What is Tsuchioki in Japanese Katana terminology?

"""Tsuchioki"" (土置き) refers to the process of applying clay to the blade of a sword before heating it for tempering, in order to create a blade pattern (hamon). This is one of the most crucial steps in sword-making, as it is the final stage where modifications cannot be made.

The clay used is called ""Yakibatsuchi"" (焼刃土), and its composition is often a closely guarded secret of each swordsmith. The clay is applied to the entire blade, with a thicker layer on the spine side (mune) and a thinner layer on the blade side (ha). After the clay dries, variations in the amount of Yakibatsuchi are used to create the desired hamon on the blade.

The areas covered with a thicker layer of clay will have a shallow and gentle temper line (yaki), while the areas with a thinner layer of clay will have a deeper temper line. This process is essential in determining the aesthetic and functional qualities of the final sword."