Wednesday, February 28, 2007
I have used waterstones to hone my tools for years. There are a few major lessons to be had with them. Number one is that the reason that they cut so fast is also their greatest problem. As the particles on the surface break down to reveal fresh sharp particles, giving fast cutting, the stone quickly becomes misshapen. For most tools, at least one side of the tool requires a perfectly flat surface which can only be honed on a perfectly flat stone. This means that the using waterstones demands almost perpetual maintainance. Luckily, it is as easy as rubbing them on a drywall sanding screen on a piece of plate glass. The good new is that many of the common problems with sharpening can be traced back to using stones that aren't flat.
Rubbing the back or bevel of a blade on a dished out stone will slightly round the edge. When moving on to the next stone, which is harder and less likely to be dished, the edge no longer makes contact. You are literally no longer honing the edge! To test for this kind of rounding, hold the tool so that light is bouncing off of the edge and move it. If the light rolls over the edge, you've rounded it. Now you must either reflatten the back or grind away the offending edge. I know that sandpaper on glass offers another solution without the constant flattening, but then you are buying lots of sandpaper and messing with gluing it down. Also, fresh particles are not constantly available, the paper simply gets dull. But, my main reason for preferring stones is the slurry.
Slurry is the mixture of particles and water that the blade rides on as you hone. The greatest benefit of this is that the more water you use, the fresher the cutting surface, the faster the cut. The other benefit is that as you hone, the slurry starts to dry and the particles in it break down. This dried slurry acts as an inbetween grit, allowing the surface you are honing to be polished to a higher level than the grit labelled on the stone. When you move up to the next higher grit stone, which cuts slower, there is less work to be done. On the last stone, normally for me an 8000 grit, I let the slurry go completely dry and turn black (that's the metal coming off the edge) and polish until the edge not only has a mirror finish but looks almost wet. During this final honing, I only pull the blade towards me, the last thing I want to do is plow a razor sharp edge into a bunch of grit! I find that combination stones are a cost effective place to start with stones. Avoid grits below 1000 as they wear too quickly to be of use, sandpaper on glass serves much better for such rough work, although, once the back of a tool is flat, it should never be necessary to return. When honing a blade with a flat polished back, never hone the back on the coarser stones, only on the finer stones to remove the burr. Stay away from any stropping of chisel and plane blades, you cannot help but round the edge. Later I'll address how this slight rounding is exactly what gives drawknives and carving tools their versatility and control.