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Bench Stones: Answers to Some Gritty Questions Print  |  Back

From: Woodcraft

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benchs1.jpgJohn Mignone is a woodcarver, classical musician, and a tree propagator. Carving professionally since 1990, he is a Contributing Editor at Wood Carving Illustrated. John lives in East Meadow, New York.

I’ve been asked, especially by newcomers, to recommend the one bench stone they should purchase as a sharpening aid. Although this sounds like a simple question, it’s not. In response, I find myself asking if the carver is using a knife and one or two palm tools or is he carving with full-size chisels and gouges? Is a grinding wheel available? And with what regularity are the tools finely honed or sharpened? The answers help determine the types of bench stones he needs. What follows is an examination of the better sharpening stones available to carvers today and some history.

In the Beginning[b1] The earliest edge tools were made from flint, which was chipped to form a hard edge. In the Bronze Age, tools were fashioned from copper and copper alloys. Edges were beaten to shape. When iron and steel tools came into vogue, they needed an abrasive to refine and restore an edge. In the history of abrasives, a naturally occurring material called sandstone was traditionally used to sharpen tools. Sandstone—”sand” being a description of particle size, not composition—came in two forms: bench stones and grinding wheels. Often rectangular in shape, the bench stone got its name because it was placed flat on a woodworker’s or carver’s bench and the cutting edge was moved across the stone’s face.

benchs2.jpgWhile either bench stones, grinding wheels or both are in almost every carver’s shop, there have been some changes in the material and technology in the last century. Sandstone, consisting of quartz crystals that have been naturally bonded, has been replaced. Man-made materials held together with resinous bonding agents and much harder but still-natural stones are commonly used today.

It was in the late 1800s that harder materials for rotating stones were sought after. Materials called silicon carbide and aluminum oxide were developed. Cost efficient and able to put an edge on steel, these materials made their way into bench stones. In the last 30 years, diamond and man-made ceramic stones have put their mark on tool sharpening. Are Bench Stones for You?[b1] Power sharpening is becoming more popular today, with horizontal and vertical grinding wheels proliferating, and I own both kinds. Needless to say, if your tool has a chipped edge or is very dull, it can be foolhardy to attempt to bring the edge back on a bench stone. Moving the tool back and forth on a stationary stone, even one that is coarse, can take an excruciatingly long time. However, some coarse stones described below will restore a dull or damaged bevel if you have the patience. What, then, are the advantages to having a bench stone?

benchs3.jpg•A good bench stone removes the bur or wire edge produced by a grinding wheel. It also keeps the edge finely tuned, saving you from going back to the wheel. I call this process finish sharpening.

• Stones give you a longer-lasting edge. While you can achieve a very sharp edge with motorized grinding and buffing wheels, scratches are usually left.

•When taking needed breaks during the carving process, touching up edges on a bench stone should be done and can actually be relaxing.

Stones for All Seasons[b1] Bench stones are available in a variety of sizes and shapes Bench stones are available in a variety of sizes and shapes. Tapered stones have both concave and convex surfaces that are ideal for gouges. The slip stones that are on the market offer either small convex edges that are excellent for small gouges or tapered edges for V tools. Some have both profiles. Square stones can be purchased that are suitable for chisels, 90-degree V tools, and large gouges. Still others are shaped like rods—useful when touching up the inside of a gouge’s bevel—and files, which are helpful for knives, axes, adzes, and drawknives.

Lubrication[b1] Let me offer a few words about lubrication. Old fashioned grinding wheels typically moved through a trough of water. Many of the wheels today have water-drip systems or water reservoirs. The water is necessary to keep the steel from heating up and losing its temper, which is the process of hardening steel through heating and cooling. For bench stones, lubrication is necessary because it contributes to the sharpening process. First, the lubricant helps prevent the stone from clogging up with the minute particles of tool steel that are left on the surface. And second, the sludge created when the stone’s loosened grit mixes with the lubricant actually contributes to the sharpening process. Be advised that the coarser the stone, the more lubrication is needed because the absorption into the stone is greater.

Many carvers have the mistaken notion that only oil can be applied to a bench stone. While it’s true that oil cannot be used with silicon carbide and some other stones, I prefer water for all other bench stones. Oil is much more difficult to clean up than water. If it gets on my hands and then on a carving, a stain is produced that can adversely affect the finish. Water, on the other hand, dries quickly and will not leave a mess on the wood, clothing, or bench top.

The Bond [b1] To help you understand how well a stone sharpens, you need to understand bonding. A hard bond results in a stone that wears slowly. Typical stones with hard bonds go by the names of Arkansas, oil and ceramic, the last having the hardest bond. In all of these stones, the abrasive grit is surrounded by and almost immovably imbedded in the structure of the stone. The nature of the bond, however, creates a comparatively slow cutting action. Instead of the top layer washing away and exposing new grit, it stays in place. What results is a glaze. When enough steel particles produced by sharpening get imbedded in the pores of the bond, the result is a dull or glazed surface. The problem of glazing in a flat stone can be remedied by regularly lapping it on a flat diamond stone or 150 to 220-grit wet-and-dry sandpaper backed by a machined metal surface or glass. When rubbing or lapping one surface on another in a circular motion, the unwanted steel is removed. Only a small amount of lapping is required to remove the glaze. You will notice immediately how the performance of the stone is improved. The Oil Stone [b1] benchs4.jpgAn old standby and still a good choice for a carver today is the oil stone, which comes in a variety of shapes to accommodate almost every carving tool from a knife to a gouge. Described as a synthetic stone, the abrasive consists of aluminum oxide or silicon carbide. The abrasive is set in a clay-like material to create a hard bond. For a lubricant, a medium-weight oil or kerosene is recommended. The aluminum oxide oil stones are available in finer grits than the silicon carbide stones, but they are not available in very fine grits. It is important when using an oil stone to wipe it with a clean, dry cloth to remove the tiny steel particles that clog the surface. When the particles do build up in the pores, then you will have to turn to a diamond stone or sandpaper. If the stone is tapered, round or V-shaped, gentle sanding with wet-and-dry paper removes the particles of steel.

