Oilstone

Also called: sharpening stone, whetstone, waterstone, honing stone, slipstone

An oilstone is a rectangular block of fine abrasive stone, used together with a thin film of oil, for putting a keen cutting edge on tools such as plane irons and chisels after they have already been ground to shape. Where a grindstone removes metal quickly to reset the angle of a badly worn or nicked edge, an oilstone works far more slowly and finely, polishing that ground angle into a sharp, smooth edge ready for use.

Why oil, and which oil

The oil on an oilstone does much the same job as the water poured over a grindstone: as Goss (1888) put it, "Oil is used on an oilstone for the same reason that water is used on a grindstone." It keeps the stone's surface clear of the fine metal and stone dust the sharpening produces, so the abrasive keeps cutting instead of clogging with paste. Hasluck (1907) recommended neat's-foot oil or sperm oil as the best choices, with lard oil an acceptable alternative provided enough paraffin was mixed in to stop it thickening in cold weather.

Coarseness and stone type

A stone's cutting speed and the fineness of the edge it leaves both depend on its grit, the size of the abrasive particles bound into it. Joyce (1987) describes the ordinary range as coarse, medium and fine: coarse for quickly restoring a bevel that has been ground out of true, medium and fine for routine honing, with double sided stones combining a coarse face and a fine face useful where a workshop only sharpens occasionally. That is a modern, manufactured abrasive. Goss (1888), writing when oilstones were cut from natural quarried rock, was concerned with a different problem: finding stone free of hard quartz grains, which could scratch rather than hone an edge. He noted that stone of that quality above about four or five inches across was scarce enough that only a workshop which genuinely needed a stone that size would pay for one.

Sharpening on the stone

A plane iron that has just been ground still carries the coarse scratches of the grindstone and needs finishing on the oilstone before it will cut cleanly. Hayward (1950) described laying the cutter with its ground bevel flat on the stone, then raising the hands slightly to tip the edge up onto the honing angle, the finer angle actually rubbed on the stone. Wearing (2010) notes that this honing angle is commonly increased to around thirty degrees on the oilstone, steeper than the angle the bevel was ground at. Because only that small honing bevel, not the whole ground bevel, is renewed at each sharpening, it grows a little wider and its angle becomes less acute every time the tool goes back to the stone, until the edge eventually needs regrinding to reset it. For curved edges such as gouges, the flat face of the stone will not reach into the hollow, so the stone is turned on its edge, or a shaped oilstone slip is used instead. An oilstone can also true up the side of a saw blade, a job Goss (1888) says can equally well be done with a file.

Looking after the stone

Because honing wears away the centre of a stone faster than its ends, repeated use can leave the face dished rather than flat, which then hones a curved edge instead of a straight one. Porter (1999) warns against letting this hollowing happen, and Bird (2004) recommends rubbing the stone flat again on a diamond plate once it has. Some workshops keep the stone in a fitted wooden box: Porter (1999) describes a design where the stone sits recessed partly in the base and partly in the lid, packed at each end so its whole length stays usable and it cannot shift about in the case.