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What's in a Chef's Knife?



What is a Chef's Knife?

A chef's knife is a multi-purpose kitchen tool that can be used to slice, dice, and chop ingredients with precision. Chef's knives typically have a sharp point and a prominent edge that features a sloping curve. The curve facilitates cutting and allows you to execute the knife’s signature rocking motion. The shape and length of the blade can vary, but a chef’s knife should feel like an extension of your body as it is being used.



Know Your Knife

Knives are a commonly used tool that we often take for granted. We expect them to perform and cut with ease, but we rarely pay any more attention to our knives than that. To understand your knife, it is important to learn about its construction and be able to identify the various knife parts.

First, it is easy to divide the knife into two main parts, the handle and the blade. But each of those two parts can also be subdivided into its own parts. With the help of the photo at right and the descriptions below, you will better understand what components make up your knife.

Point – The point is the part of the knife where the edge and spine come together. The point is often used for piercing.

Tip – The tip is the forward part of the knife and includes the knife point. The tip is used for detailed or delicate cutting.

Edge – The edge is the cutting part of the blade. It extends from the point to the heel of the knife.

Heel - The heel is the rear part of the edge, opposite the point.

Spine - The spine is the top of the knife blade, opposite the knife edge.

Bolster - The bolster is the band that joins the blade of the knife to its handle. The bolster provides balance for the knife and also helps to protect the hand from getting in the way of the knife edge.

Tang - The tang is the part of the blade that extends into the handle of the knife. It is the surface to which the handle attaches to the blade.

Scales – The scales are the part of the knife that creates the handle. Scales are often made of synthetic material or wood. Two scales are typically attached to the tang with rivets.

Rivets – The rivets are metal pins used to join the scales to the tang to form the handle.

Butt – The butt is the end of the handle of the knife.



Did you ever wonder why some kitchen knives stay sharp longer than others? How can a specific chemical composition and heat treatment improve the quality and longevity of a kitchen knife edge? What is happening on a microscopic level as a knife becomes blunt and what physical mechanisms accelerate or decelerate this process? In this text, we will try to answer these questions and provide a short introduction to the metallurgy of kitchen knives. 


Types of steel

Steel is basically an alloy of the elements iron (Fe) and carbon (C), where the latter represents up to 2% of the total mass. Even a very small amount of carbon considerably changes the mechanical properties of steel, particularly improving its strength, which is crucial for any practical application. An alloy with more than 2% of carbon is called cast iron, quite a brittle material and hence unsuitable for kitchen knives or similar tools. However, a high carbon content results in a lower melting point, which means that cast iron is easier to pour into molds and thus, for example, suitable for making cast iron pots.

Steel can also be mixed with other elements, often chromium (Cr), vanadium (V) and molybdenum (Mo), which further improve its mechanical properties and, in some cases, also corrosion resistance. This group is called alloy steel. A special subgroup of alloy steels are tool steels, the common feature of which is that they are used for tools (knives, saws, axes, drills etc.). They are suitable for use whenever high toughness, strength and abrasion resistance are required.

The exceptional mechanical properties of tool steels consequently mean that they are demanding to produce and process, which in turn makes them more expensive compared to steels with fewer alloying elements. In terms of material, most quality kitchen knives belong to the family of tool steels. The figure below shows a schematic distribution of iron alloys and the families of steels used to make kitchen knives are highlighted.



Mechanical properties of materials

In order to describe the properties of kitchen knives and their differentiation in terms of quality, it is useful to first define some basic concepts regarding the mechanical properties of the material and how we measure them.

Mechanical properties: STRENGTH, HARDNESS, DUCTILITY, TOUGHNESS

➨ Strength

One of the most basic properties of metallic materials is their strength, defined as resistance to changes in shape under the influence of external forces. It is measured experimentally by tensile tests, where a sample of an elongated material is clamped in the jaws, which are then slowly pulled apart until the sample ruptures. In doing so, the curve of force as a function of jaws displacement is recorded. To make it easier to compare samples of different sizes, the values are usually converted into a stress versus strain curve. An example of this is shown in the figure below, which also schematically presents the typical external appearance of a sample. 



The initial, very steep part of the curve represents an elastic deformation when the material returns to its original shape as the load is removed. With a further increase in load, an irreversible change in shape occurs, i.e., the material is plastically deformed. We definitely want to avoid this situation with kitchen knives, because in practice it means that the cutting edge or the entire blade bends. The sample in the tensile test can be further stretched to a certain point where the maximum force is recorded, called “tensile strength”. After this point, the force even slightly decreases due to the transverse deformation of the sample, until the moment when the sample breaks.

➨ Hardness

The hardness of a material is, by definition, its resistance to embossing or localized plastic (permanent) deformation. Consequently, this also means resistance to wear. Hardness is a different quantity from strength, although they are directly related. Strength is physically more precisely defined, but hardness is usually easier to measure in practice and is also more relevant in the case of kitchen knives. There are several different methods of measuring hardness and they are based on pressing a standard-shaped probe into the surface of the material and measuring the depth of the impression. For tool steels, the Rockwell method (HRC) of hardness measurement is often used, where the probe is a diamond cone. However, there are other methods that are more suitable for softer materials, for example measuring hardness with a ball-shaped probe.

➨ Ductility

A relevant mechanical property is also ductility or plasticity, i.e., a measure of plastic deformation before fracture. In the above figure of the tensile test curve, this means the amount of deformation at point F, whilst the stress at which the rupture occurred is irrelevant.

➨ Toughness

Toughness, however, is the property of a material to absorb a lot of energy before it breaks. This means that it must withstand as much elongation as possible at maximum force. Some materials break at high force but at low elongation. We say they are brittle. The area under the tensile test curve represents toughness and is also shown in the figure below. 


The mechanical properties of a metallic material depend on its chemical composition and thermo-mechanical treatment. The chemical element that has the greatest effect on the hardness of steel is carbon, while chromium, manganese, vanadium and molybdenum also positively affect hardness. Together with carbon, the latter elements form new, extremely hard compounds called carbides.


Line defects called dislocations are also always present in metals and they occur when one layer of atoms is inserted between other layers. Under the influence of external stresses, atoms belonging to the interlaced layer can switch their neighbors and establish a bond with other layers of atoms. In this way, the dislocations move along a metal lattice, thus allowing many atoms to permanently change their place. The movement and formation of new dislocations is a very important concept in metallurgy, as at the microscopic level it represents an explanation for the plastic deformation observed at the macroscopic level. This also leads us to the conclusion that, if we want to reduce the plastic deformation of our product or increase its strength, we must in some way inhibit the movement of dislocations.


I think that's enough science for today, as you can see, there is a lot more behind the Chef's Knife!


Cheers and have a great week!


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