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Japanese Natural Stone (JNATS) Use and Maintenance

Do’s and Do Not’s

  • Do seal the sides of your stone. All stones, even the porous ones benefit from having their sides sealed in lacquer. You may use the traditional urushi or cashew lacquer. You can use less traditional hardware store lacquers or even nail polish. For very porous stones with no backskin (e.g. virgin rock), you may either seal it up for maximum protection or leave it open to create a dual sided sharpening stone. I do not always seal the back of my stones if there is backskin, but typically will even if the backside could have been used as a sharpening surface. Instead, I would rather re-treat the single sharpening surface with a different sandpaper/diamond plate. This is covered more below in its own section.
  • Do clean the slurry/swarf from your stone before storage. As the stone dries, it will pull material back into it as it pulls water into it. Do not leave the used slurry/swarf on the surface as the stone dries. Instead, clean it off with distilled/filtered water so that it is not pulling in any questionable elements or metal traces. If this happens now and again it is fine. If it is done habitually, you may cause issues.
  • Do refresh/lap the surface of your stone before and after use. You do it before use to ensure that no dust/grit/etc is on the top of the stone which is not meant to be there. You do it afterwards to ensure nothing foreign such as polluted swarf dries on the surface. You refresh the surface of the stone with either a natural nagura, sandpaper, or a diamond plate. Both the sharpening surface and the corners of the surface of the stone should be done to avoid a sharp edge. Wash afterwards as stated above. This is covered more below in its own section.
  • Do use distilled or filtered (with no minerals) water. Because the natural stones have certain compositional makeups, we do not want to be adding new elements or chemicals to it. If you use standard tap water on your stone now and again, it will not cause an issue. Over the years though, mineral build up may swell and crack a stone, or change how it performs.
  • Do not leave your stone outside in the elements. Direct sun, extreme cold, rain, etc. are all elements which will act to degrade or ruin your stone.
  • Do not leave your stone soaking in water. Unlike synthetic stones, natural stones should never be soaked as it will drastically increase the likelihood of cracking. Most stones are splash and go; some will absorb a bit of water. For those that absorb water, just give it the bit it needs until the sharpening surface seems saturated and add water as needed to keep the friction at an acceptable level.
  • Do not contaminate your stone. Contamination is usually using oil or soap on the stone’s surface. This will seep into the stone and greatly impact its ability to perform as expected. Japanese Natural Stones are not “oil stones” and as such, should only ever be used with water.
  • Do not store your stones wet. When your stone is dry it is easy to store, just put it in a box. However, allow stone to sit in an open area and dry before storing the stone away. If the stone is stored wet, it may result in swelling and cracking. This is usually done best by wiping away the majority of water with a clean hand and setting the stone on its sealed side to air dry. No need to add heat or direct wind, it will dry fast.

Flattening the Stone

Lapping – I recommend lapping under cool or room temp water only. Not cold and not hot or overly warm water. Room temp – I believe this is best for the stone when it’s undergoing a significant amount of abrasion. Grinding away the top is harsh on the stone – I think being as gentle as possible is best here.

I’ll use an Atoma 140 to take off the bulk of the stone that needs to be relieved. After that I use w/d paper starting at 220x and I go up to Atoma 1200 sometimes. You could also stop at 600x though. It depends on the stone and progression step. There is no recipe here – things have to be figured out as you go along.

Sealing the Stone

Sealing – there are many theories on sealing the sides of these stones. I seal all of mine, whether or not someone else does is up to them. I’ve had very expensive stones shatter in my hands while lapping, and I’ve seen other unsealed very-hard stones cleave across the top in layers for no apparent reason. Water ingress causes such thing to occur, so I do believe sealing is a good thing. Sealing prevents water from soaking in on the sides which is where the seams between the layers of sediment show. Sometimes we don’t see layers – but sometimes they are there anyway. Why fool around? It takes 20-30 minutes and if nothing else it’s good peace of mind.