When talking about kitchen knives, “hardness” is a term that often gets thrown around without stopping to explain what it actually means. Although the term sounds straightforward, it’s not quite as simple as it seems, which often leads to confusion on the consumer’s end when hardness values are listed in a knife’s specifications. Although many cooking enthusiasts (especially home chefs) can select a suitable, quality knife without extensive personal knowledge of the engineering behind the blade, understanding what hardness really means and how it applies to your kitchenware will help make future shopping more accessible.
What is hardness?
Hardness, to define the term simply, is a measure of how resistant a material is to changes in shape when subjected to various types of force. In other words, materials with high hardness will deform (change shape) less than materials with low hardness when they are bent, struck, crushed, or otherwise stressed. As a simple illustration, imagine trying to bend a rod of iron versus trying to bend a rod of pewter. The softer pewter is less resilient to changes in shape, and will give way under significantly less force than the harder iron. Although your kitchen knives are not likely to undergo this type of exaggerated deformation, hardness still factors heavily into the performance and longevity of your blades.
Hard Materials vs. Soft Materials
We’ve mentioned briefly in other blog posts that the hardness of the material of which your knife is made will have a major impact on how your knife performs. Hard and soft materials both behave fairly differently, and choosing the wrong knife for a given application can significantly impact one’s cooking efficiency. While harder knives are often considered “better” among professionals in the industry, there are some trade-offs for hardness that should be considered before rushing out and buying the knife with the highest hardness value.
There is a natural tendency—likely stemming more from colloquial word associations than from anything else—to regard harder materials as also being inherently stronger. This may seem intuitive, but it is not necessarily the case. Hardness does not always indicate higher durability: only how “rigid” the molecular structure of the material is. Harder materials will naturally have less “give” than softer materials when subjected to pressure, impact, and other types of stress. On the surface, this may sound like a universally positive quality; after all, you don’t want your knife flexing and bending around while you’re trying to chop up vegetables for a stew. However, high hardness does come at a cost. While harder materials are more resistant to stress and will not deform easily, they are also more brittle: a quality that can quickly become a detriment in the wrong situation.
While pliable materials tend to release energy by bending, buckling, and denting when subjected to heavy forces, a brittle material will absorb energy into its molecular structure, allowing it to retain its original shape. However, because they are so rigid, they tend to chip, crack, and fracture more easily than other materials. To make matters worse, brittle materials generally do not exhibit any of the “warning signs” that more pliable materials display when subjected to excessive strain. Soft materials often indicate failure early by visibly warping, bending, and twisting, allowing opportunity for repair or replacement before complete failure occurs. Very hard materials, on the other hand, do not warp and deform under strain, and will resiliently maintain their shape until the applied force is greater than the material’s strength. This is great for certain applications (such as holding an edge on a knife blade), but hard materials do still have their limitations. Because hard materials do not disperse energy through deformation, they tend to fail suddenly when subjected to forces and pressures higher than they can handle. In other words, harder materials are more likely to break unexpectedly when subjected to high amounts of stress, a fault that can be detrimental—and potentially hazardous—to an unsuspecting chef.
So…what to buy?
It should be obvious at this point that the challenge in designing the “perfect” kitchen knife lies largely in striking a balance between the hardness and softness of the metal. Hard materials hold a sharper edge for longer periods of use than soft materials, but if the knife is too hard it is likely to chip and crack—possibly without any warning. Conversely, soft materials make for a more resilient structure that is less likely to break and easier to fix, but if the material is too soft, it won’t stay sharp and it is not likely to be particularly effective against other hard materials like bone or tough fruit rinds.
When shopping for a new kitchen knife, look for knives that tout their edge-holding abilities or ease of sharpening rather than knives that are advertised solely for their hardness. Companies that brag exclusively about their knives being made with “the hardest steel in the industry” have likely missed the point about what makes a truly reliable knife. There are specialty cases in which an extremely hard material is appropriate—such as delicate sushi knives or specialty pairing knives that will never be knocked against a cutting board—but most good all-purpose kitchen knives will fall within a range of moderate-hardness metals that incorporate a balance of both hard and soft material properties
