Sharp as a Knife - Part 1

Working with dull knives is as torturous as it is dangerous. So how do you keep your blades sharp? Some sharpening methods are suitable, while others are best avoided for safety reasons. This is Part 1 of the article on grinding knives for use in pedorthics.

Sharp knives are a prerequisite for many work steps in upper construction. They are the upper maker’s most important tool when it comes to creating cutting patterns, cutting upper and lining leather as well as sharpening individual parts of the upper.
The sharpening of knives was once one of the first lessons taught to apprentices. The journeymen and masters were correspondingly skilled in the sharpening of different blades. Dictated by new tends in outsole design, sharp knives became less and less important. Leather toe puffs were almost completely replaced by thermoplastically formable ones. Pre-sharpened counters can now simply be purchased, and precision work is unfortunately done more and more often on finishing machines rather than on a cap sharpening board. Completely hand- and frame-stitched leather bottoms are now produced almost exclusively in bespoke shoemaking. 
The result is that apprentices in pedorthics and the shoemaker's craft today are insufficiently trained in knife sharpening. One consequence of this development is the marked increase in "knife accidents". In 1992, 7.4 percent of all accidents were "knife accidents"; five years later, at 15.6 percent, this figure had more than doubled and this negative trend continued. In 2013, accidents caused by the incorrect handling of knives accounted for almost one-third (31 percent) of all accidents in the field of pedorthics and the shoemaker's trade. (Source: BG ETEM). The causes primarily include:

It is mandatory to wear protective goggles when working on a knife grinding machine.

  • Working with dull knives,
  • Working with reaming blades,
  • Sharpening knives on the sanding belt or buffer of the finishing machine
  • Lack of eye protection (protective goggles) when grinding knives
  • Working with knives that have a reamer/punch tool for piercing on the upper end
  • Incorrect knife stroke
  • Incorrect storage of knives in the workplace.

Sharp knives are safer
If you work with blunt knives, you automatically have to use more strength. Greater force expenditure can easily lead to uncontrollable movements if the blunt knife becomes caught, often resulting in stabbing or cutting injuries. Repetitive work with blunt blades can also cause painful tendinitis.
Disposable blades may be suitable for the hobbyist, but not for a trained and skilled craftsman. In this context, it is important to emphasize that break-off blades are not an acceptable alternative to professional tools. At best, these break-off blades can be used for cutting out patterns, but under no circumstances should they be used for cutting leather. Even slight canting can lead to uncontrolled breakage of the blade. The employers' liability insurance associations in Germany now report an alarming increase in accidents with break-off blades, often resulting in severed hand tendons. 
Grinding knives is therefore an unavoidable necessity, because sharp knives produce much better cutting results and the entire work process is faster. The knife stroke requires less effort and is at the same time substantially safer. The cutting edges are clean and accurate, which is and should remain an indispensable quality feature of any professionally produced upper.
The basic rules for making uppers should also be adhered to:

  • No grinding of knives on knife grinders without protective goggles (EN 166-compliant, optical class 1 protective goggles are optimal, as these can also be put on using the cutter's free hand);
  • knives, of whatever kind, are never to be ground on the finishing or buffering machine! There is a high risk of injuring yourself or others. In addition, the high number of rotations of finishing machines is not suitable for grinding knives and
    ruins the blades.

How do I correctly sharpen my knives?
The knives we work with in our craft are made of what is called "alloyed carbon steel". In order to preserve the hardness and structure of these knives, they should never be sharpened on dry grinding machines. Even if the blade is only slightly warm to the touch, heat build-up in the cutting edge can lead to temperatures high enough to damage the microstructure of the knife steel, causing the blades to become porous and break. Harder, multilayer blades are particularly susceptible to this kind of damage.
Knives made of softer steel can be ground faster, but also lose their sharpness quite quickly.
Japanese leather knives (Kogatana) are multilayer blades made of extremely hard carbon steels. To sharpen these, the manufacturer recommends grinding with water stones.

We distinguish between: "Belgischer Brocken” (Belgian Whetstone). Grit approx. 7000 JIS (Japan Industrial Standards Committee).

