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!

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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

Troubles	Dunting and Cracking of Clay Bodies During Firing Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.
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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