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.

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Bricks and tiles are cracking out of the kiln. How to solve this problem?

With data. Consider all the possible reasons. Die design not knitting clay together around floating elements; faulty deairing on the pugmill; clay too short to pug through the complex die, too soft or too stiff, lacks fine particle sizes, lacks dry strength; uneven or incomplete drying; too many sharp concaves or uneven thicknesses on the cross-section; kiln setting does not enable heat exposure to all surfaces; firing too uneven or too fast. Most of these properties can be measured, photographed and logged, and changes will thus be noticed. At a minimum, the SHAB test, LDW test and SIEV test and associated notes and photos will accumulate data to assist. These tiles are being made in the oldest city on earth, from the same clays as ancient Sumerian’s made their ziggurats!

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.

Links

Troubles	Cracking of Clays During Drying The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.
Glossary	Physical Testing In ceramics, glazes, engobes and bodies have chemistries and physics. To fix, formulate and adjust their relative importances in each situation need to be understood.
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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