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 submit and 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. But as a way to understand the utility of a clay for a specific purpose, this contextless report is of little use. It 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?

Factory technicians often just see clay materials or bodies as data sheets full of numbers. Similarly, 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 characteristics. I make coffee mugs, doing it the same way each time. Notice what these mugs tell me. The rims have pulled oval, the color is dark and solid and the foot is plucking: The body is too vitreous. Sandy particles can be felt on the rims (also felt during trimming and throwing). 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 on the convex side. The inside glaze is also revealing that iron speckle is excessive.

Drying cracks in bricks - why is this happening?

These bricks are being extruded in India and the plant is suffering drying cracks. A consultant recommended the 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 the value of such becomes evident - that knowledge provides direction when things like this happen. It would answer some questions. 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 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 a drying process that is not engineered 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 round corners would be better. Given the location and economic realities, the change to square holes might be enough to ignore all the other issues and get away with it.

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.

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