A process for the production of a polysiloxane containing mass incorporating surface treated filler materials comprising feeding materials to a change can mixer in which the materials are mixed; the change can mixer comprising a detachable mixing head assembly containing least two spiral mixing arms extending downwardly into the can of the change can mixer, drive means for rotating the spiral mixing arms around their own axis, and drive means for rotating the spiral mixing arms as a unit around the axis of the can and wherein the geometry of the mixing arms is such that they generate downwardly directing motion on the composition being mixed.