All Glossary

Weathering of Clays

Weathering is the practice of leaving freshly mined clay exposed in outdoor stockpiles for months or even years before processing it. Although it appears that the clay is simply being stored, repeated cycles of wetting, drying, heating, cooling and freezing are gradually conditioning the material.

Freshly mined clay can contain large, dense lumps that are difficult to crush and slow to slake. As moisture leaves them, drying shrinkage creates cracks; later rain and freeze–thaw cycles penetrate and widen these. With time, even very hard lumps and concretions can fracture and crumble, making the clay easier to crush, grind, slurry and blend. Stockpile weathering also helps equalize variations within the mined material.

Water can dissolve soluble sulfates and other salts in the clay. During drying, some of these migrate toward exposed surfaces; subsequent rain and runoff can carry them away. The effectiveness of this natural leaching depends on the permeability of the clay and the construction of the stockpile—dense plastic clays may shed most rainfall, leaving the interior almost unaffected for years.

Weathering does not transform a poor clay into a good one, nor does it eliminate the need for crushing, grinding, blending and testing. It is a slow, low-cost preliminary processing step that uses natural weather cycles to make difficult raw clay more manageable and, in some cases, reduce problems associated with soluble salts.

Related Information

A stock pile of raw, unground ball clay

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Plainsman A2 ball clay. In spite of three times the normal rainfall this summer, its natural high impermeability made it shed almost all of the water. But there is a back story. When mined, this clay was quite wet, about 10% moisture - some of the lumps are microwave-oven-sized! The outer ones dried over a period of weeks and, in the process, fractured down to what you see here. But the inner section of the pile takes years to dry out. By now it has broken down so fine that a completely dry sample put into water will slake in minutes to produce a smooth slurry. We can thus say that the pile has been "weathered".

An ironstone concretion found in a quarry in southern Saskatchewan

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These iron-rich concretions occur in the Battle Formation across Saskatchewan. Plainsman Clays mines the lower part of the Battle Formation, just above the Whitemud Formation, where a 1–2 foot layer is stockpiled as A1 clay. This clay is added to reduction-fired bodies to provide speckle, color and plasticity. The A1 contains thousands of these concretions, ranging in size from lemons to toasters. When freshly mined they are extremely hard and difficult to break with a hammer. After weathering (repeated wetting and drying, oxidation, shrinkage, and freeze-thaw), they gradually break down into small fragments that can be crushed and dispersed during clay processing. These are the same Battle Formation beds exposed in the Eastend, Sask. area that yielded the Tyrannosaurus rex fossil “Scotty,” displayed at the T. Rex Discovery Centre there.

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