Sharpening a knife

by tonytran2015 (Melbourne, Australia).

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(Blog No. 186).

To be self-sufficient a person using any knife should know how to keep his tool sharp.

It is actually more involving than it appears and some people can not sharpen their tools while some other can do it extremely well. This blog gives an explanation to all this mystery.

Sharpening a knife.

#knife sharpening, #grinding stone, #aluminum oxide, #silicon carbide, #grinding stroke

1. Knowing the original capability of the tool.

The makers of tools usually know quite well how to bring out the best capabilities of their tools, a user can rarely bring any tool to a better state than provided by its maker when new.

For this reason it would be futile for the owner of a kitchen knife to attempt to make it as sharp as an expensive razor blade. If it can be done, the maker of the knife would have done it to advertise his products.

2. The material of a knife determines its practical sharpness.

2a- The sharpness of a knife depends on its materials. The materials of the knife blades are mostly metals. Any metal is made up of grains which adhere together firmly and closely. Atoms inside each grain arrange themselves into geometric pattern fitting the chemical bond configuration of the metal. The arrangement of atoms in each grain determines all (including physical) properties of that grain (For example, in one arrangement each atom may have twelve closest neighboring atoms while in another arrangement each may have only eight neighbors.). The bond configuration depends on and may even change drastically with the introduction of foreign (solute) atoms into the host (solvent) atoms of the grains. The change from one arrangement into another of atoms in a grain is called a phase transformation by metallurgists.

When metal solidifies multitude of crystallization centers form simultaneously and they are randomly oriented. Each crystallization center gives rise to one grain. The boundary of each grain is where its orientation is terminated by meeting different orientations from neighbor grains.

2b- If the grains cannot be shaped, cut and cannot adhere together to hold the shape of a thin edge then no thin knife edge can be formed. If the grains of a knife blade deform, crack easily or don’t adhere together there is no way of making a sharp edge for the knife. The boundaries between grains in a metal are the weak points of that metal. Cracks in a metal usually develop and propagate along grain boundaries. Corrosion also begins at grain boundaries.

2c- Any knife blade made of a single grain would be stronger and more corrosion resistant but it is uneconomical to make such knives from single large grains. Practical knife blades are made of metal with multitude of randomly oriented tiny grains. Finer grains give blades with better resistance to cracking.

2d- If the grains of a knife edge are heterogeneous then the blade can crack easily: Cast iron has heterogeneous grains and it is very hard to make any good cutting edge out of cast iron.

2e- When tougher grains of steel are gouged by the sharp particles of a grinding wheel their debris fly off at higher speeds. For this reason, high strength carbon steels give off the longest sparks when ground by a grinding wheel while mild steel and cast iron give off shorter sparks. For carbon steels knives it can be said that only blades that give off long sparks can make sharp knives (This observation does not apply to stainless steel knives as stainless steel debris don’t burn in air to make sparks).

2f- If the grains are flattened in the same plane as the knife edge then they can adhere together better: Forged knives hold their sharpness better than non-forged knives.

3. Application to the proper usage and care of knives.

Do not leave sharp kitchen knives made of high-carbon steel wet. Corrosion by water can damage it overnight resulting in the cutting edge being serrated.

Do let knife blades be heated to hotter than the temperature of boiling water or the temperature for causterization. Heating a knife edge may upset the structures of the grains in the blade. Razor blades are most susceptible to degradation by heat and will certainly become dull if heated by a naked flame. (High speed steel is the exception, its strength is not reduced even after it has been temporarily heated to high temperature. )

4. Sharpening of a knife edge.

The sharpening of a knife requires adherence to the following rules:

4a- Preserve the original shapes angles of the knives. The angles of the sharp edges have been chosen by the knife makers to give optimal sharpness and durability: Thin razor blades are sharp but are not impact resistant, thick axe blades are not that sharp but are impact resistant.

4b- When sharpening a knife blade, the grains of the sharpening stone should be finer than the required thin edge of the blade.

4c- All carbon and alloy steel tools can be ground and sharpened by Aluminum oxide sharpening stones.

4d- Quality alloy steel items (made of “tool steel” having higher hardness and can be used to cut other types of steel) such as “high speed steel” drill bits and lathe cutting bits should be sharpened by “silicon carbide” sharpening stones.

4e- Silicon carbide stones grinds ordinary steel much more slowly than Aluminium Oxide stones despite its grains of Silicon Carbide being harder than Aluminium Oxide. This is analogous to a 32 teeth/inch metal saw blade cutting wet timber much more slowly than a 6 teeth/ inch tree pruning blade

Applying higher pressure between a silicon carbide stone and the steel does not make the grinding any faster but make the binding matrix of the stone break and wear down the stone.

Therefore suitable (matching) type of grinding stones should be used for each type of steel.

4f- When proper sharpening stones are not available, they can be substituted by one of the following:

Any piece of hard stone with a flat surface with fine grain structure,

Any flat back of a hard ceramic tile.

Any flat bottom edges of ceramic drink cups or cooking bowls (however no lubricant should be used on these cups or bowls as it may contaminate food and drink prepared in them),

Any piece of abrasive paper wrapped around a firm flat surface.

4g- A liquid lubricant helps removing metal and stone debris from the grinding location and keep the grinding points cool. This makes faster grinding. Kerosene or light machine oil can be used as grinding, sharpening lubricant on proper sharpening stones. Using water as lubricant may cause corrosion on some types of steel.

4h. When sharpening the cutting edge of a knife by rubbing it against a stone that edge should be trailing behind the knife and kept at right angle to the direction of motion (the direction of the grinding stroke). Tiny grooves made at right angle to the cutting edge by this method of sharpening may serve as serration and may make the knife cut better.

Rubbing a block of cheese against a cheese grafter would give you some visualization of the effect of grinding steel grains against a sharpening stone.

Figure: Direction of grinding: The blade is rubbed against the sharpening stone by pulling it towards the user with his right hand; the small aluminium oxide sharpening stone is mounted on a flat plastic bar which is held by his left hand.

Figure: Direction of grinding: The blade is rubbed against the sharpening stone by pushing it away from the user with his right hand; the small aluminium oxide sharpening stone is mounted on a flat plastic bar which is held by his left hand.

4i. Pulling lengthwise the thin cutting edge of a sharp knife against a grinding stone will make it lose its shape and may destroy its toughness. Pulling lengthwise is only acceptable for axes and chopping knives with thick blades.

Pulling lengthwise against a sharpening stone may leave lengthwise grooves and lines of residual stress which may curl a thin blade.

4j. Circular grinding motion may be a compromise motion for sharpening a long knife blade.

4k. Sharpening by a powered grinding wheel may bring the steel of the knife blade to too high a temperature and may change its material as well as creating residual stresses in it. High speed steel is again the exception.