To be able to get maximum usage as possible from your working tool and thereby make your working situation as easy as possible puts great demands on the properties which govern why a tool functions or does not function in different situations.
By attaining the right properties also ensures the avoidance of extra loading on your body when working. To be able to cut with a sharp knife instead of a blunt knife when working in different cutting positions during a whole working day will mean that the total body loading might be reduced down to only a few hundred kilos instead of a total loading of maybe tons. I
t is of course important to understand how the cutting edge of the knife must be formed and how one keeps and maintains the sharpness in the best way. A knife that repeatably or often cuts towards the bone must be able to withstand much heavier treatment than a knife which is only used for cutting and fininishing operations.
This is the reason why knives for different operations must be sharpened at different cutting edge angles. This also means that the knives with different cutting edge angles demand that they must also be finished off using different angles when applying the sharpening steel.
Different knives are also in steels with different hardnesses which means that there can be difficulties when sharpening. Here it is easier to note that a soft knife is easier to sharpen but at the same time the edge retention time will be shorter. This in turn will mean that the knife has to be resharpened more often which means that more working time will be spent keeping the knife sharp.
Knife sharpening is an art in the same way as cutting is an art and by learning as much as possible it is also possible to achieve greater satisfaction in his work.
Stainless steel knives for use in the food processing industry are of made of so called martensitic stainless steel which unlike the austenitic stainless steels can be hardened. Knife steels contain between 0.4 to 0.7% Carbon and between 13 and 18% Chromium. Carbon is necessary to make the steel hardenable and Chromium to is present to prevent corrosion ie rust. In general it can be said that Carbon contents below 0.5% are not to be recommended for professional usage since it is not possible to achieve sufficient hardness. Such knives on the other hand have excellent corrosion resistance properties which may be more important in the kitchen where appearance of the knives is more important.
Mora of SWeden uses almost solely steel from Sandvik with the Steel Grade Code 12 C 27. This steel contains 0.6% Carbon and 13.5% Chromium. This, together with a special hardening treament using liquid Nitrogen at -80°C, ensures that the knives achieve a hardness of between 57 - 58 Rockwell C. In comparison to other knives this is amongst the highest attainable hardness levels without affecting the sharpening ability to any marked degree.
Knife sharpening also demands that the the sharpening equipment is kept in good condition. All the knives must be carefully cleaned first prior to sharpening to prevent fat or meat particles becoming attatched to the grinding wheel surface which would thereby cause detrimental effects when sharpening. This should be clear common sense but unfortunately it is not always the case.
All knife manufacturers recommend wet grinding when using grindstones since the risk of overheating of the cutting edge is reduced to a minimum. Grinding using a fast operating grinding belt puts great demands on the grinding operator. The slightest miscolouring of the knife cutting edge indicates that one has reached temperatures that are detrimental to the steel. The results may well be lower hardness and micro cracks in thinner parts of the edge, both of which most certainly produce a poorer quality knife.
One should therefore ensure that grinding should take place on a slow moving grindstone with good water cooling thus avoiding damages through overheating.
When grinding knives used for the finishing operations, the total grinding angle should be around 25°. If this angle is less, whilst the knife will be extremely sharp, it will also mean that the knife condition will also be very sensitive since the extreme knife edge point will easily be folded over when coming in contact with harder objects. Boning knives on the other hand are ground to higher angle, approx 35° and thereby up to tougher usuage and heavier work on the knife edge even though this is at the expense of a somewhat higher cutting resistance. In both cases it can be good to make a honing angle on the top of the cutting edge and thereby strenghten it. This honing angle should be somewhat larger that the grinding angle. This effect can also be produced to a certain degree when polishing the cutting edge on a polishing cloth wheel due to the slight abrasive action of the polishing wax. Some experienced professional knife users can produce this “extra“ cutting angle by using a fine grinding stone and threby produce knives that keep their sharpness for weeks merely by straightening up the cutting edge using a polishing steel or by honing occasionally. What mostly happens when a knife begins to become blunt is not due to pieces becoming removed from the cutting edge but instead is due to the edge “bending over“ and thereby not cutting.
The sharpening steels have therefore the function to press the edge back into place a central position. It is therefore easy to understand why polishing steels are being more and more used. Coarser sharpening steels are really only used when standard grinding equipment is not available. It should however be noted that after sharpening with a coarse sharpening steel a polishing steel should always be used in order to even out the uneveness resulting from the coarser sharpening. It should also be noted that both operations using the coarse sharpening steel and the polishing steel should be conducted as previously recommended, i.e. that the pressure of the knife against the “steel“ be gradually reduced so that the last “draw“ be very light. Just like a stroking operation of the knife edge.
The steeling angle shall be the same as the honing angle, i.e. not larger, so as to prevent the top of the cutting edge becoming rounded more than necessary. As mentioned before knives have different grinding angles depending on their applications. The angles used when using sharpening steels shall be the same as their respective original grinding angles. Carry out these operations carefully and slowly making sure that the light is good. Place the blade against the sharpening steel and lift it until the top of the edge meets the sharpening steel surface. This is then the correct sharpening angle to apply.
Remember that all sharpening operations for cutting tools builds around the fact that one progresses from coarse operations (grinding) to fine working (polishing). However if the cutting tool is not so worn then one does not need to regrind but instead just polish.
Finally, ensure that the grinding equipment is always emptied of water after the working day so that the grinding wheels do not become locally soaked through with the result that they become soft in these parts and become unevenly worn. Heavily dirty grinding wheels can be cleaned by using a strong fat dissolving cleaning agent or spirit.
Another tip to be used at the workplace where knife sterilisation is carried out is to dry the knives carefully before placing them in the steriliser and thereby prevent any protein burning firmly onto the knife blade during sterilisation. Thus one can avoid that the knife becomes difficult to work with and thereby giving the impression that it is blunt.