Ductile Tungsten Wire: Pressed, Sintered by Current, Swaged and Drawn Hot (Coolidge 1913)

Tungsten cannot be melted and cast, and sintered tungsten is brittle. Coolidge made it into wire anyway: purify the oxide, reduce it to powder in hydrogen, press a bar, sinter it white hot with a heavy current, then hammer it down hot in a swaging machine, a little at a time. By 30 mils (0.76 mm) the metal has become ductile at room temperature and can be drawn hot through diamond dies. Source: W. D. Coolidge (General Electric), US 1,082,933 (1913). We stop at 14 mils (0.36 mm). Finer sizes need cyanide for pointing the wire, which we leave out. The sintering vessel stands in a pool of mercury, and Coolidge warns that hydrogen can explode if air is drawn in. Picture: the patent's own drawings.