The Wash brush is a wonderful watercolor brush used mainly for large washes. Where the Mop brush has a round ferrule, the Wash brush has a flattened ferrule resulting in a large Flat brush. There are also Oval Wash brushes which come with an oval shaped outer edge. These brushes are sometimes called Sky Wash brushes for painting just that, large watercolor sky washes. There are also Glaze wash brushes for glazing pottery covering large areas.