Abstract: A flame-proof product comprising a cellulosic base, and particularly a wood based product having at least one surface treated with a composition which when the surface is subjected to fire or high temperatures combines with some of the material of the base to form a protective intumescent coating on the surface which insulates the remainder of the product from thermal destruction. The treating composition includes potassium hydroxide and/or potassium carbonate alone or mixed with a comparatively small amount of sodium hydroxide and/or sodium carbonate. The composition may in addition include potassium chloride, or potassium acetate, or potassium citrate, or potassium bicarbonate, or potassium bisulfite, or potassium metaphosphate, or potassium nitrite, or potassium sulfide, or potassium sulfite, or potassium thiosulfate, or potassium phosphate (tribasic).