Unglazed earthenware, unglazed stoneware and biscuit porcelain
Using any liquid to clean unglazed earthenware risks dirt being absorbed into the body and causing stains. Although unglazed stoneware is more resistant to staining than earthenware, some fine stonewares can mark easily, for example red stoneware and Wedgwood black basalt are prone to watermarks.
To remove dust, brush lightly over the surface using a soft bristle brush (e.g. an artist's sable brush). It is a good idea to put tape over the ferrule (the metal band between the hairs and the handle) to make sure you don't accidentally scratch the surface. Avoid cotton wool because the fibres can snag on a rough unglazed surface.