Chickens love to take dust baths! They dig a shallow hole, loosen up all the dirt, and proceed to get themselves as dirty and dusty as they possibly can. They will often lay in the dirt on hot days to cool off.
Dust baths are a necessity for chickens. They prevent parasites such as mites and lice from finding a home in your chickens' feathers and legs.
If your chickens aren't free-range or their run area doesn't have a dry patch of ground where they can dig a hole, you'll need to provide them with an artificial dust bath. Place a box on the floor of their coop and fill it with 6" of a dusting powder. A typical recipe for the dusting powder: 1 part fireplace ashes, 1 part road dust, 1 part sand and 1 part diatomaceous earth.
Diatomaceous earth is a chalky power derived from the fossilized remains of microscopic plankton. Although not dangerous to the touch, diatomaceous earth contains silica, which, if inhaled, can lead to respiratory illness. If you choose to use it, make sure you research the dangers for you and your chickens.