English: Tempering colors produced on the surface of steel indicate the temperature to which the steel was heated. (Photo credit: Wikipedia)
Now we have a very hard blade, a blade as hard as a quartz crystal. But that is too hard, and though there is no danger of the edge being dented, there is very great danger of it being snapped on impact. In fact, once, I had accidentally snapped a steel blade, fully hardened, with my hands!
So, it is obvious that the blade needs to be a little softer to bring out it’s full potential. If we anneal it, it is brought down way too soft and the edge will easily dent on impact, and would have to be sharpened over and over. So, the hardness would have to be brought down just a little bit, to get halfway between easily-denting-edge-but-non-brittle steel, and very-brittle-but-will-not-deform-or-dent-on-the-edge steel.
This process is called Tempering.
To temper the steel, first the blade is brought to a near polish, and then the torch is turned on a very low flame. Often a furnace is used, but I use an acetylene torch. The torch is then applied to the blade, making sure to apply more heat to the spine and back of the blade rather than to the edge. Now, the color of the steel must be VERY carefully watched. With even a little bit of heat, the steel begins to change color. Once the back and spine of the blade is a dark blue color, and the edge is a light straw brown, the torch is taken off, and, depending on the steel, is either quenched or left to cool.
As you probably guessed, the colors were caused by, and indicate, the levels of heat. In the photo above, you can see the colors from left to right, right being highest in heat and left being fairly cool. It starts out as silver color steel is normally (not visible in the photo), and then begins as a very light brown, then gets a dull straw color, and gets an intense straw yellow, an almost clear yellow, then goes brown, then a dull purple, then intense purple, then a really dark blue, and progressively gets lighter and lighter blue, then begins to turn back to the regular steel color, and then begins to turn red and glowing.
But what causes the initial coloring, from straw yellow to light blue? As I have probably said before in some long-lost post that I am too lazy to find, when heat is applied to steel, the heat attracts oxygen to the surface, and there they bind together. Heat promotes bonding. When the heat is applied to the blade, very small levels of oxygen are bonded to the surface, and create a thin layer of iron oxide on the surface. Light goes through this semi-transparent layer, (practically microscopic) and bounces off the shiny steel underneath. On it’s way out, the light is bounced around a little extra by the layer of iron oxide, and only certain wavelengths get through, and our eyes interpret them as a certain color. As the layer thickens, more or less wavelengths get through, and so the amount of heat can be interpreted through the colors shown on the surface of the steel.
Even after cooling, these colors remain, and some bladesmiths will leave the color on, for decorative purposes. Usually, though, the oxide layer will eventually be worn off, so smiths usually sand the surface and polish it, and then the blade is ready to be given a handle and sharpened.
Because of the softness in the back of the blade and the hardness of the edge, the blade will give superior performance, and can be sharpened razor sharp with risk of the edge denting, or the blade snapping.