How Blowtorches Work

By: Jane McGrath

A welder wielding his trademark tool. See more power tools.
A welder wielding his trademark tool. See more power tools.
Antony Edwards/Getty Images

No tool is quite as cool or as fun to watch as a gas blowtorch. The showers of illuminated sparks that shoot from a blowtorch at work are mesmerizing. But don't let the pyrotechnics draw you too close -- these sparks actually are small pieces of molten metal that spit off during cutting and welding.

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A blowtorch produces an exceptionally hot flame and harnesses that heat to mold and cut metal. It does this by using highly combustible gases in the right proportions, which means that it is also a dangerous tool. Improper pressures, mishandling of the equipment, excessive sparks and messy workshops can all lead to fires, or even fatal explosions.

The invention of the blowtorch in the late 19th century thrilled pyromaniacs everywhere and revolutionized the process of welding. The earliest known evidence of welding, or connecting metal pieces by applying heat, dates back more than 3,000 years. By the Middle Ages, the process was significantly developed, and blacksmithing was a common profession. A blowtorch made it easy to cut and weld metal more precisely by focusing a high temperature on a small point.

Welders and plumbers are not the only ones who wield these fire-breathing machines. Regular folks may keep them around the house for various tasks including lighting cigars, mending jewelry and even cooking. In this article, we will discuss the components and fuel of blowtorches and how to handle one safely.

Blowtorch Components

Components of a blowtorch
Components of a blowtorch

Gas blowtorches, even professional ones, are fairly simple constructions. Their basic design has not really changed in the past century. A professional blowtorch usually consists of a cylinder of fuel gas, a cylinder of oxygen, two gas regulators, two hoses, the torch and the tip.

Regulators on the cylinder heads control the pressure and flow of the gas. Adjusting them to the appropriate pressures is essential to preventing explosions. A regulator has two gauges: the high-pressure gauge reads the gas pressure in the tank, and the low-pressure gauge reads the gas pressure for delivery to the hose and torch. Learn more about pressure and how to measure it in How Tire Pressure Gauges Work.


The hoses that connect the steel cylinders to the torch are made of strong, flexible material, usually rubber. To avoid confusion, the hose containing oxygen typically is green, while the fuel-gas hose is red.

The torchhead has two needle valves, which are circular knobs that control the flow of either the oxygen or the fuel gas. The tip attaches to the torch head, and people change it according to the task being performed and the gas used. When a blowtorch is used for cutting metal, an oxygen pipe sends an additional stream of oxygen directly to the center of the flame, increasing its intensity. Finally, an igniter, or pilot light, much like the one on a gas stove, actually lights the torch. It's a bad idea to ignite these torches with a cigarette lighter or a match because of the dangerously high temperature of the flames. 

What prevents this thing from blowing up in your face? Well, the following attachable safety devices usually come standard on blowtorches sold today, but can also be sold separately.

  • Check valves attached to the regulators and the torch head will help to prevent the gas from flowing back through the hose or cylinder. The pressure in the hose should always be greater than the pressure in the torch head. If this changes and the pressure in the torch head surpasses that of the hose, then the check valve closes, shutting off the gas supply.
  • The flashback arrestor fastens to a regulator and typically consists of both a check valve and a flame barrier, which allows gas through but not flame. These arrestors prevent flashbacks, which we'll discuss in the Lighting Up section.

Next, we'll take a close look at the different gases blowtorches consume to attain these high temperatures.

Fueling the Fire: Blowtorch Gases

Close-up of oxygen cylinders
Close-up of oxygen cylinders
Ben Edwards/Getty Images

Blowtorches can use different gases depending on the kind of work done. Professional gas torches use acetylene or MAPP gas to mix with oxygen, while smaller, home torches typically use either butane or propane.

Let's start with acetylene. Although highly effective, acetylene (C2H2) is a volatile fuel gas. The volatility stems from acetylene's triple carbon bond. This kind of bond can hold a considerable amount of energy that releases when ignited. However, the bond's unstable nature means it can suddenly explode unless it is kept at the proper pressure (between 15 psi and 29.4 psi, where psi stands for pounds per square inch). Even sudden bumps and small shocks can cause an explosion. Dissolving acetylene in acetone helps keep it more stable, but it is still dangerous and must be stored properly; that is, upright, otherwise the acetone can came come in contact with and harm the other equipment, such as the regulators, hoses or torch head.