  • Natural and synthetic grindstones
  • Water and oil stones
  • Different grits

Natural Grindstones
Stones highly suitable for knife grinding can be found in many parts of the world. These are usually exceedingly fine, hard stones with a very fine grit. They are especially suitable for final fine-grinding or polishing blades. 
In Europe, especially the "Belgischer Brocken” (Belgian Whetstone) is a popular natural grindstone. Knowledge of the exceptional properties of the grindstones from the Ardennes Mountains dates back as far as the Romans. The “Belgischer Brocken” is a 480 million-year-old sedimentary rock composed of volcanic ash that contains ultra-fine garnet crystals which dissolve during the grinding process. However, the deposits of “Belgischer Brocken” in the Ardennes Mountains have been largely exploited and it is now exceedingly difficult to obtain stones in useful sizes.
In the Slovakian Mala-Fatra Mountains, it is still possible to find the Rozsutec stone, and in Japan, there are several deposits of high-quality natural abrasive stones with grits ranging from coarse to very fine.
The Arkansas stone from the Ouachita Mountains in the USA is another example of a very good natural grindstone. This stone is ideal for fine grinding and as a whetstone.

The Arkansas stone can be used with water and with oil, but the use of oil is recommended. Translucent natural hard Arkansas bench stone. Grit comparable to approx. 10000 JIS.
The “Belgischer Brocken” is submersed in water for half an hour before the grinding process. Afterwards, the stone should be placed on a non-slip support (e.g. a wet kitchen hand towel). Now, rub with a second grindstone, which should not be too coarse, or with a second “Belgischer Brocken” on the wet stone. During this process, garnet atoms are released from the matrix and, mixed with the water, produce a grinding sludge. The knife is not sharpened by the stone, but by the grinding sludge. Only the upper light-colored layer of the stones is usable; the darker layer underneath merely consists of stuck-together pieces of host rock.
Japanese natural stones are only to be used with water. Due to their limited availability, natural stones are quite expensive, especially in a usable size of at least 15 x 4 centimeters.
Since only grindstones without cracks, fractures or quartz inclusions can be used, there is a high rate of rejection. Sometimes over 1.5 tons of rock must be removed in order to obtain a single usable grindstone.

Synthetic Grindstones
The sharpness that can be achieved with synthetic grindstones is in no way inferior to that of natural stones. Primarily silicon carbide and corundum are used for the production of synthe­tic grindstones. Silicon carbide boasts exceptionally sharp-edged crystals, which is why it is predominantly used for coarser grits. Grindstones made of silicon carbide possess a high abrasion and are therefore excellent for pre-grinding.
Corundum is an aluminum oxide and one of the hardest minerals after diamonds – sapphires and rubies consist of this mineral. It is a much finer abrasive than silicon carbide and is therefore also used for higher grits. Especially white aluminum oxide (noble corundum) is used for extremely high-quality grindstones with a fine grit.
We distinguish between soft-grade (low percentage/strength of bonding material) and hard-grade (high percentage/strength of bonding material) synthetic grindstones. Hard-grade grindstones – almost all European synthetic grindstones are hard-grade – retain sharp, abrasive grains for longer. Soft-grade grindstones – most Japanese water stones belong to this category – quickly lead to the desired result, but are significantly more abrasive than European grindstones. They have to be dressed more often and thus release abrasive grains more readily. Synthetic Japanese grindstones are predominantly pure water stones that should be placed in a water bath before use. However, most European synthetic grindstones can be used with both water and oil.

Ceramic Grindstones and Diamond Grinding Blocks
Synthetic grindstones also include ceramic grindstones and diamond grinding blocks. Both are very stable and only suitable for straight blades. They cannot be used to grind bent paring/edging knives.
Ceramic grindstones are very durable because they are hard and therefore inherently not very abrasive. They are fired at high temperatures from an abrasive material (usually aluminum oxide) and a binding agent. They can be used dry or wet.
Diamond-coated sharpening rods consist of a base plate made of steel, glass or rigid plastic, on top of which there is a thin steel plate with mono- or polycrystalline diamonds. They are quite expensive, especially the higher quality blocks with monocrystalline diamonds. The grinding blocks remain permanently flat (no dressing required). For grinding, they are usually wetted with water, but they can also be used dry. Diamond-equipped grinding stones have a very high rate of material removal, so you should not exert high pressure when grinding. Too-high pressure can cause diamonds to chip.
Knives do not become as sharp ground with diamond-equipped grinding blocks as with normal synthetic or natural grindstones. Grinding blocks with a closed surface are better for narrow trimming knives. After grinding, the stone is cleaned under running water.

