The invention relates to an improved method and apparatus for the treatment of animal skin substrates which employs a multiplicity of solid particles. There is disclosed an apparatus for treating one or more animal skin substrates with a multiplicity of solid particles and treatment liquor comprising: a rotatably mounted drum comprising an internal volume having a first portion defining a treatment volume for retaining said animal substrates and a second portion defining a collecting volume; one or more inlets for introducing solid particles and treatment liquor into said treatment volume; a partition separating said treatment volume from said collecting volume wherein said partition allows free passage of solid particles from the treatment volume to the collecting volume and prevents the passage of the substrates therethrough; a circulation path for conveying said solid particles from the collecting volume to the treatment volume; and a pumping device arranged to pump solid particles and said treatment liquor along a portion of said circulation path.