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Frequently Asked Questions

Table Cutlery & Flatware

Sharpening Technology : knife and blade sharpening


Q1 : We have recently bought a set of beautiful silver plated table cutlery, with a superb polish, but after the first two or three uses the surface is all scratched. Is there something wrong with it?

No, silver is a relatively soft metal and is very easily marked by knives, ceramic and glassware. The myriad of scratches eventually builds up to form the classic patina associated with silver.

Q2 : I would like to buy some good quality table knives and forks, is there any way I can assess the overall quality for my self?

The best option is to look for a design that suits your personal taste but carries a reference that it complies to the following quality standards or norms. ISO8442-2 is the International quality standard for table cutlery and can be prefixed by any of the following letters depending where it is made or being sold. BS for UK, DIN for Germany, FR for France and EN for anywhere in Europe. This specification is for metallic handle table cutlery and flatware only, if you want to buy some with non-metal handles such as glass, plastic, ceramic, bone or wood then the only specification available is the British Standard BS5577.1999.

Q3 : My new stainless steel cutlery has developed a rainbow type stain all over it, should I send it back to the manufacturer and complain?

No, this colouration is known as detergent staining and develops both with hand and machine washing. It occurs when the items are not adequately rinsed in clean water and the residues of detergent gradually dry hard on to the surface creating an iridescent film. This can easily be removed with a mild stainless steel cleaner and some gentle rubbing.

Q4 : The surface of our silver plated table cutlery has become stained with black/brown/blue discolouration, what is it and what can we do about it?

This is the normal tarnishing of silver, it is usually the discolouration associated with silver sulphide, which has been formed by the reaction of the silver with sulphur in the atmosphere or in food such as eggs and green leaf type vegetables. Unfortunately this phenomenon is a fact of life where silver is concerned and the only solution is to clean the items regularly. See CATRA Care of Cutlery leaflet.

Q5 : We have recently started to use a dishwashing machine and have noticed that some of our knife blades are showing small dark holes or pits in the blades is this to be expected with the dishwasher?

No, however the problem is quite common. Sometimes it is caused by poor quality of manufacturing control in the way the blade is heat treated and ground during manufacture, (manufacturers contact CATRA for advice or users contact the manufacturer) or the general damp and corrosive environment inside the dishwasher often when the items are stored inside the unit for a long time without washing. The best advice we can offer here is to not store cutlery and flatware in a dishwasher for a long period prior to washing and to remove the items immediately the cycle is over. The pits are areas of corrosion (rust), which forms in this unusual way on stainless steels. Once this has occurred the pits will gradually grow in size as further dishwashing is carried out, but generally remain dormant when the items are dry and clean.

Q6 : Our expensive new table cutlery is leaving black marks on the surfaces of our equally expensive ceramic plates and bowls, am I doing something wrong?

No, unfortunately certain types of ceramic surface glazes and particular types of stainless steel don't go well together. Very course glazes often abrade 18/8 grades of stainless cutlery items to leave thin deposits of metal as a black mark on the surface of the crockery and scratches on the cutlery. Today manufacturers and retailers of both cutlery and crockery are working hard to ensure this problem does not arise with new products, but many ceramic glazes are out there in our homes, which are just not ideal for use with certain cutlery items.

Q7 : Can I remove them?

Yes, the easy solution is to rub the effected area with vinegar, this usually is successful.

Q8 : I have inherited this old piece of table cutlery; can CATRA value it or give me some history?

We are sorry but CATRA does not have the historical expertise to answer this type of enquiry.

Q9 : Some cutlery and flatware is marked 18/8 and some 18/10, is there any difference and what does it mean?

These numbers refer to the additional elements that are in the stainless steel. The first number is the chromium content and the second that of nickel. Both elements increase the corrosion (rust) resistance of the steel. So in theory the steel with 18 % chromium and 10% nickel has a better corrosion then the one with only 8% nickel. However many specifications for stainless steels around the world have a manufacturers composition tolerance within this grade of 8% to 10% nickel, so often although the item is marked 18/10 it may legally only have, for example 8.1% nickel. So in practical terms there may be no difference in performance.

Q10 : Is it safe to dishwash all cutlery?

No. Care must be taken. It is okay if the manufacturers advice is that it will be safe, but if they do not advice it, don't do it. Dishwashers can effect cutlery in the following ways:

Blade Corrosion-rusty stains, which can be removed with mild abrasives cleaners such as "CIF". Dark pits or holes which are unfortunately permanent. Handle discoloration - particularly if the handles are made from wood, plastics or bone.Surface discoloration in the form of rainbow or white stains is due to detergent drying on the items and is usually a sign of a faulty rinse operation within the dishwasher. See Care of Cutlery leaflet.


Q11 : Why do CATRA recommend the use of Cubic Boron Nitride (CBN) for powered sharpening wheel abrasive when many other people use Diamond?

Diamond is one of the hardness substances known to man and CBN comes very close behind it, so the obvious choice would be diamond. But most knives are made from steel (either stainless or alloy steels), which contain a certain amount of carbon. Now as our school science told us diamond is in fact a carbon crystal, which when grinding along the surface of steel reacts with the carbon atoms within it and causing premature wear. Therefore in a practical sense CBN lasts much longer than diamond. Manufacturers of sharpening systems that use diamond either don't know this fact, or would rather you bought more sharpening wheels because they wear out faster, or they believe the marketing sexiness of diamond sells more products. However as CATRA is the worlds leading sharp edge technology centre we believe in using the best and most efficient material for the process.

Q12 : When the device is not powered can you use diamond then in such things as sharpening steels?

