Forge-Welding Fluxes: The Chemistry of How Straw Ash and Mud Juice Bond Steel

Why Does Steel Stick to Steel

In Japanese sword forging (tanren), tamahagane is repeatedly heated, folded back, and hammered together into a single block. This process of "joining steel to steel" is called forge-welding (tansetsu). Repeated fold-forging, as well as the construction technique that unifies the outer iron (kawagane) and core iron (shingane), all depend on the accumulation of countless forge-welds.

However, simply hammering heated steel together does not result in a successful joint. The surface of steel oxidizes immediately. The solution to this problem lies in two substances scattered during forge-welding: straw ash (warabai) and clay slurry (dorojiru).

Steel's Enemy—Oxide Scale

When steel is heated to high temperatures, the iron on its surface reacts with oxygen in the air, forming a layer of iron oxide known as "oxide scale" (black oxide). When this oxidation layer covers the steel's surface, no matter how much the steel is hammered, the metal surfaces cannot make metallic contact, leaving impurities and gaps at the joint. This causes forge cracks and other joining defects.

The key to successful forge-welding is keeping the steel's surface clean and preventing oxidation at the moment of joining. This is where straw ash and clay slurry—two "fluxes"—come into play.

How Straw Ash and Clay Slurry Work

Before forge-welding, the sword maker scatters straw ash (ash from burned straw) and clay slurry (clay dissolved in water) onto the heated steel. These are believed to function as follows:

• Create a protective coating against oxidation. As ash and clay slurry cover the steel's surface, they block contact with air and prevent further oxide scale formation.
• Melt impurities and flush them away. When the main components of straw ash melt at high temperature, they absorb the iron oxide on the steel's surface and become a flowing slag. At the moment of forge-welding, when struck with a hammer, this slag is forced out from the joint, allowing clean steel surfaces to come into direct contact and bond together.

In other words, straw ash and clay slurry serve dual roles: preventing oxidation while "washing away" obstructive oxide deposits.

The Swordmaker's Eye for Timing

Scattering the flux and heating the steel to the proper forge-welding temperature is one of the most carefully attended moments for the sword maker. If the temperature is too low, the steel will not join; if too high, the steel itself will melt and be damaged.

The sword maker observes the color of the steel in the furnace (the heat color) and the appearance of slag forming on the surface to determine the optimal "boiling point" (wakashi) for forge-welding. The moment when the straw ash and clay slurry have melted appropriately and the surface becomes glossy is when hammering should begin. This judgment is left to the sword maker's eye and experience, not to instruments.

The Quality of Forge-Welding Appears in the Jihada

The quality of forge-welding is directly reflected in the completed sword's jihada (metal surface). When joining is imperfect and forging is repeated, defects called "forge cracks" or "puffing" (fukure) can appear at layer boundaries, manifesting as fine cracks or bulges on the surface. Conversely, in a sword where forge-welding was executed ideally, the countless layers are bound together without gaps, and surface patterns such as irregular grain (itame) or wood grain (mokume) appear densely packed.

When appreciating or purchasing a sword, slowly change the blade's angle in bright light and observe the jihada carefully. Check whether the surface is dense and lustrous, and whether there are any unnatural cracks or bulges. Doing so allows you to verify with your own eyes the forge-welding skill of a sword maker from hundreds of years ago. It is not unusual for old swords to show minor forge defects, and value is not determined by the presence or absence of defects alone. It is important to evaluate the overall work, considering the location and degree of any imperfections.

Traditional Techniques Passed Down to the Present Day

In modern metalworking practices, commercially available flux such as borax is widely used. However, in traditional Japanese sword forging, the technique of using straw ash and clay slurry continues to be passed down to this day. This skill of skillfully working with readily available materials—straw and clay—is the wisdom of our ancestors born from long accumulated experience, and contemporary sword makers continue to follow its fundamentals in their craft.

Invisible Chemistry Supporting Masterblades

Straw ash and clay slurry do not remain on the surface of the finished sword. Yet without these humble fluxes, repeated fold-forging and construction techniques would not be possible, and Japanese swords could not be made. These modest, unassuming materials are the unsung heroes of the forge-welding process that unifies the steel.

When gazing upon the fine jihada of a Japanese sword, remember that each of its layers has been bound together without gaps, supported by straw ash and clay slurry. The strength and beauty of a master blade are built upon the accumulation of such invisible chemistry.

Experiencing Sword Craftsmanship

TOUKENZA (operated by M-Assets Inc.) is an online marketplace for the sale of Japanese swords. Understanding forging technique becomes a key to appreciating each sword more deeply. Swords listed in our catalog can be viewed in our online catalog and are available for purchase. If you are interested in any particular sword or have any questions, please feel free to contact us through our inquiry form.