All Glossary

Clay Stiffness

In ceramics, clays exhibit plasticity in accordance with their recipe but also the water content. Each types of forming method has an ideal combination of stiffness and plasticity.

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Clay bodies used in machine forming must have a correct stiffness in order to optimize the forming process. Great effort is expended in the body mixing departments of factories to produce a product of standard stiffness and plasticity. Some processes (e.g. pressing) require clay that is so stiff a potter would need a hammer to move it. Others need clay that is so soft it can be scooped by hand (like cream cheese). Since the body is used soon after it is made, there is no need to worry about how the stiffness might change over time. However, companies that manufacture and sell prepared bodies (either for use in large-scale production or by potters) must carefully consider stiffness. This is because wet (or mud) bodies normally stiffen over time (very important if they won't be used for months or even years). Some bodies can soften during storage; this can happen for a range of reasons (e.g. slight solubility in one or more materials, the presence of electrolytes in the water, particle agglomeration, improper particle surface wetting during mixing), then stiffen.

In addition, it is common for bodies to appear to stiffen, but re-soften as soon as they are wedged or moved. Each body has its own specific water content percentages that correspond to specific stiffnesses. Plasticity variation is the main factor. Usually, the more plastic it is, the more water is needed for a target stiffness. The percentage of water needed for a given stiffness can vary (for the above-mentioned reasons and more), so when deciding how to run a body, production departments may put more weight on the feel of the material leaving the pugmill and use the water content as a secondary guide. Other producers, especially those who tightly control water pH, ball mill and filter press, have greater confidence that 24% water, for example, will always be the right stiffness.

Although there are a variety of devices employed to gauge stiffness, the usefulness of these is often questioned by experienced people (who prefer simply judging by feel).

Cracking problems with drying rise exponentially as the water content of pugged clay rises. So it is better to avoid softer clay unless you have experience using it for the intended application (e.g. with jiggering, the mold quickly pulls water). Using very soft clay on a potters wheel means not being able to throw as thin, as high or as true as would otherwise be possible. More trimming as leather hard will be needed.

The stiffness of clay bodies is sometimes the key enabler for the kind of ware that is made from them. Yixing teapots, for example, are not so much hand-built as they are assembled from precut geometric shapes. The very stiff clay must be flattened with a mallet to make the slabs.
That inconvenience pays off because the stiff clay holds its shape despite being used to make very thin-walled pieces. On the other extreme, clay of paste-consistency is used in 3D printing. And in the making of tandoori ovens, where it is mixed with all manner of fibrous materials to give it strength. And in throwing of gigantic pots in China (where many muscle bound potters throw a piece together).

Production departments benefit when they consider the plasticity of the clay as well as the stiffness. It may be that production issues (e.g. drying cracks, forming splits) are because the clay is not plastic enough, not because it is too stiff.

If you are a hobby potter and a clay has become too stiff (because of water loss) it can likely be softened (assuming the slugs are in plastic bags). Two methods are common.
-Make some small holes on one side of the bag and set it face down on some gravel in the bottom of a water-filled basin (enough water to go a little above the gravel level). If needed, make holes on the other side(s) and put them face down for continued softening.
-Pour the amount of water needed for a slug (e.g. 1 cup) into the plastic bag and tie it. Then immerse the bag in a 5-gallon bucket of water (filled high enough to exert pressure from all sides).
After a few days it should begin to soften.

If clay has become too soft, take the slug out of the bag and wrap it with cloths for a day or two.

You will, of course, need to wedge the clay well to prepare it for use.

Related Information

Is the clay too stiff to use? Maybe not.

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Clays of very high plasticity often stiffen during storage in the bag. This is Plainsman Polar Ice, it contains 4% VeeGum. This slug is like a brick, yet it will loosen up completely. But it is far too stiff to attempt wedging. However, simply throwing it on the floor a few times (turning it each time) will pre-soften it enough to be able to wedge. Then, before you know it, it is too soft and needs to be put on a plaster table to stiffen it before throwing.

This simple device continuously gauges clay stiffness

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Here is how the pugmill operators at Plainsman Clays gauge the stiffness of the clay coming out of the pugmill. The machined nylon roller is on a slant and weighted. The softer the clay the more lines show. When they are like this (5th line steady) the operator knows the water content is around 22% for this clay, Polar Ice. For each type of clays it is different. Stiffness at pugging must also compensate when the clay tends to stiffen or soften over time. Over the years they have tried many devices to measure stiffness, but this has proven the most reliable.

What causes a clay to split after throwing like this?

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These cracks have been drawn because we were unable to get this to happen in our studio - likely because one condition was not met. This is M340, it is made from minimally processed natural mined clay. While inherently very fine it does have some sand particles. Consider the combination of conditions we created to try to make this happen:
-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).
-Not wedging the clay (or wedging it well).
Had all these been true and the clay was also soft and thrown using a lot of water (rather than slip) it could have split. Clays made from 200 mesh industrial minerals can even endure all of that. But to make this very unlikely just do the opposite of the above: Stable shapes, thrown rims, slipping later, wedging well and use a stiffness matching the shape being made.

A home made moisture sensor for pugged clay

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This is an Arduino controller board that has been wired to a display and a moisture sensor probe (which is stuck into a piece of soft clay). The sensor outputs a voltage that varies with changes in the resistance encountered between the probes (according to the material being measured). In this case the controller is directly outputting the number being returned by the probe. A study is needed to compare outputs with actual measured water content, this data would enable changing the software to output the percentage-of-water directly. However this sensor is intended for soils, it is not giving repeatability and consistency for measuring the much higher water content found in pugged clay. So we need to find a better one.

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Joining rules are different When clay is soft and plastic

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