The surface tension of clay slips leads to an unpleasant de-mixing effect known as Casting Strike. While casting the stream of slip first strikes the mould and the stream of slip becomes surrounded with a water membrane containing the finest clay particles. Wherever this impinges on the mould water is removed and the fine clay remains in the form of a wraith round the slip stream.
The fine clay usually contains Fe+ ions which after firing gives out a coloured mark. The slowly poured slips rising in the mould leads to parallel and horizontal relief lines created by the surface tension membrane.
