A method of mural painting, intended to resist the effects of damp and pollution, that had a brief vogue in the 19th century. It is essentially a variation of fresco: after the paint has been applied to the plaster, it is coated with a solution of water glass (potassium silicate or sodium silicate), which provides a protective film when it dries. As water glass is strongly alkaline it can be used only with certain pigments. Some of the mural paintings in the House of Lords (see Maclise) were executed in the medium, because it was thought that they would be proof against the damp and dirty atmosphere of London, but they deteriorated within ten years. Later the process was improved, but because of its complexity and limitations it never became popular.