In a previous section, we looked at the basics of hardening and tempering. In this section we will look at several specifics that we didn’t cover before.
Firstly, decide upon your heat source. The incinerator, a wood stove, a good campfire, even a large propane torch; all would do the job. The main criterion being that you want to heat the blade quickly. The problem you will encounter is called ‘oxidation’ and we shall look at it in a few lines time.
Get your metal tin or bucket full of water, and have it close by the fire. Tie a piece of wire to the tang. Put the knife in the fire so that the blade is in the heat source, but the tang is not. (There are no advantages in hardening the tang. Indeed, it could cause problems at a later date).
Heat the blade until it is red hot. If necessary, you can pull it out of the fire to check. It will not affect the ultimate hardness, so long as the rest of the process is carried out properly.
Bring the blade up to a bright red heat. Only when the blade is the colour of a ripe tomato, should it be quenched in the water. As we said before, jig the blade in the water, to ensure that fresh water is exposed to the blade while it is cooling. The water will hiss and spit, so keep your feet clear, or you could get scalded.
Test the hardness by lightly filing a spot on the cutting edge. If the file won’t bite, the blade is hard enough.
Now comes the good part. You have to re-polish the blade! The heat will have caused tempering colours to form on the blade. Unless you have managed to heat the blade quickly, there will also be oxides formed on the surface. (This is known as ‘mill scale’, and is the surface of the metal which has changed form).
You may wonder why we spent so much time polishing, when we will have to polish again? The point is, the blade is now in a hardened state. To achieve the same polish will now take longer. The less polishing the better, so the work you did before will stand you in good stead. At least you have to try and polish out deep scratches.
So back to the polishing bench! You should be able to achieve the same degree of polish using just your finest grade of cloth, plenty of oil, and just a little time. Also, and this is the good part, you don’t have to polish both sides. Just polish one side until it is bright silver colour again.
If possible, select a propane torch, or blow lamp for tempering. Place the blade on a dry house brick. (Note: wet bricks may explode when exposed to heat. The moisture in the brick turns to steam. Te pressure builds and the brick explodes. You could kill yourself!). We keep an ordinary ‘common’ clay house brick in the cupboard for just this purpose. It is always dry and serves as a base for tempering, brazing, etc.
Put the blade on the brick with the shiny side uppermost. Have a tin of sump oil handy. It won’t matter how black it is, as it won’t affect the steel. (You can impart a high grade black colour to steel, by heating it to just under red heat, and quenching in old sump oil. However, this process only works on low carbon steels. It will have no effect on any of the high carbon steels, and your blade falls into this group).
With the heat source, heat evenly along the back of the blade. Keep the heat moving and try to maintain an even heat. Once the temperature builds up, you will see colours form on the surface. The sliver will soon be replaced with a very light straw colour. If the heat is continued, this will turn into a darker straw colour. In turn, this will change into a very dark blue (the colour you sometimes see on motorcycle exhaust pipes). From this point on, the blue will become lighter, right up to red heat. It will never disappear completely, unless you re-polish the metal.
In this instance you want to achieve a medium straw colour. It is a bit hard to describe the colour, so it is best to heat a bit of steel, and see the colours form. You will quickly learn the colours, and then can judge the term ‘medium’.
As we said before, keep the heat moving. If the colours are forming too fast in one area, move the heat. Don’t just sit and watch the pretty effects, do something positive! Also remember that as the blade thins, the heat will travel faster. It will move very quickly as it runs down the cutting edge, so be ready.
Quench the blade in the oil, and keep it moving in the oil while it is cooling. Only when it is cool, should the blade be removed from the oil. This will be after the oil ceases to bubble, and once the smoke stops rising from the oil.
When removed from the oil, the tempering colours will still be visible. Should you find that there are blues showing on the steel, we’d suggest that you start again.
In that case, go through the full hardening and polishing processes again. It won’t harm the steel in any way, and will teach you to be more careful next time. On occasion we have had to repeat the process several times to achieve the colour that was required.
(As an aside, it is possible to harden and temper in one operation. The old craftsmen used to polish the blade, as described before. It was then heated to a good orange heat and quenched in oil. The trick is plunge the blade in oil for just a few seconds. It was then removed from the oil, and the oil on the blade ignited by the heat source. This was allowed to burn for several seconds, and the blade then quenched in oil again. In just a few seconds, the blade was both hardened and tempered. While this process works, it relies upon a degree of experience that a novice does not have. Thus we suggest that you use the first method until you gain experience. The second method dose not leave such clear tempering colours, so you can’t check your finished hardness, other than through use).
If you are satisfied with the even run of tempering colours, you are ready to do the final polishing. Use the methods previously described, and remove all traces of the tempering colours. Make sure you polish the back of the blade as well, and the downward curve of the point. Pieces of half worn emery cloth are ideal for this, as they don’t scratch. You can also re-burnish the blade if you wish. Since this is the final polish, it is worth taking your time and making sure you get the best possible finish.
When you are finally satisfied with the polish, degrease the entire blade. Remove all traces of oil and grit, particularly on the tang.
