Island Forge: Part 5 – Tsuchikabe

This series of photo essays will document the preparation, construction, and set up of a simple swordsmith style kajiba (鍛冶場, forge building) from the ground up. The main inspiration for aesthetic, form, and technique is the humble Japanese inaka naya (納屋) style of a century ago.

Earthen Plaster

The upper sections of the walls are tsuchikabe (土壁), a traditional infill technique used extensively in Japan. A mixture of natural clay, sand, and straw is applied over a tied bamboo lath (komai/kabekomai 壁小舞) attached to tie beams between the timbers.

Island Blacksmith - Crossed Heart Forge
Sandy clay soil from the ground is mixed with scraps of potter’s clay, chopped straw, and a bit of extra sand.
Island Blacksmith - Crossed Heart Forge
The materials are mixed with water to saturate the straw and dry clay then left to soak for several weeks.
Island Blacksmith - Crossed Heart Forge
Frequent mixing distributes and breaks up the straw as it begins to ferment.
Island Blacksmith - Crossed Heart Forge
As the straw ferments it begins to break down into small fibers which add strength and viscosity to the clay.
Island Blacksmith - Crossed Heart Forge
After several weeks very small straw fibers are visible in a close-up of the mixture. (area shown is just over 1″ high)

Split Cedar Lath

A grid of ~3cm wide “thick bones” are first mortised into the timber frame and then ~2cm wide “thin bones” are tied to them using rope. A lattice pattern of horizontal and vertical elements provides plenty of purchase for the earth mixture to hang on to. In this case split cedar is used instead of split bamboo for the lath and scraps of old natural fiber rope are twisted apart and used as the binding twine.

Island Blacksmith - Crossed Heart Forge
Thick bones are mortised into the frame, the top mortise is deep enough to allow insertion into the bottom.
Island Blacksmith - Crossed Heart Forge
External bones are vertical and internal bones are horizontal.
Island Blacksmith - Crossed Heart Forge
The mortises are set so that the internal and external just touch as they intersect.
Island Blacksmith - Crossed Heart Forge
Rope running along the thick bones ties the thin bones to them.
Island Blacksmith - Crossed Heart Forge
The completed kabekomai lath.
Island Blacksmith - Crossed Heart Forge
Ropes alternate top and bottom or left and right to tightly pull the thin bones against the thick bones.
Island Blacksmith - Crossed Heart Forge
There are several tying patterns that vary based on region.
Island Blacksmith - Crossed Heart Forge
Interior view showing the ropes running along the thick bones.
Island Blacksmith - Crossed Heart Forge
Front view.
Island Blacksmith - Crossed Heart Forge
Thin bones are not attached to the timbers, only to the thick bones.
Island Blacksmith - Crossed Heart Forge
The best approach to angles is to fan out the thin bones to fill the gaps while maintaining close spacing.
Island Blacksmith - Crossed Heart Forge
Northwest corner.
Island Blacksmith - Crossed Heart Forge
Ready for application of clay mixture.

Applying the Mud

The clay mixture is applied with a hawk and trowel and is about 1cm over the external lath with a reveal around the timber frame between 1 and 2cm. The rough arakabe mixture can be left as is for utilitarian purposes or can be coated with sandy middle layers and thin fine finish layers for decorative purposes (and increased weather resistance if the finish contains lime).

Island Blacksmith - Crossed Heart Forge
Application of the rough arakabe layer. The reveal around the frame is between 1 and 2cm.
Island Blacksmith - Crossed Heart Forge
Straw fibers visible under the trowel during application. (area shown is just over 3″ high)
Island Blacksmith - Crossed Heart Forge
The clay mixture should be the correct consistency to come through and hold on to the lath as it droops.
Island Blacksmith - Crossed Heart Forge
If an interior layer is applied, after the exterior has dried, it will lock into the peaks that come through.
Island Blacksmith - Crossed Heart Forge
The fresh wet arakabe layer looks like a smooth finish layer but will surface crack as it dries.
Island Blacksmith - Crossed Heart Forge
After several days the arakabe layer begins to appear dry and looks more like a tea house (or barn) wall.
Island Blacksmith - Crossed Heart Forge
Comparison of drying arakabe at left and fresh arakabe at right.
Island Blacksmith - Crossed Heart Forge
Close up of the final surface texture as it has almost dried.
Island Blacksmith - Crossed Heart Forge
Front view just after applying the arakabe.

The next steps will be to finish the interior of the walls.


Timeline

Gathering materials began in Fall 2018, site preparation in December 2018, the lumber was milled in the first week of January 2019, the frame assembled February 5th, and roofed February 7th. Yakisugi siding and tsuchikabe walls installed during March, and interior wall finishing in April. The goal is to have it operational by Summer 2019.

Thanks to all who were involved in one way or another in helping facilitate this project, providing space, time, materials, assistance, advice, and encouragement.

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