Actually not YY here; it's her other half.

I'm a metallurgist; I used to work next door to the cutlery research lab (and got to take some of their used test samples home). My PhD supervisor invented (and patented) what I think is the very best of the no-sharpening knife systems.

The first thing to think about is how you use and care for your knives. If you are a chef, butcher or similar or if you otherwise have access to training (or self-teaching) in how to sharpen a knife properly, then a traditional knife that needs frequent re-sharpening will give you the very best edge you can have. However, as the angle you are aiming for is 18 degrees, and half a degree does make a difference, those of us who are not kitchen gods may find that one of the better no-sharpening knives gives us better results than we could manage for ourselves.

I'm assuming that wld does not want to invest the time needed in learning to professionally sharpen a knife and keep it in top form, so...

There are many types of no-sharpen knife:

The oldest (Richardson's "Laser 5", "Laser 10" and their copies) have an edge like a fancy saw. It does not cut like a saw though – because, unlike wood – food is soft. These work because the tips of the teeth take all of the wear, leaving the edge in the valleys between still excellently sharp, even after years of use. As the blade slides through the food, the fibres spring up behind the teeth, encounter the still-sharp part of the edge and are cut.

Trouble is, they are a swine to clean (you get bits of washing cloth and towel stuck to the teeth) and they look ugly. When they do finally go blunt, you can't sharpen them without access to the special machine that originally made them.

The more recent type (Richardson's "Plasma" and its copies) use Physical Vapour Deposition (PVD) coatings of Titanium Nitride (the pretty gold colour) or Chromium Nitride (a more boring grey). Often this treatment is combined with the "laser" type serrated edge – sometimes not. They work because the TiN/CrN coating is very hard, and wear-resistant, so they keep their edge for a very long time. The problem is the edge should be a sharp angle between the two sides of the knife, but as the nitride deposits, it tends to take on a rounded shape, and is not faithful to the sharp angle of the underlying metal. This means they start out kinda-sharp (not, as you might hope, sharp-sharp) but stay kinda-sharp for the rest of their lives. The coating is so hard that any attempt to re-sharpen the knife is very likely to damage the sharpener.

The third type (the one invented and patented by Jess Cawley at Sheffield Hallam University and owned by Richardson's) uses a High-Velocity Oxy-Fuel (HVOF) sprayed Tungsten Carbide coating along the edge on one side of the blade only. They are marketed in the UK under the names "Fusion", "Fusion Infinity" and "Fusion Professional". They have appeared under M&S's own brand, but so do knives without this coating. They have been sold in the USA under the "Regent Sheffield" brand, but as far as I know are not widely available. The tungsten carbide is a dull grey, slightly speckled substance, laid down in a thin layer along just one side of the edge. If you run your finger VERY CAREFULLY across it, you can feel that it's rough. The Tungsten carbide is extremely hard and so it wears (and loses its sharpness) very slowly. However, when it does wear, it's the uncoated side of the steel that wears first, so the edge angle remains sharp. Eventually the uncoated edge wears so far that a microscopic flake of the edge, complete with the tungsten carbide coat, will come off, leaving a brand new and extremely sharp edge-surface – which still has the tungsten carbide protection and retains its 18 degree angle.

Because one side is uncoated, it is possible to sharpen/modify the blade by sharpening only on the uncoated edge – though it's not reccomended unless you know what you are doing.

The underlying metal is stainless steel, and (with precautions) will even tolerate being put in the dishwasher. Chefs will not make much use of them, because they can get better results by sharpening their own; they are also unpopular with those who want a knife that looks like the ones the TV chefs use (try Sabatier), but for the rest of us, the "Fusion" type Tungsten Carbide on-one-side knife is probably the most practical that money can buy – and far from being the most expensive.

The final consideration is handle type. First, it has to be comfortable for you, and people do tend to have different hand sizes – one size does not fit all. Try before you buy.

The cheapest is moulded plastic. It doesn't look so good and has an association with cheap knives, so the better ones for the domestic market don't tend to have them. However, if you look carefully at the cold meats and butchery counters at your supermarket, you will see that they are probably using moulded-handle professional knives – because the moulded handle gives no hiding place for bacteria and will survive the dishwasher.

Next up (probably) is the wooden handle – as used by Chicago Cutlery in the US and very few here in the UK. (BTW, Chicago Cutlery are not as good as they should be for the price – the ones I've seen don't even use full tang!). Wood looks good, but wears badly, is the most prone to loosening, and gives bacteria a hang-out in the gap between the wood and that metal part that is the continuation of the blade into the handle (the "tang").

Next is the hard plastic riveted-on handle. They look good, wear well, are fairly resistant to loosening (especially if the tang runs the whole length of the handle – a "full tang"). There is not much possibility for bacteria to colonise between the plastic and the metal tang, because it's riveted on so tight.

The ultimate is the fully-forged bolstered blade with full-tang three-rivet handle. The bolster (the thick piece of metal at the base of the blade on some knives) means that the meeting between the metal and the plastic/wood of the handle is a half-inch or so back from where it might touch the food. These knives tend to be hand-forged and are owned by professionals or handed down from ancestors. Buying them is seriously expensive, even if you go to the maker.

Sabatier (and many of their competitors') knives look like they have a fully-forged bolster; maybe they did in the distant past, but what's on sale now has a ground blade with a "fake" bolster soldered/brazed on.

Here endeth this week's metallurgy-for-chefs lecture!

Next week: applying the general metalworking equation to pasta-making...

Iain (YY's Other Half)