English: After all soluble material in the clay has been dissolved in hydrochloric acid, the insoluble material, largely silica sludge, is removed in the filter press (above). The filter cake is blown dry with compressed air and washed with water. The acid filtrate and washings are then evaporated in a still to smaller volume, so that less hydrochloric acid gas will be required for eventual precipitation of the aluminum as the hydrated chloride. Note the glass piping (upper right) used because of the corrosive effect of acid vapors on metal. The hydrochloric acid process for the production of alumina from was developed at the National Bureau of Standards during the war, when bauxite imports were cut off and it became necessary to utilize domestic sources of aluminum.