How Sword Making Works

Cutting to the Chase

Now that the blade is annealed, the bladesmith can engrave any designs and wor­k out the edge and tip of the blade. Using a belt grinder is the most common way of adding the edge to the sword, but some bladesmiths prefer to work with files.

Since the steel is so soft, it will not hold the edge if you try to cut anything at this point. The steel must be heat-treated to harden it. Again, the bladesmith heats the blade up to the point of austenization. The blade must be evenly heated during this process. While a lot of bladesmiths use their forge for this process, some use a salt bath.

The salts are heated to the appropriate temperature and the blade is suspended in the salt bath for a certain amount of time. The salts used in a salt bath liquefy at a temperature lower than what is needed for the steel, but will remain a liquid beyond that temperature, creating a perfect "hot bath" for the blade. Much like a boiling pot of water, the salts evenly and thoroughly heat the steel.

Photo courtesy Don Fogg Knives
A salt bath used by Don Fogg

When the blade is removed from the forge or salt bath, it must be immediately placed into the quench tank. The oil in the quench tank causes the steel to cool rapidly and evenly. If the steel does not cool evenly for some reason, then the blade can warp or even fracture. Also, the blade must not be left in the oil too long or removed too soon. Either mistake can ruin the blade. There are general guidelines for how long to quench the blade based on the type of steel, oil or other hardening medium in the quench tank, and the thickness of the blade. Most bladesmiths will tell you that it is mainly experience and instinct combined that helps them know how long is long enough. Quenching traps cementite within the ferrite and creates a very hard steel called martensite.

Now that the steel is hardened, it can be tempered. Tempering, or heat treating, is done by heating the blade again. The difference is that it is not heated to the point that austenization occurs. Tempering uses a much lower temperature, again based on the steel used. The blade is kept at this temperature for a while, then it is quenched again. Most bladesmiths temper a blade several times to get the exact level of hardness. The idea is that the metal is hard enough to maintain an edge but not so hard that it is brittle, which can cause it to chip or crack.

Photo courtesy Don Fogg Knives
A sword being coated with clay

­One common method of heat treatment, particularly favored by Japanese sword makers, is to coat the blade except for the edge with a wet clay mixture that dries out and hardens as the blade is heated. The clay retains the heat and retards the cooling process. Some bladesmiths will create thicker ridges of clay that cross the blade to further slow down cooling in those specific sections. The idea here is that those sections will be slightly softer than the rest of the sword, and will increase flexibility while the edge stays hard.