The Natural[b1] Naturally formed from a mineral called novaculite, the Arkansas stone was considered to be the sharpening stone by many woodworkers and woodcarvers. I use past tense because, in general, the quality of novaculite deposits has declined while quality synthetic stones have become available. When you do come across Arkansas stones, which are available as bench and slip stones, you will discover that they are graded coarse to fine in the following order: Washita, which is fairly coarse but fast cutting, soft, hard, black hard and translucent. Either water or oil must be used as a lubricant when sharpening on these stones to prevent metal particles from getting imbedded. And, like oil stones, they need to be wiped clean after use. Arkansas stones are still good choices. In fact, hard Arkansas slip stones and files are available and make for excellent sharpening accessories.

The Perfect Stone? [b1] benchs5.jpgUsing aluminum oxide as the abrasive, the manmade ceramic stone is set in an extremely hard bond. The advantage is that it sharpens quickly and keeps its shape for a very long time with no “dishing out” on the surface. The disadvantage is that worn and rounded abrasive grits stays in place instead of dislodging. The result is a reduction in sharpening speed. To deal with the problem, I lap my flat ceramic stones frequently with a diamond stone. Ceramic stones are available in only three grits: medium, fine and ultra-fine. While sharpening can be accomplished without a lubricant—some catalogs suggest that it is not needed for this stone—I use water. I have found that without the lubricant, the stone quickly glazes.

The Waterstone[b1] About 25 years ago Japanese waterstones hit the American market and are readily available. In competition is Norton, an American company that is now producing water stones. A decided advantage to these waterstones is their soft, porous resin bond. When sharpening, the top layer of abrasive grit is slowly but constantly washed away to expose new, sharp abrasives. This action is what accounts for the faster sharpening ability of these stones. However, they will wear more quickly than their harder counterparts described above. I recommend that the stones be lapped regularly, but not because they become glazed. Rather, the lapping keeps them flat. A diamond bench stone is needed or wet-and-dry sandpaper backed by a metal plate or glass. The most commonly used abrasive in the Japanese waterstones is aluminum oxide, although some stones with green carbide are available. The grading of their grit size is done very accurately, and they possess the greatest grit range of any stone: from about 100 to 8,000.The 6,000 and 8,000-grit stones cut and polish finer than any other type of stone. A word of caution: If you want to keep a flat and unmarked surface, don’t let a tool corner dig into the stone.

Are Diamonds a Carver’s Best Friend?[b1] Commonly available to carvers is the diamond stone, which I have recommended to use when correcting the problems of other bench stones. Since diamonds are the hardest material that can be used for sharpening devices, they will put an edge on just about anything, including carbide. But their real asset is how fast they sharpen, especially when compared to other stones. Diamond stones consist of either monocrystalline or polycrystalline diamonds. The diamonds of both types of sharpening stones are usually bonded to a metal plate that is machined very flat. In contrast to the stones described above, the grit of the diamond stone is not surrounded by any other material. The monocrystalline grit consists of single crystals while the polycrystalline grits consist of small crystals forming on top or around each other. The difference for a carver, aside from the fact that the monocrystalline stone is more expensive, is that the polycrystalline grit fractures and breaks down over time, leaving a finer and finer grit size. Consequently, it takes longer to sharpen a tool, but not appreciably so.

The typical grit range for diamond stones is 220 to 1200. Because the coarser stones cut so aggressively, they produce grooves in the cutting edge. A fine stone such as a black hard Arkansas is recommended for a second round of sharpening. If you do use diamond stones without a lubricant, be aware that metal particles will eventually clog them.

Flexible Stones[b1] A few years ago, 3M Corporation began manufacturing diamond stones. Rather than using a metal plate, the company produces a finely graded diamond grit on a piece of flat plastic. Even more novel are the flexible adhesive strips that can be cut to any size or shape you desire. This feature allows you to make your own slip stones, cone or rod-shaped sharpening devices, or files. Even a hook knife will no longer pose a sharpening problem. Another plus is that 3M diamond stones come in 1800 grit, which is fine enough for most finish sharpening.

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Stones in Storage[b1] Both oil and Arkansas bench stones usually come in wooden boxes. It’s a good idea to keep them enclosed in these containers, especially if wood dust is a problem in your shop or work area. Wood particles will clog the surface of the stone and reduce the sharpening efficiency. If a box is not available or if you want to store those specialty stones such as slip stones and tapered stones, try a plastic storage container. It is readily available at home centers and won’t damage the stones.

It is preferable to store a waterstone in water if you like to start sharpening immediately. Otherwise, the stone will have to be immersed for a while. In most cases, though, waterstones can be allowed to dry out without problems. However, they must never be allowed to freeze. Any water in them will cause cracks and breaks.

Making Choices [b1] benchs7.jpg Every carver should have at least one full-size bench stone that measures 8 in. long and at least 2 in. wide. A wide stone will provide a surface big enough for a large chisel or gouge if you own one or that you might purchase in the future. I do recommend, in addition to the flat stone, a fine-grit slip stone and a leather strop to remove even the smallest bur left after sharpening with a bench stone. If you are still uncertain about what to purchase, talk to other carvers and try their bench stones. If you belong to a carving club, it would be a good idea to have a sharpening night. Sharpening should always be the first priority after purchasing the tools. After all, how efficiently can you carve if your tools don’t have that fine edge?