A common mistake, even among people who work in architecture, is believing the Eiffel Tower is made of steel. It isn't: it's puddled wrought iron, a metallurgically different material from modern steel, produced with a process that by 1889 was already considered technologically mature, but that steel — newly available on an industrial scale — would render obsolete within a few decades. Understanding the difference between cast iron, wrought iron and steel isn't a philological detail: it's the key to understanding why the nineteenth century builds the way it does, and why certain forms (slender columns, lattice trusses, great glazed arches) become possible only within a precise historical window.
Cast iron: stiff, brittle, compression only
Cast iron is an iron-carbon alloy with carbon content above 2%, produced by melting and casting into molds. It's hard and cheap to mass-produce, but intrinsically brittle: it fails suddenly in tension, with no plastic warning, because its microstructure contains flake graphite that acts as a fracture trigger. For this reason, cast iron's structural use in the nineteenth century is almost always limited to elements in pure compression — columns, arches — while tensioned elements (chains, tie-rods) remain the domain of wrought iron. Joseph Paxton's Crystal Palace, built in Hyde Park for London's Great Exhibition of 1851, is the manifesto of this logic: 3,300 prefabricated cast-iron columns, all the same modular section, support a wrought-iron lattice structure and over 300,000 panes of glass, assembled in just nine months. It's the first building in history designed entirely around standardized, bolted components, explicitly conceived to be dismantled and rebuilt elsewhere — which is exactly what happens in 1854, when the building is reconstructed at Sydenham.
Puddled wrought iron: ductile, costly, the material of the Eiffel Tower
Puddled wrought iron is obtained by removing most of the carbon from molten cast iron through the puddling process (manually stirring the molten metal in contact with air inside a reverberatory furnace), invented by Henry Cort in 1784. The result is a nearly pure, ductile iron, able to resist well in both compression and tension — but produced by a slow, manual, expensive process that limits its production scale. Gustave Eiffel chooses puddled iron for his 1889 tower not out of ignorance of the new Bessemer steel, already available, but for practical reasons: puddled iron had a mechanical behavior well known and documented by decades of railway bridges, while industrial steel was still perceived as a relatively new material for a structure of that scale and that public visibility. The tower's 18,038 pieces, joined by 2.5 million hand-driven rivets, are puddled iron's swan song in monumental architecture — within a few years, no one will build like this again.
The Bessemer converter and the turning point
Henry Bessemer's 1856 patent introduces a process that blows air through molten cast iron to burn off excess carbon in a matter of minutes, instead of the hours required by manual puddling — and it does so on a scale of tonnes, not individual pieces. The open-hearth Siemens-Martin process, developed in the following years, offers even finer control over the final chemical composition. It's from this double innovation that industrial mild steel is born, which within a generation completely replaces puddled iron in new construction: stronger, cheaper to produce in large quantities, and — decisive for architecture — available in standardized rolled profiles that make structural calculation more reliable and repeatable. The Galerie des Machines, built for the same 1889 World's Fair that hosts the Eiffel Tower, already uses true-steel arches with a 115-meter span — a record that will stand unbeaten for decades, and a clear signal of where structural engineering was headed in the twentieth century.
What makes this few-decade transition fascinating isn't just the technical replacement of one material with a better one, but the fact that nineteenth-century iron and cast-iron architecture — covered markets, railway stations, botanical greenhouses, Henri Labrouste's own Bibliothèque Sainte-Geneviève in Paris (1850) with its slender exposed cast-iron columns — had already invented an entire aesthetic language of structural transparency before true steel even made that language structurally more solid. Aesthetics often precedes the technology that will actually make it safe.