In Japan,Wood with the pith(center of the wood) is commonly used.But with these materials, it is very easy for cracks to occur on the surface as it dries. To reduce the chances of cracks occurring on the surface,a line is sawed beforehand on one side.As the wood dries, the tension in the wood is released by expanding the sawed line(instead of cracking). This sawing technique is called Sewari. However ,if all four sides of the material is going to be visible, It is necessary to Umeki(fill) this Sewari.
To make the Umeki(filled piece)Fitted tight as long as possible, both the Sewari(saw line)and the Umeki(filling)piece is compressed.
Next is to compress the Umeki(filling)piece. Use the rounded side of the hammer to compress. Hammer the piece on top of a hard surface(such as a hard piece of wood).It is important to hammer as evenly as possible. Once compressed, check the fit.After the fit is checked.
Glue is applied to one side. Spread the glue on the surface.Hammer the Umeki piece in.
Hammer from the center to the sides.Brush away the excess glue with water. The water also helps expand the compressed parts.
Shave the away the part sticking out.
Blush with water to remove any glue and to expand any gaps.

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Sewari and Umeki technique