Cobalt can also be leached from the slag of copper smelting. Heating this material with silica in flash smelting removes much of the iron as slag. Wrought iron was a soft material that contained slag or "dross". Workers then repeatedly beat and folded it to force out the molten slag. Beryllium may be found in coal slag. Steel furnace slag is also air-cooled. Smelter slag and other residues contain significant quantities of metals. The silicate slag is a useful construction material. These oxides either escape as gas or form a solid slag. Silica is also added, resulting in the production of calcium silicate slag. This was due to the erosion of the furnace lining by the liquid slag. The aluminium oxide slag that it produces is discarded. The included slag in wrought iron also imparts corrosion resistance. TiO is produced by digesting ilmenite (FeTiO) or titanium slag with sulfuric acid. Only granulated slag (i.e., water-quenched, glassy slag) is effective as a cement component.