The Sigma Power #120 stone.

Hi again Constant reader…

As we are on a bit of a stone kick here of late, I’d like to pass on my feelings about this particular sharpening stone, that seems to be quite popular according to how much trouble I have keeping them on hand.

Here it is;

Not much to look at, is it?

Well, what you are looking at is, effectively, an aerated chunk of #120 grit Silicon Carbide particles fused together in a brick-like shape.

A simple device really, take some really hard, heavy duty abrasive in a rough and ready size and mush it together and market it for those really serious tasks.

For the grunt work of wasting off large amounts of steel quickly, this thing is king. Very VERY hard, very VERY aggressive and not at all expensive. It’s small size helps with the last part, being only 50mm wide, but you really need to see one in person to realise how angry this stone is. Believe me, 50mm is wide enough.

However, like so many amazing things, it has a flaw…

The great speed and incredible hardness means that the abrasive does become worn and being Silicon Carbide rather than Aluminium oxide, it fractures into smaller bits, and that a lot of metal needs to go somewhere. Both of these contribute to the bane of most sharpening stones, glazing.

What happens when a stone glazes is that particles of abrasive and/or the material being ground become embedded in the surface of the stone. As neither of these cut very well, they become packed into any spare space on the surface of the stone and are burnished where they lodge, leaving the surface smooth and preventing fresh, sharp abrasive from being exposed to continue working as it should.

No abrasive stone is immune to this, if you fail to keep the stone clean, then it will glaze, end of story.

There are several ways to reduce the chance of this happening. One common method is to use a ‘friable’ stone binder, which breaks up readily, constantly sloughing off spent abrasive, abraded material and a little still good abrasive in the name of constantly exposing fresh, sharp grinding material. It works very well, but in the very coarse grits, this is a double edged sword, in that yes, the stone will cut quickly, but the stone will also dish very quickly. Not so much of a problem with a grinding wheel, but potentially a big problem for a flat stone.

Another method is to use very hard, durable abrasive with a very open matrix, allowing any ground material and/or spent grit to migrate below the working surface easily. Many oil stones use this method with good success, but when the matrix (also known as ‘pores’ in the stone) become full, you run into a problem of the stone glazing as you expect, and also how to remove the now deeply embedded waste material.

Yet another is to bind an extremely durable abrasive to a substrate, and when the abrasive is spent, discarding the base and attached abrasive. Diamond plates work on this principle, as does abrasive paper. In the case of diamond plates, initial costs are high, but otherwise work very well for a long time. With abrasive paper, initial costs are cheap, but you need to constantly replace the paper as the abrasive wears rapidly.

But usually, even a glazed stone can be easily rectified by a quick scrub with a hard, durable abrasive fixed to a solid substrate such as a diamond plate, sandpaper on something hard and flat, another stone or a special flattening and conditioning stone.

The Sigma Power #120 uses the ‘durable abrasive/open matrix’ method, and it almost works. If the stone is kept wet and constantly flushed with water, then glazing is largely prevented.

However, the glazing is not the bad part. The real problem is that once the stone has indeed glazed (as it inevitably will!), becuase the stone is so hard and durable, most common wisdom as related to flattening and restoring water stones becomes null, void and moot. In short, this stone is a pig to flatten and even worse to restore to it’s former glory.

Diamond plates of medium (#300 or higher) grit don’t work. Concrete and asphalt don’t work. Soaking in a steel etching liquid doesn’t work. Scrubbing furiously doesn’t work. Attacking with very coarse sandpaper makes the problem worse than ever.

Once the stone is glazed, it does become a very good #1000 grit equivalent stone, but that’s not what we want, we need this stone to be as mean, nasty, blood spittingly angry as it was when we first used it.

How do we restore our once wonderfully coarse stone to it’s former glory, or heaven forbid, improve on it?

You need to dish out even more nastiness and aggressiveness…

Here’s the trick.

Take some very coarse Silicon Carbide or Diamond loose grit, I used #36 SiC grit myself and apply a small amount to a hard, wetted down substrate, I used a steel ‘kanaban’ plate, but plain old steel will do, just so long as it will put up with what we are going to do.

A small amount is a good thumbnail’s worth, maybe 2.

Then take your water soaked Sigma Power #120 grit stone and rub vigorously against the loose grit/hard base device.

Hey presto, the glazing has been broken up and removed and you are left with a surface that looks very similar to how the stone looked originally.

But there’s more, oh deary goodness me is there more…

I believe it to be something to do with the way the stone is manufactured, but the initial 1mm or so of stone is incredibly hard which causes the problem in the first place. Once this hard layer has been broken up and removed, you get down to a slightly softer, even more aggressive and clog resistant layer of stone. Eek!

Not as soft as say, a Shapton Professional #120 (which is harder than most coarse stones), but softer than it was originally. Because the stone is now free to shed spent grit easily, it actually cuts faster and yet, still stays quite flat and dish free. Unique among these types of very coarse stones.

How fast is it?

Well, it managed to chew through about 1mm of my 12mm wide high speed steel chisel in about 1 minute as I re-established a 30 degree bevel on it.

Ladies and gentlemen, there are few things available to the common (wo)man that will do such a task so quickly or effortlessly, manual labour excluded. And once I was done, the chisel was ready for the next stone. Folks, you might get it done that quickly with a good grinder, but maybe not too and the Sigma Power #120 is at the very least, consistent in what it does.

Now, this step of breaking up the hard outer layer is so important to the usefulness and well being of this stone and it’s owner that from this point on, a small bottle of #36 grit SiC grit will accompany every Sigma Power #120 stone I send out into the world. Those of you who already own one, and got it through the Tools from Japan store should expect a small bottle to arrive in your letter box by the end of August. I trust that you will all be more than capable of finding something suitable to use the grit with, and if not, may I suggest mild steel, ceramic tile, plate glass (only scrap glass please, no windows in current use!) or maybe even some hard, durable wood would suffice, so long as you dispose of it once you are done.

(Those of you who own one of these stones, yet didn’t get it from the Tools from Japan store, contact me through the store and I will see what I can do, might cost all of a few dollars in postage and solve your problems for good!)

So, there you have it folks. This wonderful, yet flawed stone has had it’s warts, if not removed, at least dulled somewhat and we gave it a bit of a nip and a tuck in the process.

(And yes, I am very happy I got this problem nutted out. Finally!)

Thanks for reading, as always…

Stu.