Oxy-Acetylene Torch

Note the question: why acetylene and not some other gas? The answer is a number, and you are going to measure it.

Write down the melting point of mild steel: about 1,538 °C. Any flame below that will never weld steel.

Lay two firebricks on a clear surface. Nothing flammable within two metres. Glasses and gloves on before you light anything.

Light the propane torch. Adjust until you have a sharp, clearly defined blue inner cone.

Place the thermocouple just at the tip of the blue cone. Record the maximum value. This is the hottest zone of the flame.

Move the thermocouple 10 mm away and read again. The drop is steep — which is why working distance matters so much.

Put the readings on graph paper. Compare your maximum with the melting point recorded in step 2.

Propane-air tops out near 1,900 °C. Oxy-acetylene reaches about 3,100 °C. Note the gap: it explains everything.

Record the welder's three settings: neutral (balanced), carburising (excess acetylene, white feather), oxidising (excess oxygen, short hissing cone).

Enrich the gas slightly. Heat a steel offcut and watch: the surface stays brighter, because the excess fuel consumes the surrounding oxygen.

Lean the gas off. Heat a second offcut and compare: scale forms far faster. That is chemical protection, or the lack of it.

Write down the conclusion: the torch does not only heat, it wraps the weld pool in a reducing zone. It is the first shielding gas in the history of welding.

Acetylene is unstable above about 1.5 bar. That is why the cylinders are not simple tanks: they are packed with a porous mass soaked in acetone, in which the gas is dissolved. An acetylene cylinder is stored and used upright; laid down, it lets acetone through into the regulator.

Pure oxygen is not a harmless gas. It does not burn, but it makes everything around it flammable. Never any grease or oil on an oxygen fitting: the contact can ignite spontaneously. And you never “dust off” clothing with oxygen.

Flashback. If the flame runs back up the hose you hear a sharp crack then a whistle. Shut the oxygen immediately, then the fuel gas. Modern sets carry non-return valves and flame arrestors — which is exactly the problem Fouché's injector solved.

Eye protection. The flame emits little ultraviolet but a great deal of infrared and an intense yellow glare: use proper shaded welding lenses, not sunglasses.

FR 325,403, “Gas torch”, Edmond Fouché, filed 18 October 1902, granted 28 April 1903. The granted title says neither “welding” nor “oxy-acetylene” — another gap between the popular name and the document.

The problem was not the heat, it was the flashback. It had been known since the 1890s that acetylene burned very hot. What nobody could do was stop the flame travelling back up the hose to the cylinder. Fouché's injector draws the acetylene along by the velocity of the oxygen jet: the mixture moves faster than the flame front can travel back through it.

Why this arrives in 1903 and not earlier. It needed cheap acetylene, and that comes from calcium carbide, whose industrial process dates from 1892 — Thomas Willson's patent, already published on Youblob. The torch is a direct child of the electric furnace.

What the flame won, and then lost. Into the 1950s oxy-acetylene welded car bodies, pipework and fuselages: it is portable, it needs no electricity, and the welder can see the pool. Inert-gas processes replaced it for welding because they heat a narrower zone and distort less. But oxy-fuel cutting was never dethroned: heating steel to red heat and then firing a jet of pure oxygen at it, which literally burns the metal away, remains the cheapest way to cut heavy sections.