Soaking Japanese Natural Awasedo, yes vs no

Posted on October 6, 2011

I have been asked about soaking natural japanese sharpening stones in water before use.

Although this is just an opinion, I tend to not be in favor of soaking natural awasedo stones. Someone has recently thrown out there that it is OK to soak these stones because they have spent millions of years in water already, why not now. It is true that the stones mined near Kyoto were “born” as sediment in pools of water, but the fact is that for the most recent tens of millions of years they have resided well above sea level and locked in the mountains under other rock, debris & soil as dry stone.

There is some seepage even in bedrock but because of the vertical nature of the strata that the Kyoto awasedo are mined from, any percolating water is incidental and temporary. These stones do not come to us from a pool of water, they come from a dry environment. There is one exception to this generalization however. If a mine shaft is dug below the valley’s water table, then there is a great chance of standing water accumulating in the mine itself.

I have a photo of one miner working in a raincoat within the mine, this was the last shaft dug in that mine, at the lowest elevation of the mountain. The shaft was shortly bulldozed and the mine closed. The stone extracted from this shaft is considered to be of inferior quality because of the extreme hardness of the stone when compared to the excellent quality that that mine produced at the higher elevations.

There is no reason to subject the sides and the back of a natural awasedo to a soaking in water. The top of the stone however does need to be hydrated, the purpose is two fold. The first is to use the water to carry away the swarf from the abraded steel, the second is to encourage the surface of the stone to soften thereby slightly dissolving the binder (clays) so they will reveal fresh and sharp grit particles. These actions do not require the soaking of a quality stone.

The world is full of ultra hard stone, and some will in most cases scratch and abrade steel, but if the active grit particles are so firmly embedded in the base stone and will not release they will: either become dull and ineffectual or will glaze over with swarf and in either case will stop cutting. The beauty of quality awasedo is that the friability of the grit and the fragility of the binder allows the grit to cleave and the binder to allow fresh grit to be exposed.

If you are using an super hard awasedo that is not generating any amount of slurry than there is a good chance that the cutting power of your stone is diminishing moment by moment below your blade. As this happens the actual sharpening or abrading process is exchanged with a polishing effect akin to burnishing. At that stage you will not be honing the steel but just shining it up. This could be your goal, but the sharpness and durability of the edge could be compromised by a form of “plastic deformation” to the steel where stresses may interfere with the original Martensitic formation within the steel as a displacive transformation occurs.

When you soak an awasedo your goal would do so to soften the stone. If the stone has distinctive sedimentary layer lines these may absorb water at different rates. The moisture could accumulate over a period of subsequent soakings and susceptible stones by their very stratified nature could separate along those stratification layers. A stone sealed on the sides and back would of course wick-up less water per usage, but some retained moisture could simply build behind the sealed sides and over time could create unintended results.

If there is someone out there who has soaked their awasedo stones over an extended period of time I would like to hear from them. Alx