Preparing for Yaki-ire: Study and Practice

posted in: Process, Yaki-ire | 0

What is Yaki-ire

Yaki-ire, or clay tempering, is a style of steel hardening. It isolates the hardening to the places the bladesmith wants hard (such as the edge), while keeping the rest of the piece tough (the body and spine). If the style is perfected, it results in blades that combine the best qualities of steel: hardness and durability at the edge, and toughness as the foundation to prevent breaking from brittleness in the blade. The transition zone between the two qualities of steel is called hamon. I’m curious to see if a hamon, a sort of frosty, wavy line that delineates the transition, is visible on the blades I treated using yaki-ire.

Preparing the Clay

I took the sister blades I featured in my last post and applied a clay mask to each of them using the recipe I found on the Crossed Heart Forge website (a wealth of information, by the way). The basic recipe is:

  • 1 part clay (binds the mix)
  • 1 part crushed sand/grog (prevents cracking and shrinkage)
  • 1 part crushed charcoal (prevents flaking off in the fire due to heat expansion)

The goal is to crush these materials as fine as possible, for the smallest size grain determines the minimum thickness of the clay slip or mask that can be applied. This post covers how I prepared the clay mixture.

The swage block from Crossed Heart Forge brought back into grinding service.
Sand before crushing (left) and after (right). The sand’s from a local beach and was already very fine.
Firescale from around the anvil. It serves a similar purpose to sand/grog in clay.
Firescale ground into powder
Charcoal dust from sifting pieces for the forge. Nothing goes to waste!
The dust crushed into a very fine powder. It had a tendency to float everywhere when crushing it.
I mixed some clay I got from an art class with water to make it very thin and spread it out on a sheet.
After baking in the oven at 200 deg. F for a couple of hours.
The clay is ready to be crushed into a fine powder. This is the binding agent for the mixture.
Crushed clay!

Bringing it all together

Now that I had my three (four technically) constituent parts, I was ready to mix them in equal proportion (by volume) and add water. The consistency to aim for the mask that goes on the body of the blade is pancake batter. It seemed to me to be like the texture of mortar when laying down tiles.

The materials ready to be mixed. It took about a tablespoon of each material to make enough for both blades.
The dry mixture. Add water and apply!

My next post will feature a video where I mix and apply the clay mask to the hori hori. Stay tuned!