Most older cold chisels are made from medium carbon steel, but mported chisels might be made from nearly any scrap steel rod stock. Test the chisel by holding it against a piece of mild steel (also known as "1018" steel) and striking it. If it does not bite into the steel, you should probably harden it before further use. Chisels that have been sharpened may lose temper during grinding, and they need to be hardened before use as well.
Wear eye protection, heavy leather work gloves and full leathers.
Normalise the chisel by heating the entire tool to black in a forge or using a torch. Black begins at 100 degrees C (212F) and ends around 385 degrees C (725F). This prevents crack formation in the steel during machining.
Heat the first 5 cm (2 inches) of the chisel to dull red and allow this to cool to room temperature slowly.
Heat the first 2.5 cm (1 inch) of the chisel to bright orange. Quench only that first section in vegetable oil until no red colour remains.
While still black-hot, scuff the tip of the chisel with an 80-grit abrasive wheel on a bench, belt or right-angled grinder so you can see the shine of the steel.
Watch the tip change colour until the tip is light blue. Cool it in vegetable oil.
Test the chisel's hardness by cutting into a piece of "1018" mild steel by striking the chisel with a hammer. It should cut into the steel without dulling if you have followed the previous steps correctly.
Steel often requires heat treatment to obtain improved properties; for example, increased hardness or strength, or to neutralise negative effects resulting from previous manufacturing processes, such as removing internal stresses generated by fabrication process.
Always wear wraparound eye protection, heavy leather work gloves and full leathers when heat-treating steel.
Keep a large, fully-charged fire extinguisher within fingertip reach at all times when working or quenching hot steel. Keep a 23 litre (5 gallon) bucket of sand in reach to smother fires, and another bucket of water to immerse hands, arms or legs if splashed during quenching.