Clay tempering is an art form as well as a science. Pioneered by the Japanese, traditional clay tempering techniques are perhaps the pinnacle of japanese sword design.

Today, clay tempering techniques are used by many modern smiths on swords as well as knives, both western and eastern.

The principle behind clay tempering is to achieve both hardness and flexibility, which equals strength. A hard cutting edge, and a flexible spine. A harder steel will sharpen better than softer, retain its sharpness longer, and resist denting. The flexible spine allows the blade to resist greater impact, without breaking.

These two things combined, hardness and flexibility, make for a great sword.

Clay tempering is called just that because the smith will use “clay” e.g. mud, to insulate the blades spine. Like a plastic sleeve around a wire. Where the clay is the plastic and the blade is the wire. Though it is just the spine of the sword that is coated in the clay. The edge is left exposed.

On application the smith is free to use their artistic licence. A pattern will be drawn in the clay, often appearing like an undulating wave flowing down the length of the blade

Once the clay is applied the blade is heated in the furnace. When it gets to its optimum temperature it will be glowing red. It is then plunged into a cool pool of quenching liquid (water, or oil). There is a rapid cooling of the blade. Due to the clay insulation the spine cools slightly slower than the edge which is exposed.

The differential tempering is now created, hard and flexible. It is this pattern that the Japanese call the “Hamon”. It is revered, studied and admired for its beauty.

A katana blade applied with clay, pulled from the furnace, just before it is about to be quenched. 

Clay tempered katanas are swords made according to a particular process of blade reinforcement. During the forging, there are steps to go through involving several steps and some compound mixture, used by the entire swords forger, yet each has his own formula. The compound mixture is none but a Clay recipe that each forger has a secret ingredients list. Clay tempering is supposed to be an art more than a sword works. Clay tempered Katanas are said to be more resistant and durable.

Steel Material of Clay Tempered Katanas

The Sword maker proceeds by preparing the Tamahagne, a material block made of several layers of steel with different carbon tenors. The Tamahagne is obtained through a smelting process: Folding and t10 steel katana Clay Tempered welding layers of carbon steel; the process allows getting the impurities out of the materials and result in a clean steel block.

Generally, 1095 carbon steel is the most adequate for making Clay Tempered Katana. While the other Carbon Steel types are not really being used. The difference is in the Clay mixture that applies to each type of steel and it is certainly the role of the smith to determine the combination. Traditionally, several layers of carbon steel of different Carbon tenors are folded together to obtain a primary material on which the smith will apply the process of clay coating.

Clay Application Process

The previous operation, which is folding and welding Steel layers, gives also the “billet shape” to the resulting worked steel. The smith proceeds then to clay application to start the real making of Clay Tempered Katana. The process consists in covering the blade with some layers of wet clay mixture. This mixture is prepared according specific proportions that each smith keeps to himself, but it is generally a compound of Clay, water, stone powder, and rusts and may be some ash. The clay slurry is applied differentially to the edge and the spine of the blade; the edge being destined to be sharpened later. The clay application is done while heating the material, then hammered and quenching in water or in oil according to the smith. This differential clay application and the quenching operation result in hardening the edge of the blade and keep the spine relatively soft.

Clay Tempered Katana Forging

Clay Tempered Katanas are therefore generally made of carbon steel blade professionally and differentially coated with Clay slurry to obtain a differential hardness along the blade. The differential hardening process allows the sword to acquire the beautiful curves, usually ordered by users and clients. Repetitive heating and quenching –sudden cooling in water- allows the material to go through several Steel phases which are known to be advanced forms, solid and hard. The process of heating-quenching uncovers the structure of Hamon, the line downside the blade. It is important to make it more clearly through the polishing process which comes later. It is important to notice that the processes of preparing the Steel material and Clay slurry application are particular and the smith should know what proportions to use to obtain certain characteristics of the sword such as the Hamon structure and the blade curve, and the Tang properties as well. You can then order the shape, the hardness and the Hamon of the blade, and the smith will choose the adequate combination to make your wish come true.