Thermite Welding

Write out Fe₂O₃ + 2 Al → Al₂O₃ + 2 Fe. Confirm it balances. Everything that follows depends on this equation being right.

Record the standard enthalpies of formation: Fe₂O₃ at −824 kJ/mol and Al₂O₃ at −1676 kJ/mol.

Subtract reactant from product: −1676 − (−824) = −852 kJ per mole of reaction. Negative means heat is released.

Weigh 8.0 g iron oxide and 2.7 g aluminium, the 3:1 mass ratio the reaction needs. Keep them in separate labelled dishes. They are never combined.

Check your weighing against the formula masses: 160 g Fe₂O₃ to 54 g Al is 2.96:1. Your scale should agree to within a few per cent.

Divide the 852 kJ between the products. There is enough to raise the iron past its 1538 °C melting point and well beyond — which is the entire point.

Compare your estimate with the accepted adiabatic figure of about 2500 °C. Note the difference and where your assumptions lost heat.

Note the appearance and density of each powder separately. Return both to their containers. Wash hands before continuing.

Sketch the field mould: a sand crucible above, a tapping plug, a two-part refractory mould clamped around the rail ends, and a riser to collect slag.

Compare the densities: molten iron near 7 g/cm³, alumina near 3.9. The alumina floats clear of the joint on its own, so the weld needs no flux.

Rail ends are torch-preheated before tapping. Heat a steel bar with the propane torch and record how long it takes to reach a dull red, to feel the scale of the task.

List what thermite needs on site: no electricity, no gas bottles, no generator. That is why it welds rail in the middle of nowhere and nothing else does.

Thermite cannot be put out. It supplies its own oxygen from the iron oxide, so smothering it does nothing. Water is actively dangerous: it is flashed to steam and dissociated by molten iron, and the hydrogen released can detonate. Sand does not extinguish it either; it only contains where the melt goes.

The ignition temperature is the trap. The mixture is genuinely difficult to light — which lulls people into using stronger and stronger igniters until one works, at which point they are standing over it. Industrial practice uses a remote igniter and a clear exclusion zone.

The molten product goes down. It will burn through steel plate, concrete and any floor beneath. There is no laboratory-scale version of this that is meaningfully safer, which is exactly why the reaction is calculated here and not performed.

Even the reagents deserve respect. Fine aluminium powder is a dust explosion hazard in air. The two components in this blueprint are weighed and examined separately and are never brought together.

DRP 96,317, “Process to manufacture metals and alloys”, granted to the Goldschmidt company of Essen on 13 March 1895, on Professor Hans Goldschmidt's formula. DRP 116,400 of 1899 is the one that matters to welders: aluminothermic butt welding of rails. German Reichspatente of this era are not digitised as page images the way American patents are, so the drawings are not available online — the text and the dates are.

Goldschmidt was not trying to weld anything. He wanted carbon-free chromium and manganese. Carbon reduction, the standard route, leaves carbon in the product and ruins it for some alloys; aluminium reduction leaves none. The violent heat was the obstacle he had to control, and only afterwards the product he sold. Inventions that arrive sideways like this are a recurring pattern — cemented carbide came out of a lamp factory looking for a wire-drawing die.

Why railways still use it in 2026. Continuously welded rail has to be joined in place, often kilometres from a road. Thermite needs a crucible, a mould, a bag of powder and a preheating torch, and it delivers a full-section weld across a 70 kg rail profile in about half an hour. No portable arc process matches that. The reaction has been doing the same job for over 125 years.

The other uses, stated plainly. The same reaction has military and sabotage uses, and it is used for cable-joint welding in earthing systems. That is one reason this blueprint teaches the thermodynamics and the rail application rather than a procedure.