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Knife sharpening stone

Knife sharpening stone

Introduction

If you ask a person unfamiliar with knife sharpening, or even a beginner, what is a knife sharpening stone, you are likely to hear a description of a rectangular, oblong object that can be used to sharpen a blade.

Many people have probably heard the saying «not all that glitters is gold», and it is true in our case. Technically speaking, not every sharpening stone can be called a stone, but that is just the way it is. 

The curious mind will immediately pose the legitimate question of why, all of a sudden, some objects commonly referred to as sharpening stones are, in fact, not stones. From this, one can draw the logical conclusion that there are sharpening stones and so-called "non-stones".

A number of questions follow: why is this so and what are the reasons for this

1. A brief history of sharpening stones

To answer the questions, we need to go back a little to the historical evidence that the use of sharpening stones began to develop in the late Stone Age around 5,000 BC, i.e. at the turn of the Bronze Age, and is directly related to the use of cutting tools. The first sharpening stones were made of treated rocks with abrasive qualities. According to some studies, however, people invented the ancient method of sharpening tools as early as about 75,000 years ago.

Over time, stone tools were replaced by metal tools, and sharpening stones changed in size and became smaller for easy transportation. With the transition to the Iron Age, iron became the main material from which various tools and weapons were constantly made. Later on, the development of various new forms of activity led mankind to the need to find new abrasive materials and improve abrasive tools in order to improve the quality of finishing.

Later, the extraction and processing of abrasive minerals became the subject of mining operations to provide a range of industries with the necessary grinding supplies. For centuries, well-known natural abrasives such as garnet, flint, corundum and emery have been used in the production of grinding stones, grinding tools and abrasive consumables and are still in use today.

From the age of industrialisation to the present day, all companies manufacturing sharp blades have a complete blade machining cycle, including roughing, grinding and finishing, which is done on different equipment, but grinding wheels, belts and other abrasives which are produced by large specialised companies are still used at different stages of production.

The development of specialised knife sharpening workshops as independent businesses came later in response to the widespread distribution of knife products and the growing needs of society, which could not be covered by the knife manufacturers themselves.

In more recent times, sharpening stones have been quarried all over the world (in England, France, Germany, Sweden and America), becoming widespread. Natural water stones and oil stones of various types and sizes are still used by many professionals today. The high demand for quality abrasives has resulted in many quarries reaching exhaustion levels and becoming unprofitable.

With the development of the chemical industry it became possible to produce synthetic abrasive wheels and stones with a homogeneous grain and the demand for natural abrasives began to fall.

Due to the significant growth of the industry, the volume of consumed abrasives has increased, which in turn has led to the need to establish specialised factories for the production of various abrasives based on synthetic abrasives on an industrial scale.

2. The main distinguishing factors for sharpening stones

Thanks to a brief historical overview, it becomes clear that the existence of natural sharpening stones, which can indeed be called stones, and the availability of a wide range of artificial abrasives, remain an undeniable fact.

Besides, we now also know that there are three main production steps for knife blades: roughing, sharpening and finishing.

As mentioned above, from a certain period the mined rock was processed and the stones were given a certain size that could be easily transported.

The main factor for determining the length of the sharpening stone is the length of the blade which is about to be sharpened. If the length of the sharpening stone is equal to the length of the cutting edge, this gives you a considerable advantage in the sharpening process: you can cover the whole length of the edge in one pass.

For the same reason, sharpening «stones» based on various artificial abrasives and bonding materials have almost the same classic dimensions.

In fact, technically both groups can be called sharpening stones or abrasive consumables. Some of them by all rights can be called sharpening stones, referring to their natural origin, while others artificial abrasives.

Given the above, there are a number of parameters that are clearly expressed in numbers and provide a more in-depth and detailed understanding of what an abrasive is.

2.1. Grit of a sharpening stone

The grit size of each sharpening whetstone or abrasive block is expressed in numbers and corresponds to one of the existing standards. This value gives an approximate idea of the purpose of the consumable, and places it in one of the six usual categories according to its intended use.

The main categories of grit include:

  • Extra coarse – 0 – 200 grit
  • Coarse – 220 – 600 grit
  • Medium – 600 – 1000 grit
  • Fine – 1000 – 3000 grit
  • Extra fine – 3000 – 5000 grit
  • Ultra fine – greater than 5000 grit

In fact, the three most commonly used terms are coarse, medium and fine abrasives, which correspond to the three production processes: roughing, sharpening and finishing. In broad terms, the sharpness that can be achieved with fine abrasives is more than sufficient for most people.

The grit data given is expressed according to the Japanese JIS system and you can get a better picture if you take a look at the summary tables of the various grit systems.  A good and constantly updated source would be the Grand Logarithmic Grit Chart from Gritomatic.

2.2. Abrasive material of sharpening stones

Regarding the abrasive material of the sharpening stone, it should be emphasised that there are two categories of abrasive material according to their origin:

  • Natural abrasives
  • Chemically produced artificial abrasives

Natural abrasives may be somewhat heterogeneous in origin due to grain size and may have additional inclusions of other rocks. However, the main difference compared to artificial abrasives is their somewhat different structure. Looking at the example of Japanese oil shale, the grain here looks like fine flakes rather than grains of a certain size. As a consequence, when metal is removed, the thinnest layers are removed from the surface of the secondary bevel, resulting in a more stable and evenly sharp cutting edge, rather than across the secondary bevel with the creation of scratches.

