Tempering

Tools:

• Oxy Acetelyne Torch
• Deep Baking Pan
• Clamps or blocks
• Wet Rag / Paper Towels
• Locking Pliers

Now that the edge has been hardened, the blade is almost ready to be sharpened, cleaned up and ready to go. But before we jump the gun we need to "temper" the blade a bit.

During the hardening process, although we attempted to only harden the edge of the blade, it's possible (likely) that we hardened the edge a little too far up the blade. Ideally only the bottom 1/3 of the blade will be hard, with the rest remaining flexible. This will allow the blade to bend without breaking if placed under extreme duress.

This process is a bit tricky and difficult to explain without witnessing first hand. It involves three cycles of heating with an oxy torch and successive cooling in a pan of water. Careful attention to the changing color of the metal is the key to success here. If somehow you manage to go to far, you'll ruin the previous effects of hardening and have to go back to normalizing and begin again.

Stage 1

• After clamping the locking pliers onto the tang of the blade, the Oxy torch is carefully drawn back and forth along the spine, starting at the ricasso and working towards the tip.
• Using an oxidizing neutral flame (5lbs Oxygen and Acetylene), the torch is held an inch or two from the back of the blade with the flame pointing down (parallel) and flames wrapping down both sides of the blade evenly. (Don't get it too close or the torch will blow out.)
• The torch should be in constant motion so as not to burn the metal or heat it up too much. If the metal gets red hot, it's too hot!
• As the metal heats up, the color will begin to change, beginning with a light tan and working to a dark blue. The goal is to run the light tan color down to the edge of the blade and then quickly submerge the blade in water to stop the heat from continuing down the blade.
• At the end of the process, the back of the blade has darkened from the ricasso towards the tip and the entire length of the cutting edge has turned a light brown color. This will require patience and several cycles of heating and quenching.

Video and pictures coming soon

Stage 2

• After the initial tempering, the blade must be lightly sanded to clean the coloration from the surface so you can see the original color of the metal again.
• The blade is now propped up so that it is standing in water with the edge submerged. The water line should only cover one third of the blade or less. To accomplish this the blade can rest on a wooden block under water and a clamp or a pair of bricks can be employed to hold the tang and secure the blade from falling over or moving.
• Since the vast majority of blades have a tip that curves upward, it is likely that it is not submurged in water. No problem here. Take an old rag or 2 or 3 paper towels, soak them completely in water, and drape them over the tip so that they are hanging down into the water while also protecting the end of the blade from the heat you are going to apply.
• Don't worry, the rags won't burn.
• At this point the blade is standing, edge down, in water and ready for the second heat. This time, we're concentrating on bringing a blue coloration as far down the blade as we can.
• The water will prevent the blueing from reaching the edge by creating a "steam jacket" just above the water line. I'm not quite up on my thermodynamics, but when Mike Williams explained it I kind of followed along.

Video and pictures coming soon

Stage 3

• This stage is identical to stage 2. Simply clean up the blade so you can see the original color again, then repeat the tempering process.

For more information on tempering, here is an excerpt from The 1924 edition of Machinery's Handbook and there is a good article at Engineering Fundamentals.

Next Step

Beginning - Definitions - Forging - Normalizing - Grinding - Hardening - Tempering - Summary