These are torches. Oxy-fuel torches, to be precise. Propane is one fuel, but the hotter and more common fuel is acetylene. That makes these oxy-acetylene torches. They burn a mixture of acetylene and oxygen at 6,300 degrees F.
Some torches are for welding. In 1903, two French engineers discovered oxy-acetylene welding. When you weld, you essentially heat the metals until they’re liquid. You add filler with a stick of metal in your other hand. You push the pool of molten steel with your torch, adding fill, leaving behind a line of tiny waves. This line is called the bead.
Fact: Even though we can generally see a weld from the outside, because of the bead, the inside is invisible. In an ideal weld, you could cut across it and not be able to tell which piece of steel is which. They have become one.
The torches in this picture are for cutting. You can tell because of the levers, or triggers. These levers blast even more oxygen into the flame. The science is cool. First, you light the torch with a striker. You adjust the oxygen for the flame profile you want. A perfect flame is a thing of beauty. It doesn’t lick, it stabs. And it talks to you. You can hear its ghostly whispers if you listen.
The flame heats the steel till it’s nearly white-hot. Steel glows red, then orange, then white. White is very very hot, but you can get it close with a torch. Then, dump in the oxygen. At this point, the flame no longer whispers. It roars.
What you see is liquid steel blowing out the other side of your cut. What is actually happening is that the heat and massive amount of oxygen is oxidizing the steel super fast. Basically, it’s turning it to rust, and rust melts at half the temperature of steel. It instantly liquefies and the flow the oxygen blows out a narrow canyon of molten steel. This canyon is the cut.
The odd splatters that are left over are called slag. They are knocked away with a hammer, or ground off.
And the torch, when it shuts off. The way its roar falls to a whisper, a hiss, nothingness. The way it just lays there in a drape of hoses in the sunlight.