The original Hydroton brand expanded clay is a unique, lightweight expanded clay aggregate made in Germany. This natural clay is mined, formed into pellets, then kiln fired at high temperatures to cause the clay to expand into tough, lightweight balls. Clay naturally has balanced capillary action, with an ideal surface structure. Hydroton brand expanded clay is semi-porous and generally does not float like other products can. For best results, rinse all expanded clay before use. It is reusable as long as it is thoroughly cleaned between uses. Size specification is 8-16 mm.