The bowls may be unheated or one may be a thick-walled steel shell heated internally. All bowls may rotate at the same surface speed, or one highly polished and heated bowl may rotate at a higher surface speed than the rest. In certain specialised machines, e.g. for knitted goods, two adjacent bowls may be heated, or, in the case of a laundry calender, one bowl works against a steam chest shaped to the curvature of the bowl.