The Breakdown on Heat-Treating


How Does It Work, and Why Is It Important?

By Chowser, February 1st, 2017


There are several steps to properly heat treating a sword, and many ways that the process has been achieved over the course of history. Different cultures evolved the process in different ways, sometimes through espionage and theft of technology, and sometimes independently of outside influence. Although there are many variations on the process, eventually the art of creating swords homogenized into a process with more similarities than differences, mostly due to the superior results achieved by the dominant cultures.


Annealing is a process of metallurgy that is intended to alter the physical, and even chemical characteristics of the metal to increase its ductility, and decrease its hardness. The process is generally performed by heating the metal, and then allowing it to cool gradually to room temperature. Sometimes a quenching process is employed to speed up this process, but it is dependent on the type of metal used, and the desired results intended. The annealing process is what allows blacksmiths to change the hard metal into a workable material that can then be shaped into a blade, or another implement.

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The purpose of the hardening process is evident in the name, even to the layman. The process itself usually involves heating the shaped implement to a temperature range which is higher than the metals critical temperature, and then rapidly cooling it. The cooling process is generally done through a technique known as quenching, which involves submerging the heated material in water, oil, or varying mineral water solutions, depending on the desired effect. The result achieved by this process is that the metal is brought to its maximum level of hardness.

It might seem like this is where you would stop, a super hard sword sounds great right? In terms of edge sharpness that may be so, however, the strength provided by this level of hardness comes at the cost of flexibility, which is very important when considering the functionality of swords.

While hard objects have a greater ability to inflict damage, they are also more brittle, which means that heavy impacts can cause them to fracture and break. Because the intended use of a sword usually involves repeated impacts with hard surfaces such as; armor, other swords, and bone, etc, flexibility becomes of greater import. Finding a perfect balance between hardness and flexibility is the key to making a great sword, and it is why the process doesn’t just stop with hardening.


Tempering is usually the final step in the heat-treating process, and it is intended to reduce the brittleness that the material gains during the hardening process. Tempering involves heating the hardened object to a temperature range which is lower than the critical temperature of the material, usually in a controlled environment such as a kiln, and then allowing it to cool slowly. There are varying degrees of hardness that can be achieved during this process depending on how much heat is applied. If the intent is maximum flexibility then the object is heated to a point closer to the critical point, conversely, if more hardness is desired then less heat is applied.

A powerful variant of the Tempering process is Clay Tempering. This process uses clay as an insulator to produce object which have more flexibility on some portions, and more hardness on others. This is particularly useful in the construction of swords because it allows for very hard and sharp blade edges to be coupled with flexible spines. The Clay Tempering technique involves applying layers of clay in varying consistencies prior to tempering. To achieve a hard blade edge with a flexible spine, a thicker layer of clay would be added to the edge, and a thinner layer would be added to the spine, effectively varying the temperature reached by the areas through insulation. The resulting variation creates a visible distinction on the object, which is how the Hamon (edge pattern) is created on traditional katanas.

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We hope now you have a better understanding of heat-treating, and why it is important. Thanks for reading, and feel free to comment below. Also, we appreciate and support you can give us on your favorite social media platform.

Functional Swords!