Abstract: In order to produce a decorative pattern on the surface of a layer of material in a soft plastic condition, the surface is whipped by the free end portions of a plurality of flexible thread-like members. The whipping action provides random depressions in the surface. The thread-like members are preferably fastened to the peripheral surface of a rotatable body which is spaced from the surface to be treated at a distance which is substantially smaller than the length of the thread-like members, and the surface to be treated is moved in relation to the rotatable body while this body is rotated. By this whipping action it is possible to make surface patterns imitating a slaty surface or the surface of other natural materials. Such decorative surfaces may be made on layers of materials which are cut into plates or blocks for use as building materials after firing, setting or hardening of the material. As an example, the decorative pattern or structure may be made on plates to be used as roofing plates.