wood can be rendered fire resistant by impregnating the wood with a solution of a monomer , such as a phenol , and a fire - inhibiting salt , drying the wood and thereafter treating the wood with a solution of a second monomer , such as formaldehyde , polymerizable with the first monomer . heating to dryness causes polymerization of the monomers . the fire resistant properties of wood treated this way are not affected by exposure to moisture . prior to the first impregnation , after the first impregnation and after treatment with the second monomer the wood being treated is dried to a moisture content below about 4 %.