WHY SHARPEN A TOOL?
There is an old saying that “a tool is only as good as its edge.”  This of course refers to cutting tools, and what it means is that to work properly the edge needs to be made of good steel, and it needs to be kept sharply honed to the correct cutting angle.  The craftsman is the one who determines whether the tool is made of good steel because it is he who acquires the tool.  The better quality tools will be made with the better grades of steel — the craftsman who always buys the best tool that can be afforded will never regret it.

The craftsman is also the one who will determine whether the edge is ground and sharpened to the proper angle.  Traditionally, carving gouges come from the factory ground to the correct bevel but not sharpened, though in recent years a few companies have begun supplying the tool pre-sharpened.  The idea was to put first things first:  i.e. — a good craftsman would need to master the skill of sharpening before he could master the art of carving.  In this day and time we have a tendency to jump right into the middle of a deep pool and only then see if we can swim.

However you might want to look at it, the first step in mastering the skill of sharpening is to obtain a sharpening stone.  There are several varieties of sharpening ‘stones’ available to the craftsman, and each type has its own set of devoted followers.  These are the traditional (European) oil stone, the Japanese water stone, and a relative newcomer (in the last couple of decades) is the diamond ‘stone.’  All types are good as sharpening tools, and each type of stone will do equally well at helping to keep the edge on a cutting tool sharp.  However, each type has its own set of advantages and disadvantages depending on how it will be used.

OIL STONES
The traditional (European) oil stone is made from a naturally abrasive stone, such as a sedimentary sandstone or a variety of hard metamorphic stone — until the middle of the nineteenth century the best type was considered to be a ‘Turkey stone,’ which was obtained from quarries in the country of Turkey.  The mid-nineteenth century saw the widespread distribution of American sharpening stones quarried along the Washita River in Arkansas.  These would include the commonly available soft Arkansas ‘Washita’ stones, the hard white Arkansas stones, and the black Arkansas stones.  For most of the last century man-made stones of synthetic abrasives such as aluminum oxide and silicon carbide have also been available.  The synthetic stones are now more commonly found than any other type.  The tan or brown-colored ‘India stone’ is always aluminum oxide, while the dark blue-black stones are sometimes made of aluminum oxide and sometimes made of silicon carbide.  Generally the synthetic stones have two working sides, one of which is a finer grit size than the other.  Usually this is a medium grit and a fine grit combined into one stone, though rarely a coarse and medium combination can be found.

Oil stones are so called because they are made to be used with a coating of oil while sharpening.  The oil ‘floats’ the particles of abraded metal off the surface of the stone, thus preventing the metal particles from being forced into the pores of the stone and keeping the abrasive grit exposed to the edge being sharpened.  Using a stone without the oil will allow the metal particles to lodge in the pores of the stone, building-up and consequently only burnishing the edge rather than sharpening it.  

Oil sold as ‘Sharpening Stone Oil’ is simply re-packaged mineral oil like one might buy at the drugstore, where it is sold as a laxative.  Other oils will also work — some craftsmen like to use motor oil thinned slightly with kerosene, and, in a pinch, vegetable oils will work too — although if the stone is not cleaned regularly the vegetable oil can go rancid and make the sharpening process a bit more odorous than needed.

After sharpening with the natural or the synthetic oil stones the new edge will probably still require a final honing with a very fine stone, razor hone or strop, or polishing compound.  If the black Arkansas stone is used this will not be necessary — it is so fine that it will polish the edge.

WATER STONES
The traditional Japanese water stone is made from a very fine naturally abrasive porous sandstone, although now-a-days the commonly available water stones are almost all synthetic.  The binding structure of the fine grit in the water stone is much weaker than the binding structure in an oil stone.  This means that the surface of the stone wears down much more quickly than on a comparable grit oil stone but that it also sharpens the cutting edge of the tool more quickly.  The reason water stones sharpen the blade more quickly is that the rapid wear of the stone’s surface exposes new cutting grit with unworn edges.  Water stones are usually used in sets of three or four grits in  descending sizes.

Water stones should be used only after they have been soaked thoroughly in water (at least an hour) before each use.  The water will condition the stone so the surface wears as evenly as possible — even so, water stones will need periodic re-surfacing because of their rapid wear.  The re-surfacing, which levels the surface to a uniform height, is accomplished with a slightly smaller stone made specifically for that purpose.  The very fine grit sizes of the water stones when used in sequence will preclude the use of any additional honing or stropping.