Compared to acetylene, MAPP gas, which is made up of methylacetylene and propadiene, is much less dangerous. Unlike acetylene, MAPP gas is not prone to explosion if the cylinder is hit or bumped. It also can withstand higher pressures, which means it's good for underwater work, like repairing ships. Although MAPP gas flames cannot burn as hot as those of acetylene, some claim that it meets or exceeds the performance of acetylene in welding [source: Fundamentals of Professional Welding].

Because oxygen is needed to sustain any flame, it also plays an essential role in the use of all blowtorches. But why, if the gas is naturally present in the air, is a cylinder of compressed oxygen necessary? Because without it, neither acetylene nor MAPP gas would burn nearly as hot as they do. Oxygen acts as an accelerant, which means that it helps the fuel to burn at a higher temperature.

Welding torches typically use oxygen and acetylene (hence the name "oxyacetylene torch") because together they produce flames ranging from 5000 degrees Fahrenheit to 6000 degrees Fahrenheit (2760 degrees Celsius to 3316 degrees Celsius). In fact, the oxyacetylene combination produces hotter flames than any other gas combinations. Adding pure oxygen to the flame increases the performance of acetylene by more than 1000 degrees Fahrenheit (538 degrees Celsius), and that of MAPP gas by more than 1500 degrees Fahrenheit [source: Bernzomatic].

Considering the blistering flames, it's important to know what you're doing before you decide to light up a blowtorch. In the next section, we'll take a look at the safety measures involved in starting one.

Blowtorch Safety

Before turning on a professional-grade blowtorch, welders should put on a few protective items, like gloves, an apron and, most important, goggles or full face masks with tinted lenses (because of the intense brightness of the flame). In addition, ensuring good ventilation, having a fire extinguisher handy, frequently checking for leaks and wearing clothes without grease or oil stains are critical steps in preventing injuries. Even better: wear flame-retardant clothes and hard-toed shoes.

You'll be grateful for this layer of protection if something goes wrong while you're using a blowtorch, like a backfire or flashback. Backfire occurs when the flame extinguishes with a loud pop. Insufficient gas pressure or touching the torch tip against the work causes these backfires, which, although startling, won't hurt you. Flashback happens when the flame retreats into the torch because of improper pressures or a clog. If you think flashback is occurring, immediately shut off both gas cylinders, as it is extremely dangerous.


A shower of sparks flying from a blowtorch workstation also can be trouble. Despite their beauty, an abundant amount of sparks can indicate problems. When the sparks sputter out excessively and far, this can mean that the gas pressures are too high or you may be in danger of causing a fires [source: Finch].


Lighting Up: Using a Blowtorch

This friction lighter will come in handy around step 11.
This friction lighter will come in handy around step 11.
Photo courtesy TIP Temperature Products

After gearing up, a welder follows certain steps to turn a blowtorch on and off. As different torches have different safety procedures, refer to the specific manufacturer instructions before operating a blowtorch. A step-by-step procedure for turning on an oxyacetylene torch goes like this:

  1. Turn the acetylene regulator screws out to allow gas through and adjust the working pressure of the gas.
  2. Open the acetylene cylinder slowly, but only a third to a half turn. This allows for the release of acetylene.
  3. Open the acetylene needle valve on the torch head and adjust the acetylene regulator to the best working pressure (about 5 psi).
  4. Close the acetylene needle valve. This stops the flow of acetylene so that you can start the oxygen safely.
  5. Turn oxygen regulator screws out so that oxygen can flow and you can adjust the working pressure.
  6. Open the oxygen cylinder valve slowly until completely open. This allows for the release of oxygen.
  7. Open the oxygen needle valve on the torch head. At this point, you can adjust the oxygen regulator to attain the best working pressure (about 5 psi for welding purposes).
  8. Close the oxygen needle valve so that you stop the flow of oxygen and can later light the gas safely.
  9. Pause and test for leaks in any of the valves and hoses.
  10. Open the acetylene needle valve again so that the gas is released and it can light.
  11. If the blowtorch is not equipped with an internal spark ignition device, light the acetylene with a friction lighter held about one inch from the tip.
  12. Wait for the black smoke to stop and slowly open oxygen needle valve. Adjusting the oxygen needle valve affects the type of flame you get. A neutral flame is commonly preferred and is made up of equal parts acetylene and oxygen.

