Abstract: A method of making a cast hopcalite filter. The filter is formed by adding to "green" hopcalite a given weight of ceramic fibers. Additional water may also be added as required. This mixture is blended briefly in a high speed blender and then poured into a mold. Water is drained by vacuum through a filter paper in the bottom of the mold, preferably with the aid of a vacuum. The wet preform is then placed in a high temperature (600.degree. F. to 650.degree. F. ) oven for approximately 90 minutes for calcining. In this process the basic catalyst material saturates the fiber matrix before drying and transformation into its final chemical state.