The Eiffel Tower is made of puddled iron, not steel.

The Eiffel Tower was constructed between 1887 and 1889 using 7,300 tons of puddled iron sourced from the Pompey forges in Eastern France. Puddling was a metallurgical process refined by Henry Cort in 1784 that involved stirring molten pig iron in a reverberatory furnace. This manual stirring allowed oxygen to eliminate excess carbon, reducing the content from roughly 4% to less than 0.1%.The resulting material is nearly pure iron with small inclusions of slag, which gives it a fibrous structure similar to wood. This unique composition makes puddled iron much more resistant to corrosion and fatigue than the raw pig iron of the era. Because the metal is more ductile, it can deform slightly under stress rather than snapping, which is critical for a lattice structure standing 330 meters tall.Gustave Eiffel chose this material because Bessemer steel was still inconsistent in quality and significantly higher in price during the design phase. To maintain the iron's integrity, the tower is repainted every seven years with 60 tons of pigment to prevent oxidation. This maintenance routine, combined with the structural properties of puddled iron, has allowed the monument to far outlast its original 20-year permit.