Natural abrasives include the following materials

  • Micro Quartzite
  • Novaculite
  • Sandstone
  • Slate

Natural sharpening stones most often have a large amount of silicon dioxide and aluminium oxide in their chemical composition.

Micro quartzite is a micro-grained quartzite, a type of siliceous rock with a different density, structure and colour, which can be considered a kind of quality equivalent to the well-known arkansas.

Novoculite in good quality is a rare and uncommon rock. One should know that this whetstone is a type of sedimentary rock consisting of micro-crystalline quartz, which has formed due to compression under pressure of fossilised plankton. For this reason we cannot tell the size of the grains, as they are almost identical. What is of great interest is the density of compression of this rock.

Well-known sharpening stones of this kind include washitas and arcansas, jasper, chalcedony and agates. This material is suitable for the final sharpening stage of a knife blade (up to 10000 mesh) for a very high level of sharpness, although due to its fineness, polishing takes time.

Sandstone is a well-known abrasive material, which consists of sand grains bonded together and is basically quartz. Finding the best sandstone for sharpening is a difficult task, even though its hardness and uniform grain size, as well as its brittle structure, make it a good material to use for sharpening knives and other tools.

Slates are very complex in their structure and it is difficult to judge the grain size of such structures. Very much depends on the place of origin and nature. It can be found in many countries, which will provide you with many sharpening stones with different qualities, textures and grains. Slate makes an indispensable natural abrasive, commonly used for finishing operations for creating a fine and very sharp cutting edge.

Artificial or synthetic abrasives are represented by the following compounds:

  • Silicon carbide
  • Aluminium oxide
  • Diamond stones
  • CBN stones

Silicon Carbide (in the form of powder with a certain grain size) is sintered with a ceramic bonding agent and later used to produce bars of the required size. Silicon carbide stones are versatile and can be used for sharpening forged steels with a hardness of up to 62-64 HRC. Silicon Carbide stones are considered to be water stones.

Aluminium oxide is produced from bauxite clay and sintered with a ceramic or magnesite bonding agent. Magnesite-bonded bars are considered to be of professional grade. Often manufacturers colour the aluminum oxide stones in certain series for better brand recognition in their own colours. Such stones work delicately with steels with a hardness of up to 60 HRC and are also considered to be water stones.

Diamond stones are manufactured with several different bonding agents (electroplated, metallic and organic), but their main advantage is their speed of working with almost all existing steels and they have a much better wear resistance than the previous ones. Diamond stones can be used for sharpening as a whetstone, with a light soapy water solution.

CBN bars are made of an ultra-hard material - Cubic Boron Nitrite consisting of boron and nitrogen atoms, which is second only to diamond in hardness. The stones are suitable for sharpening both soft and very hard steels. As well as diamonds, they are made with a variety of bonding agents.

2.3. Cooling liquids for use with a sharpening stone

There are three categories of sharpening stones: those used with oil, those used with water and those that do not require the use of coolants.

Sharpening stones used with oil can be either natural or artificial abrasives. This detail is specified in the manufacturer's recommendations for use.

The same applies to a whetstone. In the case of a whetstone, Japanese natural and artificial sharpening stones are most commonly used for making sharp Japanese kitchen knives, including top-end knives. If these are stones of European origin, it is worth reading the instructions for use and/or asking about them before making a purchase.

There are stones that do not require a long soaking in water and are used on the Splash&Go principle.

Among all artificial abrasives there are some that can be used without a coolant. Such abrasives are not used very often and are only recommended for use by experienced users.

2.4. Bonding agent for sharpening stones

Generally speaking, bonding is necessary only for artificial abrasive stones, while natural abrasives, on the contrary, have their structure created in the course of geological changes in the earth's crust.

The most common bonding types include:

  • Ceramic bonding - used for the production of silicon carbide and aluminium oxide based stones
  • Magnesite bonding - a more advanced bonding type for the production of aluminium oxide-based bars. It is more brittle and softer than ceramic bond
  • Electroplated bonding - mainly used for the production of diamond stones for intensive metal removal. Has a low wear resistance.
  • Metal bonding - the most popular copper and/or tin based bonding type for the production of diamond and CBN bars.
  • Organic bonding – one of the most versatile bonding types, with a longer service life than electroplated bonding and better performance than metal bonding. Designed for the production of diamond and CBN based bars.

Conclusions

All existing differences in the size of the sharpening stones, the grit size of the abrasive material, the material itself and the bonding agent of the sharpening stones mostly depend on the place of origin, the country of production including industrial standards and the typical knife traditions of one geographical area or another.

All of the sharpening stones available today have their advantages and disadvantages, but when choosing a sharpening stone, which can be sold both individually and in a kit, the right one for you is the one you need to work with your knife.

You just have to properly examine the condition of your knife blade. In other words, if the cutting edge of your knife is really dull or heavily chipped, you will of course need a coarse abrasive for roughing. If the cutting edge is in good condition but you need to make it sharp and polish it, then a medium and a fine abrasive is the right choice. This way you can carry out three sharpening stages: roughing, sharpening and finishing.

If you are a more sophisticated user, you can always increase the number of sharpening steps in each of the three stages as you wish by increasing the range of sharpening stones in an appropriate format using a sharpener.

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