a sand collection system includes a column of coarse sand within a cylindrical container coaxially mounted to the output end of a furnace conduit through which heated gases containing sublimate will pass . a layer of gas permeable material holds sand in position and an aperture in its center assists in directing gases generally to the center of the sand column . the sand column is spaced away from the furnace a distance such that the temperature differential will cause condensation of the heated gases on the surfaces of the sand particles .