Abstract: An improved inorganic fiber mat containing mineral wool, preferably a composite mat of mineral wool and fiberglass, is prepared by:(a) forming an aqueous slurry of the inorganic fiber raw material containing non-fibrous, particulate contaminants;(b) agitating the slurry to separate heavier contaminant materials;(c) passing the slurry through a unique cleaning apparatus whereby the moving slurry is cascaded and subjected to air induced hydraulic turbulence to separate any remaining contaminant materials; and(d) thereafter transferring the fibrous material, preferably as an admixture of cleaned mineral wool fibers and glass fibers, to a mat forming device.