Oil or Water?
In Europe, oil is most commonly used in the shoemaker craft. If you want to sharpen your knives with oil, you can also use normal sewing machine oil. The level of abrasion can be controlled during the grinding process by varying the viscosity of the oil. Very low-viscosity oil leads to a high level of abrasion; the highest abrasion level is achieved when grinding with petroleum. High-viscosity oil reduces abrasion, but the higher lubrication effect allows for finer grinding. Incorum combo oil stone. Grit 500 – 240 FEPA.
The mixture of oil and the abrasion of the grindstone sharpens the blades more gently than when using water. Over time, however, the pores of the grindstones grow somewhat. For cleaning, the stones can be rubbed or brushed off with petroleum (wear protective goggles). Not all synthetic grindstones are suitable for sharpening with oil – please follow the manufacturer's instructions! From an ecological point of view, synthetic grindstones are preferable to natural grindstones. Grinding with water is also more environmentally friendly than grinding with oil.
Rules to Follow When Grinding:

  • It is important that the stone always is slightly wetted with oil or water.
  • If oil is chosen, the stone is no longer suitable for use with water.
  • It is imperative that the correct size of the grindstones is taken into consideration! A size of 150 x 40 millimeters is sufficient for trimming knives. For paring/edging and other knives, the stone should be 200 x 50 millimeters. The rule of thumb is that bigger is better than smaller.

Combination stone of silicone carbide.  Grit 400 - 150 FEPA (can be used wet & dry).

Different Grits The grinding process is not carried out on a single stone; it is divided into several grinding steps on different stones. The duration of the grinding process primarily depends on the condition of the blade. If this is still quite good, the first pre-grinding step can be skipped. The appropriate grit is selected according to the specific grinding process. The different data on the grain size provided by different manufacturers can sometimes be confusing.

  • European grindstones: Grain size according to "FEPA" (Federation of European Producers of Abrasives [Fédération Européenne des Fabricants de Produits Abrasifs]),
  • Japanese grindstones: Grain size according to "JIS" (Japanese Industrial Standard),
  • American grindstones: Grain size according to "ANSI" (American National Standards Institute).

Types of Grindstones
Depending on grit, we distinguish between four types of grindstone:

Coarse Shaping Stones
Grit: FEPA 120 – 280/JIS 360 – 400/ ANSI 280 – 350 
Application: Pre-grinding, basic shaping, repairs of chips in the blade... During pre-grinding, the cutting edge is given its basic shape and the thickness of the cutting edge is reduced for the respective application.Super Arkansas high-grade corundum.  Grit 500 FEPA.

Sharpening stones
Grit: FEPA 300 – 500/JIS 600 – 1200/ ANSI 360 – 600
Application: The main grinding process, during which the grinding marks from pre-grinding are removed and the cutting edge is given its final shape. It is important that grinding is performed at the correct angle and that this angle is maintained during the entire grinding process.

Combo stone of high-grade corundum.  Grit 800 & high-grade corundum 150 FEPA


Grit: FEPA 600 – 2000/JIS 1500 – 4000/ANSI 800 – 1200
Application: Fine grinding, which gives the knife its requisite sharpness and is the last mechanical grinding step carried out on the blade. The final steps of the grinding process are not made counter to the direction of cutting edge, but now only towards it. At the end of this grinding process, the last burr is removed (whetted). The sharpness achieved by this process is perfectly adequate for the knives we use.


Polishing stones
Grit: FEPA 2000 – 4000/JIS 6000 – 10000/ANSI from 1200 Combo water stone of high-grade corundum. Grit 1000 – 400 FEPA.
Application: Polishing (honing), giving the knife optimal final sharpness. A well-ground knife is only run along the polishing stone two or three times per cutting edge. This step as well is NOT done counter to the direction of cutting edge, but rather only towards it.

Combo stone of high-grade corundum. Grit 500 & normal corundum. Grit 180 FEPA.

Hartmut Seidich, Seidich Schäfte, Herne, Germany.

Part 2 in the next foot & shoe

The grindstones shown here were all produced at the Friedrich Müller grinding materials plant, which has been producing grindstones since 1869. We would like to thank Friedrich Müller for their kind support.

Stefan Trittler, Master Cutting Toolmaker, Esslingen; Schleifmittelwerk Friedrich Müller GmbH; Friedrich Herder Abraham Sohn GmbH; Dictum, Germany; BG ETEM.