You can temper an untreated chisel by heating and cooling it. A steel chisel needs heat-treating or “tempering” like this to make it hard enough not to lose its cutting edge when it is used. However, you must avoid overheating the steel, as this can weaken it. You must also cool the steel carefully, or it may fracture. Follow some straightforward steps and with a little practice, you will be able to achieve the perfect temper for your chisels.
Light the blow torch. Adjust the wheel control on the side of the blow torch, until it produces a short, steady blue flame.
Hold the tip of the chisel in the flame, using long iron pincers or tongs. Leave the chisel there, until it glows a dark cherry-red colour. This shows the steel has reached 788 degrees Celsius, the critical point at which crystals in its internal structure undergo a change.
Remove the chisel from the flame. Hold the chisel, with the heated tip downward, over an iron bucket filled with water.
Dip the last inch of the chisel's point swiftly into the water. Move the chisel end up and down in the water for a few seconds. When you take the chisel out, it should have cooled to the point where it takes two seconds to dry off in the air.
Rub one side of the hot chisel briskly with a house brick. This will reveal bands of colour on the steel. These bands move down the chisel toward the end that was cooled in the water. You should see pale yellow first, the colour of straw. Then a darker yellow band appears, followed by brown, purple and finally, blue.
Dip a woodworking chisel back into the bucket when the tip of it is pale yellow. Dip a cold chisel--a heavy, dark tool used to cut metal--back into the bucket when the tip of it turns blue.
Keep the tip of the chisel submerged until the whole body of the tool is cool to the touch. Do not keep the chisel tip still in the water, though. Move it up and down slightly all the time it is submerged.
Try the chisel on an odd piece of wood or metal--depending on whether you have tempered a wood chisel or a cold chisel--once the steel has cooled completely. If the edge of the chisel becomes dull, loses its shape or chips, then it has not hardened properly. Simply repeat the heating-and-cooling process again, paying close attention to the bands of colour.