This term is using in ceramics to describe how a dry clay disintegrates when it is immersed in or exposed to water. Different clays have different slaking rates.
'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).
A slaking clay bar (of a typical pottery 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, the corners are disintegrating. Watching this process can be addictive! In about 20 minutes this bar will disappear into a pile on the bottom. Slaking happens most quickly when the sample is completely dry. High plasticity clays take longer (like this one). Slaking can be used to prepare clays for use: Dry the lumps, break them down using a hammer, put them in water, wait (less than 30 minutes for typical pottery plasticity), propeller mix, screen out impurities, then dewater to plastic state.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.
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