Abstract: In order to provide a steel door with a fire rating sufficient to satisfy the Fire Regulations in North America which require in the fire rating endurance test that the temperature of the unexposed side of the steel door not to rise by more than 250.degree. F. over a period of one hour, the steel door is provided with a high density bonded mineral fiber sheet as its core which is bonded to the inside surfaces of the steel sheets forming the outside surfaces of the door by a binder comprising a mixture of an alkali metal silicate and a mineral powder selected from aluminium trihydrate and wollastenite, the powder being present in an amount sufficient to cause said binder on being subjected to high temperature to intumesce thus protecting said sheet against the heat.

Abstract: The present invention provides a composition for use in forming a fire retardant core for a wooden door by curing and pressing said composition to form a sheet, said composition consisting essentially of (a) 30 to 75% by weight of an inert mineral filler selected from perlite and vermiculite; (b) 10 to 30% by weight of wood chips and (c) from 3 to 15% by weight of a binder which is a mixture of an alkali metal silicate and a curable phenolic resin, at least one of which is present in dry form.