This is from the small stock journal:
Dust Bathing
Dust bathing refers to the pattern of behaviour where chickens scratch in an area of fine, loose, dry soil and then takes a "bathe" in it - allowing the fine soil particles to trickle through the feathers. This is an instinctive action and helps to get rid of external parasites in the feathers and on the skin. Most freely ranging birds will find a dry, sunny spot where they can make their own dustbaths and use these repeatedly. However, if they are in a run where this is not possible, dust-bathing facilities must be provided.
A dust bath can be constructed using a large shallow box filled with fine sand or earth. The addition of ash from the fire provides extra fine material that is ideal for dust bathing - but make sure it is completely cold first!! This dust bath should be placed under cover or provided with its own roof, since it is more or less useless when wet.
Start with a bucket of sand, add a bucket of ash. Adjust quantities as needed. Ive even heard of ash only in confined runs as a dust bath.