DIAMOND SHARPENING ‘STONES’
The relatively recent developments in abrasive technology have allowed diamonds, the hardest abrasive known, to be used as sharpening tools.  A uniform layer of sifted diamond grit is attached to a metal plate using electroplating to bind it to the surface.  The metal plate is in turn laminated to a rigid plastic structure to mimic the size and shape of a traditional sharpening stone.  Diamond ‘stones’ have the distinct advantage of sharpening very quickly with the least amount of effort — even stainless steel and tungsten-carbide cutting edges, which are difficult or almost impossible to sharpen on regular oil or water stones.  Diamond ‘stones’ must be used with water as a lubricant and coolant to prevent the diamond dust being worn off the surface of the metal plate.  Diamond ‘stones’ are usually available individually in two or three grit sizes.  They also cost several times as much as a comparable traditional sharpening stone.  Depending on how fine a grit is used the cutting edge (of high-carbon tool steel) may or may not need additional honing or stropping.

PROCESS: HAND SHARPENING TECHNIQUES
The techniques of sharpening with the hand stones is the same no matter which type of stone is used.  The stone needs to be held steady so it doesn’t slip around on the workbench or table.  Traditionally the stone is fitted in a shallow box with sharp nail points projecting to grip the surface of the workbench, though the commercially available boxes used now have rubber or plastic non-slip feet attached.  The stone can also clamped gently in a woodworker’s vise or a vise with padded jaws.

The table top should be set at a comfortable height when standing, about waist high.  Stand with the legs a foot or two apart and either roughly parallel to or at a slight angle into the near edge of the workbench.  
Hold the tool to be sharpened with both hands: if sharpening a gouge hold it much like you would hold it during carving,  if a chisel hold it upside down to a carving position,  if it is a knife hold the handle as you would during carving and place the other hand lightly on the back edge of the blade.

HOLDING THE ANGLE of the blade axis so it meets the surface of the stone correctly will vary depending on the specific tool.  Usually you would want to copy the same angle as the existing bevel on the cutting edge when you re-sharpen.  When you have the angle set with the tool in your hands, try to keep the relationship of your two hands the same.  This is best accomplished by keeping your arms fairly rigid and rocking your body back and forth on your feet.  Do not use the muscles of your arms to create the strokes of the tool over the sharpening stone — it is too easy to lose the correct angle you have set up. 

STROKE DIRECTION when sharpening might vary depending upon the type of stone being used, the type of tool being sharpened, and the skill or preference of the user.  The most common of these are the forward push stroke, the backward pull stroke, and the sideways stroke.  The pattern of the strokes can be straight forward and back, as well as in a figure eight.  Some craftsmen use one type and pattern of stroke for all their sharpening, and some, like myself, prefer to vary the type and pattern of stroke used for each particular tool along with the type of stone being used.  Just as in choosing the type of sharpening stone there are advantages and disadvantages for each method to be considered. 

The STRAIGHT BACKWARD PULL STROKE is probably the simplest technique to master.  The STRAIGHT FORWARD PUSH STROKE requires more control, but has the advantage that it will require less steps to complete the sharpening of a particular tool.  The SIDEWAYS FIGURE EIGHT STROKE is often used because it is relatively easy to master and it will also require less steps than using a pull stroke, and the movement is especially well-suited to sharpening curved-edge tools such as gouges. 

The techniques described below can be done on any of the types of sharpening stone, with any appropriate cautions noted for each type.  The process can be used for coarse sharpening if needed, and then repeated for the final sharpening with finer grits.  Very fine stones used on the final stages will give the edge a polished appearance.  If the finest stone available leaves visible scratch marks on the surface being sharpened then a further honing is necessary to polish the edge for razor sharpness.

BACKWARD PULL STROKE
The technique of the straight backward pull stroke is simply to repeatedly draw the edge back across the stone taking care to retain the proper angle. On a curved-edge tool like a gouge the tool is rolled during the stroke so that the entire edge is in contact with the stone for an equivalent amount of time.  Determining the exact amount of pressure used is not critical, and the strokes are continued until the edge has turned-up a burr on the side away from contact with the surface of the stone.  When the burr, or ‘wire-edge,’ protrudes more-or-less evenly across the width of the edge the initial step of sharpening is complete.  

The next step requires the ‘wire-edge’ to be removed.  This can be done several ways depending on the type and shape of tool being sharpened.  If the tool is flat on one side and beveled on the other (like a chisel or plane blade), then the tool is turned over and drawn back across the stone with the un-beveled side flat upon the surface of the stone.  This will either break-off the wire-edge completely, or it will turn the wire-edge so it protrudes from the beveled side.  If it turns it, repeat a few of the initial sharpening strokes with a light pressure — this will turn the wire-edge back to the flat side, where it is again drawn flat across the stone to break the edge off completely.
If the edge is being sharpened on both sides (like a knife blade), then the ‘wire-edge’ will naturally disappear if the strokes are taken alternately from one side, then the other.

