The main issue of this article is the grit of the sharpening whetstone, which determines the quality of processing the secondary bevels, which affects the level of sharpness of the cutting edge and the possible level of the sharpness retention of the cutting edge of the knife blade.
It would seem that everything here is quite clear, but what exactly characterizes and determines the grit of a particular abrasive?
The word "grit" refers to tiny particles or pellets that form the structure of a sharpening whetstone.
The mere fact that a knife sharpening whetstone consists of a certain kind of tiny particles does not reveal the essence of the question: in what way the grit size affects the cutting qualities of the blade edge. Nevertheless, it is obvious that different grit sizes have different effects on the cutting properties of the knife.
Chances are that if you go into a hardware or a knife store and ask a salesperson about a sharpening whetstone, you will be immediately offered several options. You will probably see a double sided sharpening stone or perhaps a uniform sharpening stone of different abrasiveness level of synthetic origin. Besides, they might offer you a more homogeneous and relatively smooth sharpening stone of natural origin.
Still, the questions remain open: How did it come to this? What is the difference between natural and artificial materials and what is the structural and practical difference between the grains of different abrasives?
In fact, the abrasive properties of various natural stones have been used by mankind since time immemorial. The extraction of abrasive minerals has been the subject of mining operations. About two hundred years ago, before the invention of artificial abrasives, the deposits of stone from which grinding wheels and stones could be made largely determined the economic development of the region, like gold mines.
For centuries, well-known types of natural abrasives such as garnet, flint, corundum, emery and pumice have been used and are still used in the production of sharpening tools. These materials are still the universal means of manual finishing and polishing knife blades.
A number of natural materials with different hardness of the grit were taken as the basis of the relative hardness scale proposed by German scientist Friedrich Mohs, which can be used to measure the hardness of synthetic abrasives.
In the progress of industrialization and with the development of the chemical industry, it became possible to produce synthetic abrasive wheels and sharpening stones with a certain grit and the demand for natural abrasives began to fall.
Due to the significant growth of industry, the volume of consumed abrasive materials increased, which in turn led to creation of specialized factories for the production of heterogeneous abrasive materials based on synthetic abrasives on an industrial scale.
Like any other material, both natural and synthetic abrasive grit has its own unique physical characteristics such as hardness and size that are important when choosing a sharpening stones for sharpening knife blades.
The Mohs scale (also known as the mineralogical hardness scale) is a set of reference minerals for determining the relative hardness by scratching. Ten natural minerals, arranged in ascending order of hardness, are taken as references.
It consists of 10 hardness references: talc - 1; gypsum - 2; calcite - 3; fluorite - 4; apatite - 5; orthoclase - 6; quartz - 7; topaz - 8; corundum - 9; diamond - 10. Minerals with an index below 7 are considered soft, those above 7 are considered hard. In general, the main part of the natural compounds has a hardness between 2 and 6.
The hardness of a whetstone reflects the resistance of its surface when scratched by another stone or other object; hardness is a measure of the atomic structure of a substance. The hardness of the same stone can be different. The big difference in hardness in different scratching directions among the other minerals is typical of kyanite: its hardness varies from 5 to 7, it can be scratched in some directions but not in others.
Mohs Hardness Scale
|
Hardness |
Mineral / Chemical Formula |
Absolute hardness |
|
1 |
Talc (Mg3Si4O10(OH)2) |
1 |
|
2 |
Gypsum (CaSO4·2H2O) |
3 |
|
3 |
Calcite (CaCO3) |
9 |
|
4 |
Fluorite (CaF2) |
21 |
|
5 |
Apatite (Ca5(PO4)3(OH-,Cl-,F-)) |
48 |
|
6 |
Feldspar (KAlSi3O8) |
72 |
|
7 |
Quartz (SiO2) |
100 |
|
8 |
Topaz (Al2SiO4(OH-,F-)2) |
200 |
|
9 |
Corundum (Al2O3) |
400 |
|
10 |
Diamond (C) |
1500 |
The list of materials with abrasive properties is very wide, but the basic abrasive materials, which have industrial importance even today, are the same. Knowledge of properties and areas of use of natural abrasive materials allows their use even in the ultra-modern productions.
One can guess that if the source rock of natural stones is different, then the size of the particles in different natural stones will also have a different size. Besides, even within the limits of one rock the grains can be affected by natural factors and have different sizes. The reason for this may be: pressure in the layers, temperature effects, as well as various inclusions.
The problem is also that it is in human nature to limit many things to certain units, because it is easier this way and there is a chance to understand things through unified values.
If you seriously ask the question and try to understand the existing industrial standards on the basis of summary tables, you will not get a complete exceptional clarity either. The reason lies on the surface: the different countries have different classifications of abrasives.
In terms of chemical composition, natural sharpening whetstone types most often contain a large amount of silicon dioxide or aluminum oxide, which can also be seen in the chemical formulas of the Mohs scale.
Besides, when comparing grit sizes, as already mentioned, it is common to use summary tables, but they do not reflect the whole picture, and you have to check a lot yourself.
The grit of the abrasive material or, in our case, sharpening stones or bars should be understood as the size of the average grinding particle in the fraction, which is measured in certain units and is a discrete value, that is not continuous.
