The washcoat is the porous intermediate layer applied to the substrate of a catalyst, such as a ceramic or metallic honeycomb. It usually consists of metal oxides such as alumina, titania, silica or ceria-zirconia mixed oxides. The smooth substrate on its own offers little surface area. The fine pore structure of the washcoat multiplies the internal surface and provides space for finely dispersed precious metal particles or other active components. Its composition and thickness affect activity, light-off behaviour, temperature resistance and tolerance to catalyst poisons. Good adhesion to the substrate is important, as temperature cycling, vibration and flow forces place mechanical stress on the layer. Spalling or loss of pore surface through overheating permanently reduces performance. When catalysts are cleaned, methods and cleaning agents must therefore be chosen so that deposits are removed without attacking the washcoat.