Flattening Oilstones: An Experiment

I own a 3″x8″ medium India oilstone by Norton that I bought from Tools for Working Wood, maybe five years ago. While honing a 2 5/8″ plane iron a few days ago, I noticed that the stone was dished–on both sides. I remember flipping it, but because it was clogged.

India oilstone, dished

There are plenty of suggestions out there for truing an oilstone: lapping on a piece of sandpaper [1] backed by a flat substrate; rubbing two stones together until both are flat; rubbing the stone on concrete; lapping on a diamond plate… and the list goes on.

Given that both sides of the stone needed truing, and there is so much varying advice on how to go about it, I decided to try various methods and judge their effectiveness. I limited my research to the archives of the OldTools mailing list, which, given the group’s longevity, is a wealth of information.

Since most people have free access to concrete, I decided to try that approach first. While it did seem to have some cutting action to it, I quickly abandoned that approach, due to the deep scratches it left on the stone.

From there, I tried various sandpaper options, eventually settling on 60-grit cloth-backed aluminum oxide, which I got in a box lot at an auction several years ago.

20121020-124018.jpg

It was slow-going to begin with, but I could feel an arris forming along the edges of the stone, which seemed a good indication of progress.

20121020-124403.jpg

I had read that lacquer thinner was effective at cleaning oilstones [2], so I began using that as a means of cleaning away the swarf, pouring it directly on the abrasive surface. This seemed to help keep the abrasive from becoming clogged, and appeared to be an effective solvent for removing the “gunk” from the stone.

Even with the lacquer thinner, though, I was getting approximately 15 minutes of continuous lapping before the cutting action was noticeably diminished. When I noticed that the quart of lacquer thinner was half-empty, I switched to WD-40.

For Side B, I tried using a 2″x6″ diamond stone. Initially, I tried using it with the lacquer thinner, but it began to dissolve the plastic substrate of the diamond stone, so I switched back to WD-40.

Results
Initially, I was concerned that I would need to be careful to flatten the stone evenly, lest I end up with the world’s largest slipstone. It turned out that the stone flattened evenly, reinforcing the India stone’s reputation as a tough-wearing stone.

20121020-124738.jpg

The cloth-backed aluminum oxide was slow but effective as a lapping abrasive, requiring several fresh sections of abrasive to flatten Side A over the course of two hours. The lacquer thinner was effective for dissolving the black gunk clogging the stone. However, I was alarmed to note that the stone seemed to have no bite when I was finished on the aluminum oxide. I used the diamond stone to create a fresh surface, and it cut like new.

20121020-124950.jpg

On Side B, the diamond stone alone was effective at renewing the surface, but it was not large enough to work as a lapping surface.

The roll of cloth-backed aluminum oxide, had I purchased it retail, would have cost about $13 for a 50-yard roll, of which I used five yards. Along with the pint of lacquer thinner at $16.28 per gallon, this turned out to be a cost-effective approach to restoring a 3″x8″ India stone which today retails for $16.95 plus shipping.

Conclusions
I noticed that Side A, the side of the stone I used when I first purchased it, was in much worse shape than Side B. Some sources suggested that the significant layer of black gunk that had accumulated on Side A may be saponified vegetable oil. I do recall using vegetable oil early on when sharpening kitchen knives, concerned that WD-40 might contaminate my knives. (Ah, the irony.) Since then, I’ve dedicated separate stones to kitchen knives.

The lacquer thinner, while effective, evaporated quickly and was irritating to skin. Had my box of latex gloves not gone bad (just remember kids, heat plus latex equals bad), I would have donned a pair for this operation. WD-40 was as effective at keeping the abrasive surface of the aluminum oxide from clogging, and had the advantages of drying more slowly, being gentler on the skin, and smelling better.

Directions for Future Research
The limitations of the methods attempted suggest the need for an improved maintenance regimen that prevents clogging and leaves a fresh cutting surface. For now, I will be sticking to WD-40 as my honing oil, although a friend did recommend a 1:1 mixture of 30 weight motor oil and kerosene [3].

A collector of old oilstones mentioned his success using Goop hand cleaner to clean oilstones [4]. Others mentioned kerosene may be an ingredient, adding that kerosene may be an effective cleaning agent. Future stones in need of cleaning may benefit from this approach.

Given the effectiveness of the diamond stone at leaving a fresh surface, a large lapping plate would be ideal. At $92.50 (Lee Valley), the 4″x10″ plate would be an expensive purchase for maintaining a $17 stone, but it might be useful beyond this purpose, especially given my affection for old tools. The 3″x8″ plate, at $52.50, would still be large enough to maintain the surface of my oilstone, but is still expensive for the single purpose, and might not have the utility of the larger plate.

References
[1] http://swingleydev.com/archive/get.php?message_id=79526
[2] http://swingleydev.com/archive/get.php?message_id=224277
[3] http://swingleydev.com/archive/get.php?message_id=202063
[4] http://swingleydev.com/archive/get.php?message_id=202042

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