Process of Clay Tempering a Tanto Blade

Once the steel is shaped as much as possible in its softer state, it is coated with a thin layer of clay along the edge and a thicker layer on the body and spine. During the hardening process, the split second difference in cooling time caused by the clay layer creates two different hardness areas in the same piece of steel. The edge cools faster and forms a very hard steel structure called martensite while the body cools slower and forms a very tough steel structure made of ferrite and pearlite. The boundary between these two areas is called hamon and is commonly seen as a frosted wavy line down the length of a polished sword blade.

When the clay is fully dry, a charcoal fire is used to heat the steel slowly and evenly, taking care not to overheat any part of it. First the spine is heated to bring the whole blade to just below temperature, and then it is flipped over to focus heat on the edge. When the entire edge is at the correct temperature, it is plunged into a water bath, edge down, and held until cool (yaki-ire). The hardness is checked with a file and the process repeated if necessary. After hardening, the clay is removed and the steel is heated slightly again to remove some of the internal stresses (yaki-modoshi). Once this process is finished, and if the steel survives, the blade is ready for Togi, hand polishing.


Preparing the Clay Mixture

The clay mixture does not need to have secret or exotic ingredients, but there are a few properties that are desirable in the final blend. The basic recipe is approximately a 1:1:1 ratio of regular clay, ground stone/sand, and charcoal powder. The clay provides the stickiness to keep it together and on the blade, the stone/sand prevents shrinking while drying, and the charcoal helps prevent flaking off in the fire due to heat expansion.

The soft water source is snow or rainwater. Each ingredient is ground in small batches between stone and steel or two stones. Grinding and screening the ingredients as finely as possible is important, especially for the slip layer, as the layer can only be as thin as the largest sized particle.

Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
The partially dried clay before pulverizing. Clay sticks the ingredients together and onto the blade.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
Ground fine sand, silt, or polishing stone powder provides a filler that will counteract the tendency of clay to shrink and crack as it dries.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
Powdered charcoal being reduced to dust. Charcoal provides some micro pores as it burns out, allowing for some heat expansion and also affecting cooling time. More charcoal is added to the slip mixture than to the body clay mixture.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
In this case, fine steel filings and powdered iron oxide provide some thermal mass and make up part of the non-shrinking filler material.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
The steel and stone mortar and pestle stand in.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
Powdered rust being ground between two stones, this will go into the body clay mixture.
Island Blacksmith: Traditional handmade nihonto style Japanese swordsmith techniques
Adding the rust to the body clay mixture before adjusting the water and thickness prior to application. A fairly runny but not watery mixture helps create a thin layer (think pancake batter not butter).

Applying the Clay (Tsuchioki)

The body and spine are coated in a thin layer of clay that will prevent the steel from cooling too quickly in these areas. A uniform thickness is important for even drying, heating, and cooling during various stages of yaki-ire. An almost runny but not watery mixture helps create a thin and even layer (think pancake batter not butter). When this layer is dry a very thin slip layer of clay that is higher in charcoal content is applied to the exposed edge, the charcoal burns out in the fire and the resulting porous clay surface has been found to cool steel faster than if it was bare. The clay slip also helps prevent oxidation and decarburization (loss of carbon at the surface) while in the fire.

Island Blacksmith: Hand forged tanto made from reclaimed and natural materials
The blade has been shaped with sen scraper and files to set the approximate geometry but the cutting edge is still 1-2mm thick. The extra thickness will help prevent cracking, warping, and carbon loss. The clean, rough surface will help the clay adhere to the surface.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
The thicker layer (~1-2mm) helps to define the boundary of the hamon but is not directly related to it. Steels with manganese harden much deeper than files or traditional tamahagane and the mask must be much closer to the edge.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
A visible illustration of the two different clay layers when dry. The darker coloured thin slip layer is higher in charcoal and should have a very fine particle size. It covers the exposed part of the edge that will not be protected by the thicker body layer clay. Note that the spine curves down to counter the upwards effect of yaki-ire.

Yaki Ire

This is the moment of truth for the knife, if it survives intensity of the quenching process it will become a live blade. If it succumbs to the stress and cracks it will become a piece of scrap steel again. Yaki-ire requires intense and singular concentration and even the best smiths lose one out of every four or five sword blades to the process. Even though all the work up to this point may be lost, the benefits far outweigh the risks when a good blade has been born.

In order to provide visual consistency for judging color and temperature, yaki-ire is done at night time or with doors closed and lights off. A charcoal fire is built and the water (heated up almost to boiling for modern spring steels) set near the forge. Not heating too quickly or too slowly, the blade is brought up to its critical temperature and committed to the water with a prayer.

Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
After drying overnight. This century-and-a-half-old shear steel has very minimal alloying ingredients and responds more like steel made from tamahagane. The clay is farther back from the edge than it would be with modern steel. (see example above)
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
When working with steels less than a century old, the rain water bath is heated to the temperature of a proper ofuro to lessen the thermal shock to the steel by way of the vapour jacket.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
Making sure the clay is completely dry by resting it on or above the glowing charcoal with no air blast for several minutes or longer.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
A fresh, deep fire is built for even heat. Heating slowly with the spine downwards at first to protect the edge from overheating, constantly moving through the charcoal. This stage is done in near complete darkness for consistancy.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
When the correct temperature has been reached, the blade is immersed up to the machi until it has cooled to the temperature of the water and then removed.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
The clay is partially fired after quenching, it is normal for some of it to come off in the water. A file is used to test if the entire edge has hardened fully, and if not the blade is cleaned, normalized, and new clay is applied to begin the process over.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
A soft mild steel scraper is used to remove the clay mask. If there are no signs of stress cracking, the blade is cleaned and tempered over the remaining hot coals.
Island Blacksmith: Hand forged reclaimed tanto made from reclaimed and natural materials
A test polish to inspect the hamon on a blade after a successful water quenching.

Yaki Ire Performance Testing

Though there is great risk of losing a blade to the traditional water and clay tempering process, the gains in performance are great. Rather than finding a mid point that is an appropriate compromise for a particular blade and steel, clay tempering allows for the best of both worlds in a single blade. If the whole blade were as hard as the edge, it would be too brittle for most uses, and if the whole blade were as tough as the body, the edge would be too soft for most uses.

The images below depict some informal performance testing for a traditional charcoal forge, clay, and water differential hardened blade. The blade was approximately 1/8″ thick at the spine, the testing was done just behind the tip and a hammer used to drive the blade through various metal bars. Though the edge did not chip, bend, crack, or dent, this is not a recommended practice with any knife except perhaps in a life threatening circumstance.

Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Clay tempered blade hammered through annealed copper pipe.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Work hardened aluminum block, note that a cutting plate was used to protect the anvil face for all tests.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Cold rolled steel sheet.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Large hardened medium carbon steel nail, as found.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Normalized 3/8″ steel rod.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Hot rolled 1/8″ x 1/2″ steel bar.
Island Blacksmith: Water quenched Japanese clay tempering hamon performance testing
Hardened 1/2″ logging bridge spike vs yoroidoshi tanto. (note that these potentially destructive tests should not be performed on blades destined for use due to the risk of unseen internal stress)

The next process is Togi, hand polishing.