mud bath

Wikipedia has an entire entry on medicinal clays, though many are of the mud-bath variety.

Then after remounting, we were edged off the road into the mud again by a heavy transport lorry, and enjoyed a second mud-bath.

How thankful I should be to Judge North for trying to purify me in that mud-bath of rascality.

He walked in behind some willows and, as I discovered afterward, lay down in some water to take his mud-bath.