The atoms in a solid object are far from stationary -- they constantly vibrate. In fact, they vibrate more intensely as their temperature rises. When this happens, the repulsion between the atoms outweighs their attraction, which means that they spread farther apart and the solid expands in volume. This effect, called thermal expansion, is why it is easier to open the lid to a bottle after it has been run under hot water. But this process inevitably causes problems for welders. When only one side of a piece of metal is heated and then cooled, the whole thing becomes warped and misshapen. That's why welders make sure to heat the entire piece of metal uniformly before they begin.
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].