Annealing

A torch is not only useful for soldering or brazing, but can also be very useful for annealing metals. Metals don't have a uniform structure - they're crystalline. If you bend a piece of metal back and forth until it fatigues and breaks, and look closely at the broken surfaces, you'll see the metal crystals exposed on the surface. You might need a magnifying glass, but they're there. When heated to a high enough temperature - usually red hot - these crystals tend to grow together and form larger crystals. The friction between the crystals creates much of the strength in many metals, so making the crystals larger makes the metals softer and easier to work. Hammering or bending the metal breaks the crystalline structure and causes metals to work harden.

Probably, the only metal you'll want to anneal is copper. Fortunately, annealing copper is extremely simple. All you need do is heat the copper to red heat and then let it cool. You can leave it to cool in the air, or quench it suddenly in a bucket of water, because copper doesn't temper like iron or steel. After annealing the copper will be much softer and easier to work, but if you keep bending a piece of copper or hammer it, then the large crystals will break up again and the metal will harden. You may find annealing necessary if you have trouble winding a copper tube to make a condenser coil. Too many attempts to correct bends will harden the metal, and the only answer then is to heat the hardened section until it is red hot and let it cool down. We mentioned rivets earlier when dealing with sealing the top of a large pot to make a boiler. Solid rivets are hammered into shape when applied to a job and so harden very quickly. It often pays to ensure that they are as soft as possible by annealing them before beginning a job.

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