Copper tubing is available in rolls of soft annealed tubing, or in straight lengths of hard tubing. Pure refined copper straight from the furnace emerges in the annealed or ductile state and has a large molecular grain size. Bending, rolling or hammering copper reduces its grain size, allowing the smaller grains to lock together in a tougher "work-hardened" state. Heating hard copper to cherry red and allowing it to cool increases copper grain size and softens the material. This process is known as "annealing."
Connect the welding torch hoses to their respective tanks on an oxyacetylene welding set -- the red hose to the acetylene tank, and the black hose to the oxygen tank. Turn both pressure regulating valve handles fully counterclockwise to close them and to prevent damage to the gauge diaphragms. Open both tank shut-off valves with the tank key provided with the welding set. Adjust the pressure-regulating valve handles clockwise until the pressure on both tank gauges register 34.5 kpa (5 psi). Open the acetylene valve on the torch half a turn and ignite the gas with a flint torch lighter. Slowly open the oxygen valve on the torch. Adjust both valves to the desired flame intensity. Reduce oxygen until the flame has a visible 5 cm (2 inch) "feather" at the end of the blue inner cone. This is known as a "carburising" flame.
Grip the end of the copper tubing between the jaws of a pair of pliers. Heat the area of tubing you wish to soften evenly with the flame. Keep the flame moving while rotating the tubing until it glows cherry red.
Drop the heated tube into a bucket of cold water to anneal it to the softest, most malleable state. Alternately, allow the heated tube to air cool if you prefer to anneal the tube to a slightly harder, less malleable state. The faster you cool the copper down, the softer and more malleable it will be.
Wear safety goggles and thick leather welder's gloves when annealing copper. If you are working with smaller-diameter thin-walled copper tubing, a propane torch will supply sufficient heat to bring the copper to cherry red for the annealing process.