An acetylene torch is a handheld tool that mixes acetylene gas with oxygen to produce a flame hot enough to cut, weld, and braze metal. The flame reaches approximately 3,100°C (5,612°F) when burning with pure oxygen, making it one of the hottest fuel-gas flames available. Also called an oxy-acetylene torch or oxyfuel torch, it has been a staple in metal fabrication, auto repair, plumbing, and construction for over a century.
How the System Works
An acetylene torch isn’t just the handheld piece. It’s a complete system built around two separate gas cylinders: one containing acetylene fuel and the other containing pure oxygen. Each cylinder connects to its own pressure-reducing regulator, which steps down the high pressure inside the tank to a safe, usable working pressure. From the regulators, color-coded hoses (red for acetylene, green for oxygen) carry the gases to the torch body, where the operator controls the mix using two separate valves.
When both valves are open and the tip is lit, acetylene provides the fuel while oxygen dramatically increases the flame temperature. By adjusting the ratio of the two gases, you can change the flame’s characteristics to suit different jobs. The torch tip is interchangeable, and selecting the right size for the work is critical. A tip that’s too small for a thick cut will starve for gas and cause problems.
Three Types of Flame
The balance between oxygen and acetylene at the torch tip produces three distinct flame types, each suited to different work.
- Neutral flame: Equal parts oxygen and acetylene. It produces a clear, well-defined inner cone and is the go-to setting for welding most steel and iron. It doesn’t add or remove anything from the metal.
- Carburizing flame: Excess acetylene. You’ll see a feathery secondary cone around the inner one. This flame adds carbon to the metal surface, which is useful for hard-facing and welding high-carbon steel.
- Oxidizing flame: Excess oxygen. The inner cone shrinks and the torch makes a noticeable hissing sound. It works well on brass and bronze but will make steel brittle, so it’s used selectively.
What Acetylene Torches Are Used For
The oxy-acetylene torch is one of the most versatile heat tools in metalwork because the same basic setup handles several different jobs. Welding is the classic application: the flame melts two pieces of metal together, sometimes with a filler rod added to the joint. Brazing uses a lower temperature to bond metals with a filler material that melts below the base metal’s melting point, which is useful for joining dissimilar metals or thinner pieces.
Cutting is where the torch really stands out. A specialized cutting attachment adds a lever that sends a jet of pure oxygen into the heated metal. Once the steel reaches a bright cherry red, that oxygen stream reacts with the hot metal and blows the molten material out of the joint, slicing through plate steel cleanly. The torch also handles heating and forming, letting you bend metal by softening it in a specific area without melting it entirely. This makes it common in repair shops, pipefitting, and sculpture work.
Why Acetylene Burns Hotter Than Other Fuels
At 3,100°C with oxygen, acetylene outperforms propane (2,820°C with oxygen) and most other fuel gases. That 280-degree advantage matters in practice because it means faster cuts, quicker welds, and the ability to work on thicker material. In open air without supplemental oxygen, the gap widens further: acetylene burns at about 2,400°C versus propane’s 1,980°C. This is why acetylene remains the preferred fuel for precision cutting and welding even though propane is cheaper and more widely available.
How Acetylene Is Stored Safely
Acetylene has a unique and dangerous property: in its pure gaseous form, it becomes unstable and can violently decompose at pressures above 15 psi. That means it can detonate from pressure alone, without any spark or ignition source. This is why you’ll never find acetylene simply compressed into a tank like oxygen or nitrogen.
Instead, acetylene cylinders are filled with a porous material, similar to a dense sponge, and then saturated with a solvent, typically acetone. The acetylene dissolves into the acetone and distributes evenly throughout the porous filler. This combination eliminates any open voids where dangerous pockets of gaseous acetylene could collect. It also allows the cylinder to hold far more acetylene than could safely be compressed into the same space without the solvent. Cylinders range from 3 to 60 liters in internal volume and are pressurized to a maximum of about 275 psi at 68°F, well above what pure acetylene could tolerate, because the dissolved state keeps it stable.
This is also why acetylene cylinders should always be stored and used upright. Tilting or laying them on their side can cause the liquid acetone to flow into the regulator and hoses, contaminating the gas supply and creating a hazard.
Safety Devices in the System
Two key safety devices protect against the most dangerous failure in an oxy-acetylene system: flashback. A flashback occurs when the flame travels backward through the torch and into the hoses or even the regulators, which can cause an explosion. Flashback arrestors are small cylindrical valves installed on the outlet of each regulator, on the torch inlets, or both. They contain a mechanism that instantly blocks gas flow if the flame reverses direction. Most modern setups use combination flashback and check valves at these points.
The regulators themselves are also gas-specific by design. Oxygen regulators only fit oxygen cylinders, and acetylene regulators only fit acetylene cylinders. The hose fittings use different thread directions for each gas, making it physically impossible to accidentally connect the wrong hose to the wrong regulator.
Lighting and Shutting Down
The startup and shutdown sequence matters more than most beginners realize, because doing it wrong can cause backfires, sooting, or worse. Before opening any cylinder valve, you first make sure the regulator adjusting screws are fully released so no gas rushes through when the tank opens. When opening the cylinder valve, you stand so the cylinder body is between you and the regulator, never in front of or behind it.
To light the torch, you typically open the acetylene valve slightly and ignite it with a striker (never a cigarette lighter), then gradually introduce oxygen until the flame is properly adjusted. Shutting down is where opinions vary even among experienced welders. Some prefer to close the acetylene valve first, reasoning that the remaining oxygen clears soot from the torch internals. Others close the oxygen first, arguing that cutting the oxygen supply immediately stops any combustion inside the torch body and also reveals any fuel gas valve leaks, since a small flame would keep burning. Torch manufacturers sometimes specify a preferred sequence for their equipment, and following those instructions is the safest approach.
After closing the torch valves, you close both cylinder valves, then open the torch valves again briefly to bleed the remaining pressure from the hoses and regulators before closing everything for storage.