Will soil testing help you assess a wild clay for pottery?

No, soil testing is not helpful. Soils normally contain clay but it is so diluted with sand, rocks, silt and organics that overall plasticity is just a dot on a graph - not even close to what modelling or throwing clays exhibit. Pottery clays easily hold a shape and can be adjusted to a new shape without splitting. They dry slowly with substantial shrinkage. Highly plastic clays need more water to achieve working consistency, silty non-plastic ones need less (typical pottery clays need 18-23%). The reports shown here are typical for soils. But almost nothing here would look familiar to a potter.

-The Shrinkage Limit (SL) is the water content where further loss of moisture will not result in any more volume reduction.
-The Plastic Limit (PL) is minimum water content at which a soil is considered to behave in a ‘plastic’ manner, i.e. is capable of being moulded.
-The Liquid Limit (LL) is the maximum water content a silt or clay can have before becoming a liquid, i.e. turning into mud.
-The Plasticity Index (PI) is the range of moisture contents where the silt or clay remains plastic (PI = LL – PL).

Potters don't care about the amount of water needed, they care about how plastic the clay is once enough water has been added to get the right stiffness.

Related Pictures

Clay lab report. Is this really what you need?

If you are trying to use local clays for brick or tile or even pottery production, characterizing the available materials is the first step. But how? This is the kind of data a lab might return - perhaps you wonder about its value? We feel traditional ceramics technology is fundamentally relative. A history of many reports like these, in context with other data, might be good for mining companies to determine if new stockpiles have any shifts in certain specific properties. Or a tile company evaluating a new ball clay. But as a way to understand the utility of a clay for a specific ceramic purpose, this contextless report is of little use. For example, the physical properties, the whole reason for using a clay, are unrelated to the chemistry. This is also a tunnel vision view, looking at only one temperature. On the other hand, simple procedures, like the SHAB test, provide a hands-on way to understand what a clay actually is.

These were just lumps - but I gave them meaning

This clay is exceptional in multiple ways, it clay is from Flintoft, Saskatchewan. It holds together in lumps (center picture) but when broken its sandy nature becomes clearly visible. Other sandy clays in the area are similar but when water is added this one is different: It becomes plastic, plastic enough to form well (notice the texture of the plastic material in the closeup photo on the upper left). And it dries quickly with low shrinkage (the drying test disk upper right shows perfect performance). This combination of properties is what brick makers look for. No wonder that a clay similar to this nearby was used at the Claybank brick plant for 75 years.

I ran a series of physical tests to characterize this material (data shown lower left), that brought to light other good properties:
-It is super refractory, a fireclay. The SHAB test bars (lower right from cone 10R and 10 down to 6 oxidation) correspond to the SHAB test results in the chart. Even at cone 10 this has an amazing 19% porosity. With almost no shrinkage.
-The top bar is reduction-fired yet barely darker than the one below it at the same temperature in oxidation. This indicates low iron content.
-This is low soluble salts. Even though they concentrate on the outer edge of the DFAC test disk (upper right), that part fires only slightly darker at cone 10R (inset).
-Centre-bottom: G1947U clear glaze on it fired at cone 10R. It is neither crazing or shivering. This is unusual.

All of this information is preserved in our Insight-live account. Not shown are all the picture-specific and general notes I took. I compared this with about 10 other clays, doing the same for all of them, preserving a treasure trove of data that enables comparing all of them side-by-side.

Links

Materials	Ball Clay A fine particled highly plastic secondary clay used mainly to impart plasticity to clay and porcelain bodies and to suspend glaze, slips and engobe slurries.
Materials	Kaolin The purest of all clays in nature. Kaolins are used in porcelains and stonewares to impart whiteness, in glazes to supply Al2O3 and to suspend slurries.
Materials	Bentonite Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
Articles	How to Find and Test Your Own Native Clays Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.

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