On a curved-edge tool like a gouge the ‘wire-edge’ will need to be broken using either a shaped slip-stone or a wooden dowel.  The inner curved edge of the gouge is drawn lightly back across the curved surface of the slip-stone or a dowel.  Wrapping the dowel with a piece of 600 grit sandpaper will substitute for a slip-stone.  If a plain wood dowel is used more pressure can be applied to turn the edge back onto the bevel side.  The slip-stone or dowel should be held as parallel and flat to the inner surface of the gouge as possible.  If the ‘wire-edge’ is only turned and not broken off on the first attempt, repeat a few of the initial sharpening strokes on the bevel side with a light pressure — this will either break the wire-edge or turn it back to inner side, where it is again drawn across the slip-stone or dowel to break the edge off completely.  A parting or V-tool is sharpened the same way, excepting for the shape of the slip-stone or the appropriately-shaped piece of wood instead of a dowel.

When the wire-edge has been completely broken off all that remains to be done is to create a micro-bevel at a slightly steeper angle (about 5 degrees more than your original bevel).  To create the micro-bevel hold the tool at the correct angle and draw the edge straight back with very light pressure for one or two strokes.
Using very light pressure on the straight backward pull stroke will minimize the size of the ‘wire-edge’ created during the process.

FORWARD PUSH STROKE
The straight forward push stroke requires one to gently push the tool across the surface of the sharpening stone with just enough pressure to abrade the surface of the beveled cutting edge without digging-into the surface of the stone.  This type of stroke is well suited to sharpening with hard oil stones, but is less successful with Japanese water stones because they are relatively soft.  Using the forward push stroke on a diamond ‘stone’ requires a light pressure so the diamond is not scraped off the metal plate it is adhered to.  

The ‘wire-edge’ created in the pull method does not occur when using the push method, so the steps to remove the ‘wire-edge’ are not needed.  However, a micro-bevel should still be added at the conclusion of the process.  The micro-bevel makes that very fine cutting edge slightly more durable.

SIDEWAYS FIGURE EIGHT STROKE
The sideways figure eight stroke can be used for any type cutting edge, though it is especially well-suited to sharpening curved edge tools such as gouges.  In sharpening with the sideways figure eight stroke the gouge is held at right angles to the stone, maintaining the angle of the bevel, so that the contact of the edge of the tool describes a figure eight on the surface of the stone.  

During the figure eight stroke the tool is rolled so that the entire edge is in contact with the stone for an equivalent amount of time.  This rhythm is suited to rolling from the right side to left while describing of the first half of the ‘eight’, and rolling from the left to the right while describing the other half of the ‘eight.’
The ‘wire-edge’ created in the pull method does not occur when using the sideways figure eight method, so the steps to remove the ‘wire-edge’ are not needed.  However, a micro-bevel should still be added at the conclusion of the process.  The micro-bevel makes that very fine cutting edge slightly more durable.

CAUTIONS: SHARPENING WITH POWERTOOLS
The use of powertools to sharpen can speed the relatively tedious process of grinding a chipped or extremely dulled edge, but it requires a good eye, a steady hand, and a cautious attitude.  The friction of powertool grinding causes a rapid heat build-up in the contact areas.  If the heat builds up to the point where the temper colors on the surface of the metal appear, then the edge will dull very quickly when used.  If the delicate edge starts to turn red hot it can burn the carbon out of the steel and so prevent the edge from ever being sharpened properly.  If the metal has begun to change color the tool will require re-tempering.  

To prevent this from occurring, the craftsman must keep the pressure on the grinding wheel (or sanding disc) as light as possible, and must also be aware of how hot the blade is getting.  Use bare hands (no gloves) to hold the tool while grinding, and keep a bucket of water handy to quench and cool the tool when it begins to warm up.  

Though there are water-cooled slow-speed grindstones made specifically for the sharpening of tools available, most beginning carvers are not likely to have one — so the choice of power tool to use for sharpening will probably depend on what is available: the small benchtop grinder, or portable tools such as an electric drill, a belt sander, or a right-angle grinder could all be used (with the proper accessories) to do the job.  All the portable tools will require some sort of holding or clamping device to free-up both hands so they can be used to grasp the tool being sharpened.  If a stationary power tool is available it would be a better choice than a portable version, if for no other reason than the stability of a stationary tool.

If possible the surface of the abrasive wheel or the sanding disc to be used should be of a relatively fine grit (either “fine” or in the 100 to 150 grit size), with a flat and uniform surface.  If an abrasive sanding disc is being used then a new piece should be used.
Class:  Carving in Wood and Stone                               all materials copyright 2019 Don Dougan
Don Dougan                                                               www.dondougan.com
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Sharpening of woodcarving chisels and gouges