Og had a couple of questions/points I thought I'd address here on the subject of heat-treating knives.
I mentioned before it's a two-step process, hardening and tempering. Hardening is the real pain, in that you have to get the entire length of the blade up to temperature at the same time without over/underheating it in places. Several ways to do it:
1. Spend a couple of thousand on a heat-treating furnace. The good ones can be had with a compartment up to 36" long, the temperature can be set exactly, and if you get the computer controller you can even set how long to take to get to full heat, or bring it up in steps, etc. Or if you have a gas forge, many of them can be adjusted to give just the right heat. If you don't have the money or enough time/interest to invest in one of these,
2. For small/medium blades you can take some firebrick and stack them to create a long empty box in the center. Cut/drill a hole in one of the side bricks, a little off-center, and stick a torch tip into the hole. Adjust things right and the flame/heat will swirl around inside; by moving the piece back & forth you can get a nice, even heat. However, this only works well with short blades; long ones require a long chamber, and probably more than one torch. Or in the forge, you can
3. Set up something to make the fire longer & thinner. I use a piece of pipe cut in half lengthwise with a bunch of holes drilled in it. Set it right over the air hole at the bottom, build the fire over it and it works pretty good. You can make it better by getting a piece of sheet steel and cutting it to an hourglass shape, then bending it in the middle and sitting it in the air hole below the pipe. You want it to stick up a little above the hole but not touching the pipe. It forces the air to blow to the sides as well as straight up, and by spreading the air blast more evenly you get a lot more even heat over the length of the fire. Big blades you'll need to move back & forth, but you can, with care, get it done. Or you can
4. Build a firebox specifically for this. A rectangular box however long, with a smaller piece of square tubing welded to the bottom for the full length, and a lot of holes drilled through. Hook a blower up to one end of the tube, and make a plunger to insert from the other end; by moving it back & forth you can control how long an area gets air, and you can stack firebricks at that end to set the length of the fire. For most knives the box would only need to be 6" wide, and if you find something suitable that's too wide you can stack fire bricks along the sides to leave a narrower channel down the center.
To be continued