Recovery of heat for industrial processes was first widely used as hot blast in blast furnaces to make pig iron in 1828. Later heat reuse included the Siemens-Martin process which was first used for making glass and later for steel with the open hearth furnace . (See: Iron and steel below). Today heat is reused in many basic industries such as chemicals, oil refining and pulp and paper, using a variety of methods such as heat exchangers in many processes.  Multiple-effect evaporators use vapor from a high temperature effect to evaporate a lower temperature boiling fluid. In the recovery of kraft pulping chemicals the spent black liquor can be evaporated five or six times by reusing the vapor from one effect to boil the liquor in the preceding effect. Cogeneration is a process that uses high pressure steam to generate electricity and then uses the resulting low pressure steam for process or building heat.