During food preparation, knives are mainly used for slicing, chopping, and piercing. The function is to sever a nerve, muscle or tendon fibers, or blood vessel to disable or kill the adversary. A simple blade intended for cutting has two faces that meet at an edge. Ideally this edge would have no roundness but in practice all edges can be seen to be rounded to some degree under magnification either optically or blades in the dark 8.1 pdf an electron microscope.
Force is applied to the blade, either from the handle or pressing on the back of the blade. This necessitates the blade being strong enough to resist breaking before the other material gives way. The angle at which the faces meet is important as a larger angle will make for a duller blade while making the edge stronger. A stronger edge is less likely to dull from fracture or from having the edge roll out of shape. The shape of the blade is also important. A thicker blade will be heavier and stronger and stiffer than a thinner one of similar design while also making it experience more drag while slicing or piercing.
The curved edge of an axe means that only a small length of the edge will initially strike the tree, concentrating force as does a thinner edge whereas a straight edge could potentially land with the full length of its edge against a flat section of tree. A splitting maul has a convex section to avoid getting stuck in wood where chopping axes can be flat or even concave. Similarly, pushing on a rope tends to squash the rope while drawing serrations across it sheers the rope fibres. Drawing a smooth blade is less effective as the blade is parallel to the direction draw but the serrations of a serrated blade are at an angle to the fibres.
Saw blade serrations, for both wood and metal, are typically asymmetrical so that they cut while moving in only one direction. This loss of material necessarily weakens the blade but serves to make the blade lighter without sacrificing stiffness. Fullers are only of significant utility in swords. In most knives there is so little material removed by the fuller than it makes little difference to the weight of the blade and they are largely cosmetic. A balance must be found between the sharpness and how well it can last.
This method yields an edge that can hold its sharpness as well as a body that is tough. Prehistorically, and in less technologically advanced cultures even into modern times, tool and weapon blades have been made from wood, bone and stone. Most woods are exceptionally poor at holding edges and bone and stone suffer from brittleness making them suffer from fracture when striking or struck. In modern times stone, in the form of obsidian, is used in some medical scalpels as it is capable of being formed into an exceedingly fine edge. As non-metals do not corrode they remain rust and corrosion free but they suffer from similar faults as stone and bone, being rather brittle and almost entirely inflexible.
They are harder than metal knives and so more difficult to sharpen, and some ceramic knives may be as hard or harder than some sharpening stones. Both require diamond stones or silicon carbide stones to sharpen and care has to be taken to avoid chipping the blade. As such ceramic knives are seldom used outside of a kitchen and they are still quite uncommon. Plastic knives are difficult to make sharp and poorly retain an edge. They are largely used as low cost, disposable utensils or as children’s utensils or occasionally in environments where it is considered that even a bread and butter knife would be undesirable.
They are often serrated to make up for their general lack of sharpness but, as evidenced by the fact they can cut food, they are still capable of inflicting injury. Though copper is soft for a metal, and would easily deform, it would not hold an edge well and was poorly suited for working stone, though was more durable than earlier materials. Iron replaced bronze once sufficient smelting technology was achieved to reach iron’s melting point. Various alloys of steel can be made which offer a wide range of physical and chemical properties desirable for blades.