If you try to solve the problem of choosing grit by narrowing down the values to certain units of comparison, you are on the right track, but there are disadvantages and limitations within all standards.
Among the common grit classifications showing the value from extra coarse to ultra fine values are the following systems:
Within each standard, and depending on the manufacturer, a different set of grit is defined. For example, FEPA-F has F500 and F600 grit, but there is no F550 or F650.
This is all because any abrasive consists of a very large number of particles, and the value indicated is that which is predominant in a given sharpening stone. In fact, no industrial particle filtration process during production can deliver an abrasive in which all particles are exactly the same.
This natural compound is better known in sharpening as stones with quartz base, which include arkansas (chalcedony), washita and jasper. The best known natural silicon-based whetstone series are Arkansas soft 600 - 800 grit, Arkansas hard 800 - 1000 grit, Arkansas translucent 3000 - 8000 grit, Arkansas hard black 2000 - 3000 grit.
The natural stones in this group are novaculites, which are sedimentary rocks composed of microcrystalline quartz formed by compression under the pressure of fossilized plankton. Because of this reason, we cannot talk about grit size, as it is almost the same. Among the characteristics of this rock, the density of compression is of great interest to us.
What this actually means is that the sharpness result depends on the amount and density of the cooling liquid and the pressure applied to the secondary bevel. The more fluid the oil is and the less pressure is applied to the abrasive, the cleaner and more uniform the secondary bevel will be.
As mentioned above, there are both natural and artificial aluminum oxide. Artificial aluminum oxide is produced from bauxite clay and sintered on a ceramic or magnesia bonding agent. Magnesia-bonded bars of professional series are popular in Japan and are used for sharpening steels with hardness up to 60 HRC and are also used as a whetstone for kitchen knives of the highest quality.
There are also some equivalents to the whetstone and oil sharpening stone series more familiar to the United States market.
Aluminum oxide sharpening whetstone examples: Norton India Stone (FEPA-P): 150, 240, 400. Boride T2 (ANSI): 150, 220, 320, 400, 600, 800, 1000, 1200. SHAPTON Ha-no-kuromaku: 120, 220, 320, 1000, 1500, 2000, 5000, 8000, 12000, 30000 grit. The following stones from Japan are high quality water-based abrasives Naniwa Professional: 400, 600, 800, 1000, 2000, 3000, 5000, 10000, Suehiro Cerax: 320, 700, 1000, 5000, 6000, 8000 grit.
Silicon carbide is a synthetic substitute for the rarest mineral moissanite, which is found in nature only in deposits of corundum and kimberlite, and even less frequently as part of meteorites that have fallen to Earth from outer space. Natural moissanite is found in large quantities only in carbon-saturated dust clouds near stars.
As a synthetic material, silicon carbide adopts the properties of natural moissanite. It is also much easier to get. The synthetic inorganic binary carbon compound forms crystals similar in appearance to anthracite, but with rainbow-colored iridescence. Usually crystals are colorless and shiny, but technical carborundum sometimes acquires different color shades due to the presence of iron impurities.
Silicon carbide in powder form of a certain grit size is sintered with a ceramic bonding agent and cut into bars of the required size. Silicon carbide is versatile and can be used for sharpening steels with a hardness of up to 62-64 HRC as well as kitchen knives. Silicon carbide whetstone types are considered to be the type of a water whetstone.
A good example of a sharpening whetstone series based on silicon carbide - Boride CS-HD series of stones (ANSI): extra coarse 120, 150, coarse 220, 320, medium 400, fine 600, extra fine 800, 1000, 1200 and Norton Crystolon Stone (FEPA-P): 120, 180, 320.
Cubic boron nitride was first produced in 1957 and has no analogues in nature, its chemical formula is CBN or cubic boron nitride. CBN is somewhat inferior to diamond in terms of metal removal, but it works finer and is considered a more expensive and higher quality abrasive, especially if it is made with an organic bonding agent. The CBN grit itself has a more consistent grain size and shape than diamond grits.
Today, cubic boron nitride based bars are not very common and are available in limited quantities in the United States. and European markets.
Depending on the grain size, diamond powders are used for grinding or polishing items made of natural and artificial stones, ceramics, glass and metals of high hardness.
Diamond powders are used for sharpening carbide tools; cutting, grinding, finishing and polishing hardened steel, hard alloys and ceramics.
Here are some outstanding examples of a diamond sharpening stone: Edge Pro Matrix, Naniwa, as well Venev Diamond Abrasives and a number of other manufacturers.
Diamond abrasives are used just as often as a water whetstone. Soap emulsion is used as a coolant with diamond abrasives.
The choice of grit for sharpening any knife is directly related to the quality, steel grade and purpose of the knife blade edge. These specifications determine the choice of an abrasive stone. There are three stages of knife blade processing - roughing of the dull blade edge with coarse grit abrasive, sharpening with medium grit abrasive, finishing and polishing with fine or superfine abrasives.
It is possible to sharpen or hone a dull knife edge using an artificial double sided abrasive in a coarse to fine combination. You can use one side or you can use both sides one after the other. You can find such abrasives from Venev Diamond Plant or Norton.
Natural and artificial Japanese water stones, on the other hand, are designed for fine finishing blade edge with a stone above 6000 grit and are designed for experienced users.
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