Intumescent flame retardants form a foamed-char barrier layer when exposed to a flame. Conventional systems are based on the acid catalysed dehydration of carbonifics such as dipentaerythritol. Metal oxides also have utility as catalytic flame retardants. For example, both antimony and tin have been used to impart flame resistance to cellulosics without any assistance from halogen compounds. Apparently they alter the condensed phase thermal degradation pathways in such a way that more non-volatile char and less flammable gases are generated. Recently it was discovered that potassium carbonate enhances the charring of polymers containing pentaerythritol-silica combinations as flame retardant.