After the blade is hardened and tempered, the final geometry is created and the surface smoothed and polished with various abrasive stones. In contrast to other methods, stone polished blades have a different surface look and retain their crisp edges and lines. Polishing is broken into three distinct stages, the rough polish occurring before the fittings and scabbard are made, and the foundation polish and final polish once the rest of the knife is complete.
The word togi (研ぎ) does not differentiate between the action of polishing and the action of sharpening, for a Japanese sword the operations are one and the same, an integral process. A combination of Japanese waterstones, both synthetic and natural, are used to remove smaller and smaller amounts of steel and give the knife its final shape and surface. Each stone is progressively finer and is used with a different orientation so that the scratches from the previous stones may be clearly seen. Once they are erased, the next stone can be used, each time refining the geometry and surfaces towards the final goal. Once the rough polish is finished, work on the blade is halted until after the habaki, seppa, tsuka, and saya are created and fitted. This prevents accidental scratches while working on other parts of the knife.
Rough Polish (Kaji-Togi)
At this stage, the large volume of steel left around the edge for the process of yaki-ire must be removed, and the blade and tang are given their final geometry. The edge of the blade is much harder than it was during the rough shaping stage and steel cannot be removed with any metal tools. A combination of natural and artificial waterstones are necessary for this process.
Care is taken to refine each part of the blade geometry and bring the planes into proper alignment and proportion beginning with coarse stones and quicker removal and ending with very fine stones and subtle adjustments. Tagane-ha (chisel edge) is a common technique for first establishing the centre line of the edge. Both sides of the edge are honed away on a 45 degree angle and then the excess material in between the edge and the spine is removed in sections, similar to the method of using a sen to set the pre-quench geometry after forging.
Once the fittings and scabbard are complete, the blade is first given the rest of its foundation polish and then the final polish. Depending on the condition of the blade, the last used stone (#300, #500, or #700) may be repeated again to check that there are no new scratches from the workshop and then finer and finer stones are used to complete the finish.
Most of the foundation polish is carried out with natural (or high-quality synthetic) Japanese waterstones which cause the hamon and other steel activity to show up against the body of the blade. A Japanese waterstone forms a slurry like fine clay with suspended particles in it, water (sometimes with a little baking soda to combat rust) is used to control its viscosity and how much stays on the stone during use. A natural stone gives a nice final finish with a unique look due to its slight variation of hardness and grit size.
Finishing Polish (Shiage Togi)
Depending on the steel and the desired look, the final stages may take almost as long as all the other stages together. An uchigumori stone is used to bring out all the internal aspects of the steel surface and then small finger stones are used to work over the entire surface again to remove scratches and give an even look. Finally sashikomi nugui made from satetsu (iron sand) and clove oil is applied to darken the surface and highlight the edge. Sashikomi (差し込み研ぎ) is an older style of polish that is known for its subtlety and honest view of the activity in the steel. The blade is carefully cleaned and oiled frequently with a clove oil blend for the first few days and weeks after polishing.