Thread: How to grade old oilstones

Gidday Fuzzie,

the easiest way I know if is to simply run your tools over them in a particular order and check the depth of the scratches on the blade.

If one stone's scratches replace another's with deeper marks, then it is coarser - if you get a finer polish off one then the previous one - then it is finer.

I don't know of a more "scientific" method than that - but I would assume that test shouldn't take you too long - then just chuck a label on 'em so you don't forget!

This won't determine the exact grit rating of the stone, obviously - however many of the older stones (such as india stones, I think) don't have grit ratings - they just have "coarse", "medium" and "fine" or somesuch.

The charts linked on http://www.knifeforums.com/forums/sh...hp?tid/763651/ this page might assist you to place them in the almighty list of micron ratings and abrasives.

Best of luck,
sCORCH



sCORCH

Scorch is on the money I reckon, maybe compare the scratch marks that a particular stone leaves on an edge with the scratches from a series of known grits, eg. wet & dry abrasive.

Using a 30x loupe or similar you should be able to figure it out.

Good question.Maybe examine and compare the stones to graded sandpapers with magnifying glass ?

Thanks guys. Sounds like visual inspection is the only way to go. I've go a small pocket folding magnifier that will probably do the job.

I was after a 10" coarse/extra coarse diamond stone on ebay to use to flatten things. I won the item, but unfortunately the seller disappeared before the item was shipped. Something bad must have happened to the seller. At the time of sale the feedback profile was for 2400 items sold since 2001 with 99.9% positive. Since January the seller has racked up 14 no deliveries and no more items on sale. Thankfully Paypal refunded the purchase through the dispute resolution process.

I tried again and took a punt on a buy it now item for US$25 that was described as 10" but had no grit information. It turned out to be an 8" coarse/extra fine. I can't seem to get it right on ebay! In any case I didn't hassle the second seller I figure even an 8" at $50 all up is not that bad.

Gotta say, inspection of sharpened blade IS the only way to determine the performance of the natural stone.

I have recently tried making of sharpening stones using various types of fillers with different concentrations of binder, and results have been quite interesting.

For example, if you take let's say silica-based ordinary beach sand, dry it up in the oven, let it cool down and then mix it with the epoxy and compress a stone slab in a mold, you may or may not get a good stone.

The starting "grit" of non-sieved beach sand is for example somewhere between 10...200 um as seen in the light microscope. One would by default expect pretty illogically behaving scratching coarse stone as a result. But in one case the damn thing sharpened a chisel really well. Slurry was good, and everything went well. I examined the slurry, the largest particles were around 10...25 um and then finer distribution. As a conclusion, there must happen such things that those initially large particles are not presenting primary grit, but are more like agglomerates (although not looking like that). They will be crushed under bevel pressure and are divided into finer ones.

Then another one appearing like the previous sand made no such effect, just scratches.

To me, natural abrasives and natural stones can't be really evaluated by anything else than by sharpening some blades on them. Grain size on stone surface does not necessarily say anything yet. Or that's how it looks like at the moment.

*********'

In my experiments, I have been using air-classified alpha-aluminas and long pot life epoxy systems. I have used no actual coupling agents, but some smaller amounts of wetting agents. Long pot life gives me time to finish the batch mixing and then to apply and bake them into slabs in the compression mold. It looks like I am getting pretty greedy stones even with finer grits. Stones are pretty large, I get two stones from one slab, and after clean-up they are about 85mm wide and 310mm long, usually 10-12mm thick. Pretty generous area for sharpening jigs, too.

I wish to write some kind of empirical report during next summer. I gave away samples of these stone-experiments to my woodworking pals, and they have liked them, although there has been complaining that such big stones are pretty tedious to true. Oh well .

kippis,

sumu