Abstract: A filter medium is formed by preparing a pH 3-5 aqueous suspension of cellulose fibers in a pulper or beater with a concentration of between 3% and 5%. A pH 2.8-3.8 aqueous suspension of glass fibers is prepared in a pulper or beater with a fiber concentration of between 0.5% and 1.2%. A first pulp consisting exclusively of cellulose fibers is prepared, to which a moisture-resistant resin is added. A second pulp is prepared consisting of cellulose fibers and glass fibers in proportions of between 30% and 70%, to which a moisture-resistant resin is added. Two superposed layers of the pulps are formed by methods used in paper manufacture. The two layers are collected on a layer of material able to absorb water. The two superposed layers of pulp are pressed to a pressure of between 0.1 and 1 kg/cm2. The resultant sheet is dried to a moisture content of 1%. The dried material is impregnated with a 50 wt % solution of acrylic latex, and dried again. The dried material is then wound on a reel.