Cutting Edge

Anyone with a fair bit of cooking experience knows the joy of working with sharp knives. With the proper technique, a sharp blade makes slicing, dicing, deboning a chicken or gutting a fish more efficient compared to a dull one.

At its most rudimentary level, a knife’s edge is an inclined plane with a small surface area that amplifies a small force into massive pressure. For those of you who already forgot, pressure is calculated using this formula, where P = Pressure, F = Force and A = Surface Area:

The smaller the surface area, the greater the pressure. A sharp knife has a very small surface area of its cutting edge and that means less force is required to cut through stuff.

In reality, however, the science of cutting through stuff appears to be a bit more complicated than that. A good knife blade will fulfil the following criteria:

  • Good edge geometry
  • Good heat treatment
  • Good steel

Let’s dive deeper into each criterion.

Edge Geometry

Not all blades are made equal. Identical blade material can give you vastly different cutting performance depending on how they are ground. The thinner the edge, the easier it is for the blade to cut stuff. Thin edges, however, tend to be more fragile and are more prone to chipping. Knives that are meant to chop (like a cleaver) usually have a thicker edge. Western-made knives and Japanese knives are also different in terms of how the blade tapers to an edge. Western knives tend to have a double bevel while Japanese knives tend to have a single bevel. “Bevel” refers to the inclined plane that forms the edge of the knife.

A double bevelled blade will distribute cutting force equally to the left and right, resulting in an even cut. A single bevelled knife tends to have a biased cut – the flat side will cut more cleanly compared to the bevelled side. The diagram above shows a single bevelled blade meant to be used by a right-handed person. The cut food will have a slight slant while the uncut food will not. If a left-handed person uses this, the opposite will happen – the uncut food will have a slight slant that adds up, resulting in a messier cut. Using the proper cutting technique, this slight slant can be compensated. The single bevelled blade allows the edge to be thinner, since only one side needs to be sharpened and honed. A highly trained Japanese chef will be able to make long, thin and unbroken cuts, which is very important when preparing certain Japanese vegetable such as daikon. Think of using a chisel on a piece of wood. It is very difficult to achieve unbroken cuts with a double bevelled knife.

Heat Treatment

Traditionally, knives are made by forging a piece of steel. Forging is the process of heating the metal up to make it softer, and then hammering it into shape. When steel is heated beyond a certain temperature (usually around 800°C) and then quenched (cooled rapidly) in either oil or water, it undergoes a change in its structure resulting in the steel becoming harder.

In science, hardness is not a simple topic to discuss. However, for our context, you may think of hardness as the ability for a material to leave a scratch mark on another material. If material A can scratch material B and leaves a deep mark, material A is “harder” than material B. Hardened steel can easily leave a scratch mark on wood, and that is because steel is harder than wood. However, steel is “less hard” than ceramic so it cannot leave a mark on your ceramic plate. In fact, doing that will just dull the edge of your knife. This is why certain types of ceramic can be used as a knife blade. The downside is that ceramic knives are really brittle, and it is highly difficult to sharpen them once they get dull.

After quenching, the steel blade is usually too hard for any practical use. A blade that is overly hard, while it can cut better, is very brittle. The blade hardness is then lowered by tempering it. During tempering process, the blade is heated to around 200°C for a few hours. Any error made during hardening or tempering will significantly affect how the blade performs.

Steel

Steel is basically an alloy (mixture) of iron and carbon. Stainless steel is a type of steel that contains chromium in the mixture. Unlike stainless steel, carbon steel (non-stainless steel) is prone to rusting if not properly cared for.

A few years ago, I enrolled in a 2-day knife making course in Tharwa, ACT, Australia. In it, each participant designed their own knives using templates and forged them into shape. We were given 2 pieces of 1055 steel. The last 2 digits of the steel name indicate the carbon content in the alloy – 55 means the steel contains 0.55% carbon. 1055 steel contains virtually no chromium, making it a type of carbon steel. It is less than ideal for kitchen use in Singapore considering how humid it is, as humidity makes rust form more easily. However, 1055 is a beginner-friendly steel and is quite easy to work with.

Being a knife collector myself, I am very particular about the steel of my knives. I personally prefer stainless steel over carbon steel, simply because stainless steel does not rust as easily as carbon steel, thus requiring less maintenance.

My go-to steels for kitchen knives are VG-10, 440C and X50CrMoV15. Knives made of these steels are readily available and won’t break the bank. They are all stainless steel but they are not the same. VG-10 is popular among Japanese knifemakers and is known for its ability to hold an edge and durability. 440C is commonly used by western knife makers. Containing up to 18% chromium, 440C has excellent corrosion resistance. X50CrMoV15, on the other hand, is popular particularly among German knife makers. It is affordable and has good all-around performance.

Among these 3, my personal favourite would be 440C. In fact, the knife that I always bring with me when I go camping is made of 440C steel. Since I tend to camp near the beach, I will always go for a knife with the highest corrosion resistance. For most people, the 3 steels have comparable performance as kitchen knives when used and cared for properly.

For a pocket knife, my favourite steel would be LC200N. This is a special steel that contains nitrogen in the alloy. LC200N is virtually rust-proof and is best known as NASA’s preferred material for the ball bearings used in their high-performance aerospace equipment. Hey, if it’s good enough for NASA, it’s good enough for me.

I particularly enjoy using a sharp knife until it gets dull. I enjoy sharpening a dull knife even more. So how do I sharpen my dull knives? Well, that is another topic for another day. Stay sharp!

Written by Ferry Kurniawan
Illustrations by Toh Bee Suan