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.

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I notice small details about the clays I test? Want to know why?

Knowing the statistics of a pro sport can make one feel like an expert. But there is no substitute for actually playing! Likewise, taking the time to actually slurry up a clay body and hand-make and fire ware using it is the way to really notice it’s physical and firing characteristics. Engineers call this characterization of a body. I make coffee mugs, doing it the same way each time - so I notice differences. Notice what these cone 10R ironware mugs tell me. The rims are warping oval, the color is dark and solid and the foot is plucking: The body is too vitreous. I felt excessive coarse particles during throwing and trimming - and even on the fired rim. When pulling the handles splits formed, indicating low plasticity (this was also evident during throwing, I was unable to pull the walls as thin as normal). Dry strength is low, the lower handle-join dry-cracked along the convex side. The inside glaze is also revealing very high iron speckle (likely from the coarse particles).

Drying cracks in bricks - but no data to determine best response

These bricks were being extruded in India and the plant was suffering drying cracks. A consultant recommended a high percentage addition of lignosulphonate, at a cost of $800/ton, as a solution. But before adding such a large expense, some obvious changes seemed in order first. The technician knew the plasticity index of the clay (a measurement used for soils) but he did not have records of its drying shrinkage, water permeability, drying strength or drying performance - when problems like this arise such data provides direction and help answer questions. For example, is cracking happening because of lack of drying strength or plasticity or because drying shrinkage is too high. The splitting along the corner of the extrusion is a clue that plasticity could be lacking - that could be solved by a small bentonite addition or reduction in grog. If permeability is low an increase in grog might be needed (if the pugmill can still extrude slugs with a smooth edge and corner). Notice the cracks that start from those splits (lower left). But also notice how the top edge has shrunk while the center section has not. That indicates the drying process is not tuned to subject all surfaces to equal airflow (sure enough, these are being dried outside in the sun and wind). Another factor is cross-section: The round holes create variations in thickness that exceed 300%, square holes with rounded corners would be better. Given the location, economic realities and past success this one change might be enough to make a big difference.

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.

Links

Glossary	Characterization In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.
Glossary	Brick Making Brick-making is surprisingly demanding. Materials blending and processing, forming, drying and firing heavy and thick objects as fast as possible are like no other ceramic manufacturing challenge.
Articles	Simple Physical Testing of Clays Learn to test your clay bodies and clay materials and record the results in an organized way, understanding the purpose of each test and how to relate its results to changes that need to be made in process, recipe and materials.

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