Step 5: Heat Treating--for the little pyro in all of us
Here's probably the most technical part of the entire project--heat-treating the blade. You can use either a coal forge (as I did), a gas forge, or a torch. The last should only be used on small knives--maintaining high heat on a big blade would be hard with just a torch. See picture one below to see me starting the fire.
Heat-treating consists of two steps, hardening and tempering. In hardening, you heat the blade to a critical temperature and then quench it. This changes the structure of the steel so it's extremely hard but also pretty brittle. A knife in this stage, if dropped, can crack or shatter like glass. The next step, tempering, is done by heating the knife to a lower temperature, around four hundred degrees. This makes the knife less brittle, while still keeping a relative amount of strength.
Now, You'll need a hardening bath. For 01 steel, you should use oil. Different types of steel require different methods of quenching--oil quench, water quench, air quench, etc... again, I recommend 01 steel because it's easy to heat treat and doesn't require anything more complicated than a bucket of motor oil. See picture two. You should be able to immerse the blade completely. The second thing you'll need for hardening is a magnet. This will help you determine the hardening temperature, because at that point the steel becomes non-magnetic. See picture three--I keep the magnet on the hood of my forge, specifically for this purpose.
Now to start. Make a fire on your coal or gas forge or light up your torch--heat the blade by the spine, so as not to burn off the edge. Steel will burn off or melt into an unusable foam-like metal mousse if it's heated too high.
So, you're going to heat the metal to a medium-high orange heat, until the steel becomes non-magnetic. Just tap it against the magnet while it's glowing, and if it doesn't stick, it's ready. At this point you'll want to let the steel cool slowly in the open air, a couple times. This is called annealing, and relieves stresses in the steel cause by the rolling and milling process. After you've annealed (three times is a good round number), heat it to the same temperature you have been, but instead of annealing it, plunge it into the oil bath. Wear gloves because there's going to be some fire here. See picture eight. When you take the knife out it'll be smoking and the entire room should smell like the French fry tent at the county fair. To test the edge, run a sharp file over it. If it's hard, the file should skitter over the edge without making a mark, as in picture ten. You've hardened the blade at this point, so be careful. It'll break if you drop it.
Now, there's not much you can do with the blade until you temper it. Put out your fire, go inside, and preheat the oven. Your steel might have come with tempering information on it. If it did, chose your hardness from the sheet and use that temperature. You'll want a medium hardness for a knife. The eleventh picture of this step is an illustration of the tempering colors--these are a visual aids for measuring the temper of the blade. The higher the temperature, the softer and springier the blade will be. Try to shoot for a brown or purplish color, which will usually show up at about 400-450 degrees. If you don't know exactly what temperature to use, go for 425 degrees fahrenheit. Put the blade on the middle rack and let it cook for one hour. When the hour's up, the knife is ready. Congratulations. You've officially made a blade--though to turn the blade into a fine piece of cutlery, you'll need to do a little more work.