But to switch off the torch, you follow these steps:


  1. To switch off the torch, close the acetylene needle valve first, which extinguishes the flame.
  2. Close the oxygen needle valve. This cuts off the flow of oxygen.
  3. Turn off the acetylene cylinder valve and then turn off the oxygen cylinder valve. This stops the release of the gases from their cylinders.
  4. Open the acetylene needle valve until gauges read zero pressure and turn out the pressure adjusting screws on acetylene regulator. Close the acetylene needle valve. This drains any remaining acetylene pressure left in the hoses and torch.
  5. Open the oxygen needle valve until gauges read zero pressure and turn out pressure adjusting screws on oxygen regulator. Close the oxygen needle valve. This drains any remaining oxygen pressure left in the hoses and torch.

[Source: Oregon State University]

Intimidated by all those steps? You may want to try arc welding, which is eliminating the need for welding with gas blowtorches. Arc welding heats metal by means of electric discharge and is generally faster and more efficient than the blowtorch process since it focuses heat more precisely [source: Fundamentals of Professional Welding]. However, gas blowtorches still are used in certain fabrication processes, as well as in other capacities, such as plumbing, which we'll discuss next.

Other Uses for Blowtorches

Hot blowtorch soldering right-angle elbow to 15-mmcopper tube
Hot blowtorch soldering right-angle elbow to 15-mmcopper tube
© Price

Plumbers use blowtorches to solder and fix copper pipes. The word "solder" (pronounced "sodder," with a silent L) refers both to the process of joining the pipes and to the substance used in the process. To solder, a plumber heats the pipes and then applies solder material, which softens in the heat. As the solder melts, it flows into the gap between the pipes and creates a tightly fitted joint. To learn about a different method of soldering, read about cold heat soldering irons.

In addition to fixing leaky pipes, many people use smaller, handheld blowtorches for cooking, stripping paint and thawing frozen pipes. These simplified torches are relatively inexpensive. To produce a flame, they use pressurized fuel gas, like propane or butane. Rather than having a separate cylinder of oxygen, they rely on the oxygen in the surrounding air to sustain their flame. These kinds of torches can reach temperatures of 3200 to 3800 degrees Fahrenheit (1760 degrees Celsius to 2093 degrees Celsius), lower than those attained by oxyacetylene torches, but still plenty hot.


A blowtorch browning crème brûlée to perfection
A blowtorch browning crème brûlée to perfection
© Ferguson

Handheld butane or propane blowtorches can come in handy in the kitchen. Most commonly, cooks fire them up to caramelize the top layer of sugar on crème brûlée -- a custardy dessert -- add a glossy exterior to chocolate ganache or brown the top of lemon meringue pie. Their focused heat also works well for roasting vegetables and melting cheese. But remember, if you do keep a blowtorch for home use, it's always smart to have a working fire extinguisher and smoke detectors nearby.

In the past, blowtorches commonly were used for stripping paint. After being heated with a blowtorch, paint is soft enough for easy removal with a scraper or putty knife. However, several potential dangers can come from this practice. For instance, the flame can ignite the wood or dust on the paint, injuring the user. In addition, if you apply the blowtorch to a lead-based paint, it can release toxic fumes, which are dangerous to inhale. Because of these reasons, new and safer techniques have been developed for paint removal, such as electric heat guns, and the use of blowtorches for this purpose has generally been abandoned.

Similarly, some people grab their blowtorches to thaw out frozen water pipes. However, experts strongly discourage this practice as it is a safety hazard. If the water inside the pipe starts to boil, the pipe can explode. People have started fires trying to thaw out pipes this way [source: CBC News]. For more ideas on thawing frozen pipes, check out the related section of this article.

Although blowtorches can be a dangerous instrument in the hands of untrained amateurs, people continue to use them. If you're one of them, read more about related subjects, including how fire works, in the Lots More Information page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

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