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.

Related Pictures

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 - 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.

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.

Got a Question?

Buy me a coffee and we can talk