A few decades ago, American Barbers' organisations distinguished several stones.
- Dark colored waterhones, with a grey or clay-like appearance, producing a very fine edge, used with a rubbing stone that created a fine polishing sludge.
- The Belgian oil stone, top-light and mounted on a dark base, or bicolored ones cut from one piece of light rock and adjacent alkali deposit rocks. They cut slightly faster than the water hone.
- Synthetic hones, the oldest of which was the Austrian Swaty, a medium fast cutter, causing rapid overhoning in the hand of the unexperienced.
Present hones are natural or synthetic and can be used dry, with oil or water. Only very fine hones are suitable for razor sharpening; we will not discuss the coarser carborundum and sand stones here, because they are not suitable for razor honing. Fine and very fine natural stones widely available are the Arkansas, Belgian, and Japanese water stones. They are quarried and cut and polished into the preferred size. Most natural stones quarried in the 19th century were good. Particles were densely packed and highly regular. The newer natural stones all carry the risk of containing clustered particles, which form cuboid inclusion bodies that damage the edge; this is a side effect of the dense packing of particles. The figure shows a magnified natural hard Black Arkansas stone with a cuboid inclusion body. This may also be present in all other modern natural stones, the Japanese natural waterstones (Awase Toshi) included. A good natural stone will cost between a few hundred to over thousand dollars. Another problem with natural stones is that the tight bonding of particles prevents rapid release of worn particles. Therefore, used stones lose some of their cutting ability.
The Washita is a coarser type of natural Arkansas stone. They are graded from coarse to fine: Washita, soft Arkansas, hard Arkansas, Black hard Arkansas.
The Belgian stone, also called the Belgian Old Rock, used to be a superb natural hone for razor sharpening. It is used dry, with water or with oil, and has been the standard razor hone for centuries. It is still quarried in Belgium, in the caves around Liege, but due to some earthquakes the production process has been delayed. However, the best stones are already gone, and it is getting increasingly difficult to acquire modern ones with the same quality as those from the previous century. Therefore, due to irregularities, all present natural stones carry a considerable risk of damaging the edge of the razor.
Synthetic stones contain bonded abrasive particles. They are either completely synthetic or reconstituted from crushed natural stones. Some are to be used with water, some with oil, but all natural and synthetic stones might also be used dry. Examples of man made oil stones are silicon carbide and aluminium oxide stones with vitrified bonds. Woodworkers use the coarser ones to do some quick grinding, but for fine honing there are better alternatives. Oil will relatively soon mask the abrasive particles in a synthetic oil stone because of the type of bond. The mix of worn metal, rounded abrasive particles and the oil can stay fixed in place, causing quick glazing.
In contrast, the synthetic waterstones wear rapidly due to loose bonding, permitting easy and regular removal of the remnants with water, exposing a new surface continuously. See the flake like appearance with open structure and loose bond of the japanese waterstone below. The rapid wearing of a waterstone is generally considered a disadvantage, because narrow cutting tools can create furrows, and with heavy use its life is limited. However, this is relatively unimportant with the delicate use for razor sharpening. Synthetic waterstones came to the West in the seventies.
Synthetic waterstones cut faster than other stones of the same grit number, increasing the risk of overhoning at lower grit numbers, but also permitting the choice of an ultrafine polishing hone that has cutting abilities despite its high grit number. Overhoning is the process of creating a too thin edge, which will break off at first contact, exposing a rough, dull surface. The synthetic waterstones give a much finer finish than would be expected on the basis of their rapid cutting rate. In the fine and ultrifine range (#6000 to #8000), there is no comparable Western synthetic stone. 3M's 0.5 micron chromium oxid on Mylar film cuts exceptionally fast and gives a high polish. It is a #16000 abrasive, which should be glued on glass. Whether one still needs such a fine abrasive with the even finer strop as the last phase, is debated.
Diamond hones are extremely hard, and are available at grits up to #1200, but not in the fine and ultrafine range. They cut very fast. Therefore, they are not suitable for razors. Ceramic stones are sold in very fine grits. However, the bond is so hard that the abrasive particles, which cannot dislodge, get rounded after some time. They should be regularly lapped with a diamond stone to renew the surface.
It remains speculative which type of abrasive compound a synthetic hone should consist of. The most common types are silicon carbide and aluminium oxid. However, chromium oxid may give the edge its ultimate polish. Supported by woodworkers' research, the conclusion seems valid that razors should be honed with the finest available synthetic waterstone, preferrably consisting of chromium oxid, or as a good second choice with an antique natural hone. Modern natural hones probably should be avoided, while oil stones will not reach the same amount of polish as a very fine waterstone.
Rubbing stones create an abrasive sludge on the hone. Their usefulness is debated.
Oil and water
The purpose of a honing oil is to prevent glazing, and to flush dislodged abrasive and bonding particles. It shoild not clog the stone, and it should be a non-drying type. Light mineral oil will do, as will diesel fuel. In stones with an open structure and soft bond, glazing is primarily caused by abrasive particles. Water will flush these particles easily. You can use a water stone with water or dry, but oil will dissolve the resin bonds in a waterstone. In contrast, oil stones can be used both with water and with oil; only hydrofluoric acid will dissolve the vitrified bond of an oil stone.
Waterstones produce the best edge in the least amount of time. For polishing, no oil-, ceramic-, or diamond stone can match a waterstone.
A different discussion is whether you should hone dry or wet. Juranitch did research on this and found that liquid dispersed particles blunted the edge while honing. He advocates dry honing, and cleaning the hone with a cloth or water afterwards.
Flattening a hone
It will not be necessary with normal use with straight razors, because the surface is wearing regularly, but if you have an irregular hone you can flatten it. For waterstones you need sandpaper mounted on a glass plate: stones up to #800 need #120 wet- or dry sandpaper; #800 - #1200 stones need #220 paper; #4000 - #6000 stones need #320 - #400 paper. #7000 - #8000 stones: at least #400 paper. You soak the hone and the paper, then stick the paper to the glass with water. Rub circular, untill the surface is even. Hollow places will appear darker than the surroundings.
Oil stones are difficult to flatten. You need carborundum powder on glass.
Comparative grit numbers
The #1000 waterstone corresponds to soft white Arkansas oil stones, the #4000 waterstone (fine) to a hard black Arkansas oil stone, and the #6000 (S1 or silver stone), the #7000 Takenoko stone, and the #8000 G1/gold/North Mountain stone have no equivalent among natural stones.
Coarse is defined as up to #800, e.g. for flattening stones.
Medium is #800 to #1200: a sharpening stone for general use; however, it is too coarse to sharpen a razor.
Fine: #4000 - #8000, polishing stones.