Process of Carving & Shaping a Tanto Tsuka
Once the habaki is made, it is time to create a handle for the blade. Before starting to carve the wood core, any additional hardware such as seppa (washer), tsuba (handguard), fuchi (ferrule), or kashira (pommel) are prepared. Because of the tapering shape of the tang, the handle can be designed to be easily removable for cleaning and sharpening the blade. If the handle is to be finished by wrapping with rawhide, leather, or ito (flat silk cord), these must be also planned into the design.
A tsuka is created in two distinct stages, first the wood is split and the inside carved to receive the tang, and then it is joined back together and the outside is shaped. To increase strength, the tang is placed slightly off centre in the block so that the nakago-ha (bottom edge of the tang) is fully supported by wood rather than resting against the glue joint.
Choosing and Preparing the Blank
The core of the handle is traditionally honoki (朴の木, hou wood, Japanese Bigleaf Magnolia, Magnolia Obavata), chosen for its low moisture, low acidity, stability, relatively straight grain, and balance of cushioning and strength for tsuka and saya.
The wood should be dried and seasoned for as long as possible, usually five years or more and at least a couple of them right in the working space. Shirasaya require very clean and clear grain but nurizaya can contain some cosmetic artifacts or colour variations as they will be lacquered over later.
Grain that runs relatively parallel to the curve of the koshirae is both structurally and visually advantageous, particularly in the handle and upper area of the scabbard where the most stress will be. It is possible and sometimes necessary to remove a slight wedge shape between tsuka and saya to keep the grain aligned in the best way possible without interrupting the flow. Creating a tsuka is begun by selecting an appropriate block of wood and sawing it into halves and planing the inside surfaces flat.
Carving the Nakago-ana
After the blank is sawn into halves and planed smooth, the decision is made where the line between handle and scabbard will be cut, then each half of the tsuka is carved out carefully to seat the tang in place. Carving is done with purpose-designed chisels called saya-nomi (鞘鑿).
The nakago mune should be centered in the block of wood but the nakago-no-ha (bottom edge of the tang) should be fully seated in the omote half of the wood, slightly to one side. The purpose of this is to keep the tang from placing strain on the glue joint, resting it fully against wood. When the fit is just right and the tang sits tightly in place, the two pieces of wood are joined back together with sokui (rice paste glue), and the handle carved and shaped once dry.
Making Sokui (Rice Paste Glue)
Sokui is a simple wood glue made from rice that is traditionally used for joining wood from tsuka to sliding panel shoji. The all natural glue contains nothing but delicious Japanese rice and a little bit of extra water. One of the reasons rice glue was originally chosen for making tsuka is that it is non acidic, does not degrade either the steel or the wood over time, and does not retain moisture. Another is that, while quite strong, it is not stronger than the wood itself. This allows a scabbard or handle to be split open for cleaning or repair without damaging the wood.
A bite or two of cooked rice is placed on a board and worked with a bamboo or wooden wedge to break all the grains into pulp. As the rice is squeezed under the wedge it becomes like sticky dough. Once there are no pieces left, a few drops of water is worked in to bring the glue to the desired consistency. It should not be runny but should be thin enough to spread evenly on the parts to be joined.
Gluing the Core
A thin layer of glue is spread on one or both tsuka parts, depending on the consistency of the glue. The two parts are tightly bound with a cord or leather strap and wedges further increase the pressure. Using a strap rather than clamps provides an even, non-marring pressure even when the block is not yet perfectly square and true.
Shaping the Tsuka
Planes, chisels, knives, rasps, and coarse files are used to turn the block of wood into a graceful handle shape and smooth the contours. Starting from the profile of the fuchi, the shape of the handle is carved at the fuchi and then carried back by stages, working from a squared taper to a rounded one. Finally, extra wood is removed to make room for the wrapping to come.
Applying the Samegawa
An additional layer of strength, grip, durability, and embellishment is often added to the wooden tsuka using a rawhide wrap and leather or silk cord. Deer rawhide is sometimes used, but most often samegawa (ray skin) is seen under formal wraps or on its own. Rawhide is stiff and strong, and samegawa in particular has a good grip for the hand or for locking in place any cord that wraps over it.
A paper pattern is made to determine as near as possible the shape of the samegawa needed and then a piece is cut slightly oversize and trimmed down bit by bit. The samegawa is dampened and formed around the tsuka several times as it is adjusted. Once it is finally dry and perfectly fit, the skin is carefully glued in place with sokui and bound with string until dry.
Making the Mekugi
The Japanese sword is unique in its handle engineering as well. The tsuka and nakago are shaped and fit together in such a way that the strength of a single bamboo peg holds the whole unit together and so that removal of the peg allows complete disassembly. This allows the blade to be easily removed for cleaning and sharpening and allows for replacement of the mounting without damaging the blade in any way. In addition to being inserted from the omote side, it is important to note that mekugi also have a correct orientation in the mekugi-ana. Looking endwise, the part of the mekugi with the most dots is the outside of the bamboo plant and the strongest. It should be rotated towards the back of the handle, where the nakago places the most strain on the peg.
Mekugi for swords should be made from smoked susudake bamboo for strength, but horn, hardwood, and sometimes metal mekugi are found on antique tanto koshirae. Leaving as much of the denser exterior material as possible down one side, the bamboo is shaved to a rough taper with a chisel and then smoothed with coarse and fine files as it is slowly adjusted to a final fit before being cut to length. The ends are sealed by soaking or saturating with 100% pure tung oil and allowed to cure for two or three weeks.
The next step in the process is Carving the Saya, or scabbard.