Yes, the above mentioned effect is most noticeable at high speed and power and so in the case of sharpening steels and proprietary sharpeners operated by hand, diamond is OK. We at CATRA do use diamond on the low powered CATRAHONE knife sharpener.

Q13 : What is the correct sharpening angle for my knife edge?

The answer to this is not simple; it depends on the type of knife, its particular use and your facilities/ abilities to resharpen. However through many years of Research and Development CATRA recommends these following included edge angles as the best compromise between sharpness and edge life.

Type of knife and its use Included edge angle in
Table Knife 55 to 60
Domestic Kitchen Knife 30 to 35
Professional Cooks and Kitchen Knives 25 to 30
Commercial Meat Processing 25 to 30
Commercial Fish Processing 25
Commercial Vegetable Processing 35
Folding & Sporting Knives (High Sharpness) 30 to 35
Folding and Sporting knives (High Durability) 40 to 45

These angles are the ones to give the best compromise between sharpness and life and have been optimised by extensive testing on the CATRA knife sharpness test system.

Q14 : Some people argue that electric sharpening of knives is bad for the blade, is this true?

The answer is yes and no. Some types of knife sharpener with conventional grindstones or abrasive belts can overheat the blade edge ruining the hardness and therefore the ability of the knife to retain its sharpness. If you see sparks comming from a sharpening operation you can be sure the cutting edge has been overheated and so is soft, but even some sharpener without sparks also soften the edge, so beware! But the CATRA CBN system of sharpening machines generate very little heat, even without water and don't loose the blades high hardness and sharpness retention properties.

Q15 : How can I measure the sharpness and life of a knife or blade?

CATRA has developed a number of sharpness measuring systems and equipment over the last 50 years. This research has led to a number of devices that can measure the cutting tip geometries and condition of the cutting edge and also machines that measure sharpness and life in actual highly controlled cutting operations. For more details go to the CATRA AET for knives/blades from 1.5" (35mm) in length upwards or the CATRA REDS tester for smaller blades such as razor, surgical and needles. Alternatively talk to us if you have a special case with a difficult or unusual cutting edge.

Q16 : What is the CATRA Gothic Arch edge system?

Normally when a knife or blade edge is sharpened by drawing the blade through interlocking abrasive sharpening wheels the edge bevel (or cannel in old Sheffield cutlery industry language) is slightly concave in shape following the curvature of the wheels themselves. However many knife experts believe it is better to have the bevel in a convex form, which gives an apparently sharper edge for the same tip angle. (Some experts also believe the edge is stronger, but we at CATRA don't subscribe to this point of view). CATRA's range of spiral interlocking wheel sharpeners can with our subtle engineering modifications sharpen all types of blades and knives with this convex (or GOTHIC ARCH shaped) edge, thereby giving that extra boost to an already very high level of sharpness.

Q17 : Is it easy for me to sharpen a ceramic bladed knife?

Yes and no, conventional sharpening steels and domestic knife sharpeners have virtually no effect on a ceramic blade especially if the edge has become chipped. Diamond sharpening steels do work, but the best option is the CATRA spiral knife sharpeners such as the CATRASHARP. These units quickly and accurately restore a worn and chipped edge to its factory condition. However they do require fitting with finer diamond wheels than the standard ones supplied originally.

Q18 : Why do knives become blunt

This may sound a silly question, but in fact its not. In the case of most domestic and professionally used knives for cooking, virtually all of the loss of sharpness is as a result of damage and edge rounding to the cutting edge caused by contact with items you don't want to cut. These are other knives and implements in washing, dishwashing and storing( always use a knife block or rack, never a drawer with loose knives in it). Cutting boards are a major source of damage, when the blade is forced down on to it, try to cut if possible with only the tip of the blade contacting the board, the worse possible boards are glass and granite ( they never should have been invented !) Plastic chopping boards are ok, wooden boards are better and generally cause slight less damage than plastic also they have a natural bactericide which keeps them infection free. Paper or cardboard cutting boards are now available which are disposable and reduce cross infection between foods as they a low cost and many boards may be used and then discarded. They have a similar wear effect on the knife as that of wood.

Q19 : What is the difference between taper ground and hollow ground blades

Taper ground blades are the conventional way that knives of many types have been always made and consist of an a single apparently flat surface on each side of the knife. This surface is not actually flat but slightly profiled so that the back edge of the blade is thicker, especially near the handle end and thinner near to the tip, whilst the cutting edge is thinner and of the same thickness along the cutting edge. A hollow ground blade on the other hand is usually made from a flat strip of metal and has no tapering either along the length or across the blade from back to cutting edge. The cutting edge is made thinner by a process known has hollow grinding which creates a second curved or hollow surface on the blade side usually between 8 and 20 mm wide ( 3/8" and 3/4").The actual edge sharpening is usually similar on both types.

The advantage to the manufacturer of hollow grinding is that it is cheaper to make, as there is less metal to grind away and normally needs less metal to make. The advantage to the user of hollow ground knives is that the edge is usually thinner for a distance of 1 to 2mm back from the as new cutting edge compared to a taper blade, which means it is easier to sharpen as there is less metal to sharpen away. However ultimately it becomes more difficult to sharpen as the blade get thicker very quickly after 5 or 6 mm back from the original edge. Another disadvantage to the user is that hollow grinding tends to be used on cheaper quality knives so that overall quality is often lower than on the more traditional taper ground blade.

Sometimes hollow grinding is a technique that can be applied to an old taper ground blade to thin down the cutting edge and make it easy to resharpen. CATRA model V100, V 140 and U200 knife sharpeners act as hollow grinders for this type of operation.

Please contact us by phone or e-mail for our latest brochure and prices.

CATRAHONE Diamond Power Knife Sharpener made by CATRA

Every home should have one !


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