefore produce a body having lower plasticity, poorer particle bonding and reduced dry strength. The same issue can occur at the end of a production run if the equipment is allowed to continue grinding and screening the accumulated oversize until all of it passes. Unless this material is separately held, tested or thoroughly blended with the main production lot, it can create localized homogeneity problems in a storage hopper or pugmill feed. Of course, batching errors in production could be a cause (e.g. forgetting to add the bentonite or putting in kaolin instead of ball clay). Another is issues with deairing equipment (although these can be alleviated at the ware production stage by thorough wedging).

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).

What causes a clay to split like this?

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

This is M340, it is not made from a mixture of 200 mesh refined minerals and ball clays, rather, from minimally processed natural mined clay. It is ground at 42 mesh, so the material contains a wide range of particle sizes. Potters who have only used this type of body are at home with it. Its advantages include excellent dry strength, pleasant texture, natural fired character, easy glaze fit and low cost. But downsides are lower plasticity, higher drying shrinkage and splitting. But a case this bad is not normal, it only happens when there is a "perfect storm" of factors:

An unstable bowl shape (flared out to the edge of the clay's ability to support itself).
Cutting the rim off with a needle tool and leaving that flat surface uncompressed and with water on it.
Applying slip just after throwing the piece (rather than waiting until leather-hard). Or, simply leaving excessive throwing slip in place.
Not wedging the clay (or not wedging it well).
Using the clay when it is too soft.

The best way to prevent this from ever happening is to do the opposite of all the factors just mentioned.

PayPal

No tracking, No ads, No paywall, No transient content!
Just organized, concise information constantly updated and improved.
Was this helpful? Consider supporting me.

By Tony Hansen
Follow me on

Got a Question?

Buy me a coffee and we can talk