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Slake, Slaking

'Slaking' refers to the breakdown that normally occurs when you immerse dried clay chunks or lumps in water (damp or wet lumps will not normally break down in the same manner because the wet clay resists the penetration of water). Typically the water attacks the surface and particles simply fall away. When slaking is complete a pile of fine material will be found settled on the bottom of the container of water, power mixing will then produce a slurry. Clays that slake well will break down in minutes if chunks are less than about 1 cm in size and all have exposure to the water. Very plastic clays may not slake since the wetting of the surface will cause swelling and act as a barrier to further water penetration.

The slaking phenomenon makes it possible to slurry a raw clay mix and screen impurities from the slurry and then dewater, all without the need of grinding equipment.

Slaking can be prevented by adding a hardener to a clay. Xantham gum, corn starch, polymers, sodium silicate are examples. Hardeners are commonly used in earth plaster and when making rocket stoves and cob ovens. Self-hardening clays used in schools also have hardeners added. Hardeners affect the plasticity and workability of the clay, often quite dramatically (even for small additions).

Slaking. What is that?

A slaking clay (a typlical potters clay). On the left the clay bar has been in the water for around 10 seconds. On the right, after a couple minutes, the rate of slaking has increased dramatically. Within about 5 minutes this bar will disappear into a pile on the bottom. Slaking happens most quickly when the sample is completely dry and the clay has low plasticity. Very high plasticity clays can take hours to slake. Slaking can be reduced and even eliminated by the addition of a hardener (like Xantham gum).

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By Tony Hansen

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