Natural Earth Plaster Walls – Crossed Heart Forge

In contrast to the rough walls of the kajiba, which are arakabetsuchi (荒壁土), the traditional infill technique using a rough mixture of natural clay, sand, and straw applied over lath (komai/kabekomai 壁小舞), the interior walls of the shiageba are finished with a finer natural earth plaster top layer.

This small shiageba will provide work areas for carving saya (scabbards) and tsuka (handles), polishing blades, and other finishing work such as lacquering with natural urushi and tsukamaki (handle wrapping). The main inspiration for aesthetic and technique of the interior design is the humble Japanese inaka naya (納屋) style or minka style of a century ago.

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Materials
Plastering
Finishing

Making the Plaster

This plaster is something like a finer version of a kirikaeshi finish. Kirakaeshi (切り返し仕上げ), sometimes called nakanuri shiage (中塗り仕上げ), imitates the second last layer of plastering being used as a final layer. The three ingredients are clay soil, sand, and straw fibers. It is finer than a true nakanuri but a bit rougher and less refined than a kyo kabe finish such as mizugone. It has a rustic appearance but is fine enough to be used as an interior wall in a teahouse or home.

As a general rule, the largest size of soil or sand particle should be about half the thickness of the plaster coat. For example, a nakanuri coat is screened through a ~5mm screen, kirakaeshi through ~3mm, and mizugone through ~1mm. For this project a window screen was used for the sand so the largest particle is about ~1mm. The finely chopped straw was screened through a fireplace screen which has about 3-5mm openings.

Dry sand is sifted through a window screen (~1mm openings) to remove small rocks and other impurities.

The main ingredients for this plaster are sand, clay, and cattail/bulrush fiber (蒲, gama), with the addition of some fine straw for texture, some fine charcoal for colour, and some iron filings for a potential kyosabi effect from winter humidity. The proportions are approximately 2.25 sand, 1 clay, 2 parts cattail fluff, 0.3 parts straw, a few handfuls of iron filings and 1 handful of fine charcoal powder in the form of lampblack. The iron filings and lampblack are intended to affect the white kaolin colour slightly towards the tones of a warm gray jurakukabe (聚楽壁) appearance.

After screening the sand and straw, the ingredients were dry mixed until evenly distributed and then water added a little at a time while mixing. The cattail fibers will tend to clump together when dry or very wet but will separate with the mechanical force of a stiff mixture. Once they are broken apart more water can be added. The plaster is stirred thoroughly again after sitting overnight to fully hydrate the clay.

2.25 parts (~18L) sand (~1mm and smaller, mixed size).

1 part (~8L) clay trimmings and scraps from the potter’s ground (mostly white kaolin in this case).

0.2 parts (~1.6L) fine chopped straw (<1-2cm), sifted through fireplace screen (~3-5mm openings).

A few handfuls (~0.2L) of iron filings and 1 handful of fine charcoal powder (in the form of lampblack).

Antique lampblack that came from the old tractor shed several decades ago.

The cattail/bulrush heads from last fall are ripe and dry, seeds ready to fly.

3 cattail/bulrush heads were used for this mix.

Fluffed apart using a stem, the volume is about 5L for each head, 2 parts (~15L) total.

Ingredients are mixed dry, then >1 part water (~5L) added and mixed while thick to disperse fibers better.

After mixing, extra water is added to adjust consistency and plaster is left overnight to soak.

Plastering the Walls

The amount of water in the mixture is adjusted before applicaton and throughout the process as evaporation occurs from the bucket. The plaster is applied to the walls using a Kyoto style wooden hawk and a steel trowel. When working on a traditional substrate this finish is applied while the arakabe is still slightly damp, or water is lightly sprayed on the walls before application to help bonding.

Wooden edges can be taped before application, and should be wet with a damp brush just before applying plaster to each section to make clean up easier. If applying two layers, the first should be a drier mixture and the final more watery. A small test patch applied the night before helped determine the appropriate viscosity and thickness of the plaster layer. In this case a single layer was used with an average thickness of about 1.5-2mm, certain areas required more due to the condition of the wall underneath.

Wooden hawk, made from handplaned driftwood, nails must be sunk to prevent damage to the trowel.

Back of the wooden hawk, angled handle facilitates flipping up the hawk to load the trowel.

The next morning the mixture is stirred thoroughly again and water added to prepare for application.

Applied about 1.5-2mm thick with a hawk and trowel, edges can be taped for easier cleanup.

Working one section at a time, water is added to the remaining mixture as needed to keep it from thickening.

By the end of the day the first areas of application are drying, thicker areas dry slower.

The next morning the later sections are also drying.

Finishing & Clean Up

Major clean up should be done immediately (thick sandy plaster drops removed from the bottom sill) before drying, and then usually the fine clean up (thin layer of clay slip removed from around the edges) is done with a damp brush right after application. Using a damp cloth is another approach, in this case it worked well for removing the fine white kaolin clay from the grain of the dark charred yakisugi style wood, even when time did not permit it to be done the same day.