Knife

A knife is a sharp-edged hand tool used for cutting. A knife usually consists of a blade, commonly less than 12 inches (30cm) in length, attached to a handle. The blade of a knife is usually pointed and may have one or two cutting edges. Knives have been used as tools and weapons since the Stone Age.

The first knives were flint or other rock, chipped or ground to an edge, sometimes with a handle. Later on with advances in smelting and metallurgy the blades were made of bronze, iron, then steel. While the materials have changed over time, the basic design remains the same.

Together with the fork and spoon, the knife has been a common eating utensil in the Western world since, at least, the Middle Ages. Today, these are not frequently sharpened, and most sharp knives are used in kitchens. In the West, multi-purpose pocket knives are commonly carried, to be available anywhere. The importance of the knife as a weapon has declined as more specialized weapons have been developed, but the knife remains the universal tool.

Anatomy of a knife

A knife consists of a tang, a bolster, a blade and a handle. The tang is an extension of the blade into the handle. The bolster functions as a balance point and handguard, to prevent fingers from slipping onto the blade. The blade in turn consists of a tip, a spine, an edge and a heel.

A fuller, sometimes called a blood gutter or blood groove, is a groove on the side, along a blade. According to a popular myth, it lets bleeding occur from an artery without removing the knife. In reality, its only function is to make knives and swords lighter while sacrificing little strength; on most knives it has more decorative worth. Additionally, a groove on the blade can reduce surface tension between the blade and the item being cut, thereby allowing easier movement or removal of the blade. See 1 Some knives also have a shoulder in which the blade thickens as it meets the handle. In piercing, this helps keep the knife from jamming, for example in bone. In kitchen knives, it keeps chopped items from moving back toward the hand.

The handle of a knife should be made of a non-slip material. For a large knife, it is desirable that the handle is thick enough that one's fingers just meet one's palm when the knife is gripped tightly. A hole in the end of the handles allows the knife to be hung or placed on a lanyard.

Blades

Materials

Knife blades are usually made of steel; although there are a few knives using materials like high tech ceramic and titanium, these are very uncommon. Stainless steels have gained popularity in the latter half of the twentieth century because they are highly resistant to corrosion (so they usually only rust under extreme conditions). Tests done by Razor Edge Systems, and described in their book "The Razor Edge Book of Sharpening" indicate that stainless steel knives hold an edge better than regular steels. Stainless and semi-stainless steels include D2, S30V, 154CM, ATS-34, and 440C. Chromium is the major alloying element in stainless steels, giving them the 'stainless' quality.

Steels having high carbon but low chromium content are prone to rust and pitting if not kept dry.

A variety of exotic steels and other materials can be used to form blades. Knife manufacturers such as Spyderco and Benchmade typically use 154CM, VG-10, S30V, and CPM440V (also known as S60V), as well as high-speed high-hardness tool steels like D2 and M2. Other manufacturers sometimes use titanium, cobalt, and cobalt containing alloys. All three are more ductile than typical stainless steels, but have quite a vocal support group despite concerns about health effects of cobalt content. The original craft of Damascus steel may be lost, but not being a registered trademark, the name is today used to apply to the equally old but less exotic pattern welding, which creates layered and admired patterns. The cost of the process restricts it to high-end knives. There is typically more demand for exotic alloys in the utility, outdoor, and tactical or combat knife categories than there is in the kitchen knife category.

Forschner/Victorinox make decent, inexpensive kitchen knives; high-end manufacturers include Wüsthof, Global, Henckels and Böker (Tree Brand). Some manufacturers, particularly of kitchen knives, make ceramic blades; these are harder and stay sharp longer, but because of their brittleness, chip and break more readily.

All knife steel is tempered martensite, which means that a fine-grained crystal structure with lattice irregularities that make it hard. It is formed as it is quenched, changing it from the austenitic structure that it has at high temperature to a hard, but brittle martensitic structure. The blade is then tempered by heating to an intermediate temperature for a period to make it less brittle. Knife steel has fairly low nickel content, because nickel tends to keep steel in the austenitic structure, even when cold. Stainless knife steels are high in carbon, but "carbon steel" means there is not also a lot of chromium.

Stainless steel is steel with very high (12–18%) chromium content. It is corrosion resistant (though knife steel is less so than higher nickel stainless steel) because, except in acid, one of the metals or one of the oxides is always stable. Stainless steel usually has particles of chromium (or other alloy metal) carbides. These explain its reputation for long wear (the carbides are harder than the metal) and for being harder to sharpen and not taking as good an edge as rustable, low alloy ("carbon") steel (the ceramic particles themselves cannot be sharpened easily.) The bulk hardness and toughness of stainless steel tend to be lower than those of low alloy steel.

Vanadium and molybdenum are important alloy metals because they make the gain size smaller, which improves hardness and toughness. Vanadium, and perhaps molybdenum, also increase corrosion resistance.

Shapes

There are a variety of knife blade shapes; some of the most common are listed below.

(1) A normal blade has a curving edge, and flat back. A dull back lets the wielder use fingers to concentrate force; it also makes the knife heavier and stronger for its size. The curve concentrates force on a small point, making cutting easier. Therefore, the knife can chop as well as pick and slice.

(2) A curved, trailing-point knife has a back edge that curves upward. This lets a lightweight knife have a larger curve on its edge. Such a knife is better for slicing than a normal knife.

(3) A double edged or spey blade has two edges. The idea is to make a blade that cuts in either direction, with a strong sharp point. This shape is primarily used for fighting knives (daggers, bayonets) because it can cut in both directions and point in line with the handle.

(4) A clip point blade is like a normal blade with the tip "clipped" to make the tip thinner and sharper. The back edge of the clip can have a false edge that can be sharpened to make a second edge. The sharp tip makes the blade exceptional as a pick, or for cutting in tight places. If the false edge is sharpened it increases the knife's effectiveness in piercing. The Bowie has a clipped blade.

(5) A sheepsfoot knife has a straight edge, and a curved dull back. It gives the most control, because the dull back edge is made to be held by fingers. Sheepsfoot knives are good for whittling, including sheep's hooves.

(6) An Americanized tanto style knife is thick towards the point. It is superficially similar to the points on most Japanese long and short swords (katana and wakizashi). The traditional Japanese tanto knife uses the blade geometry of (1). The edge is straight. The point is actually a second edge on the end of the blade, with a total edge angle of 60-80 degrees.

An ulu (Inuit woman's knife) knife is a sharpened half-circle. This sort of blade is all edge, with no point, and a handle in the middle. It's good for scraping, and sometimes chopping. It is the strongest knife-shape. An example is a head knife, used in leatherworking both to scrape down leather (reducing thickness), and to make precise, rolling cuts to form shapes.

A drop-point blade is very similar to a clip point, but it features the back convexed down, rather than having a clip taken out of it. It handles much like the clip-point.

Types of knives

Knives can be categorized based on either form or function.

Form

Knives exist in several styles:

Fixed blade knives 

Folding knives 

Dorsal vs. Ansall

Function

In general, knives are either working knives (everyday-use blades), or fighting knives. Some knives, such as the Scottish dirk and Japanese Tanto function in both roles. Many knives are specific to a particular activity or occupation:

For whittling (artistic wood carving) a blade as short as 25mm (1 inch) is common.

Serrations on a blade "saw" through the item being cut and stay sharp for a long time. The points protect the slicing areas from nicks. A good serration pattern will stay sharp several times as long as a straight edge.

The edge is sharpened at different angles for different purposes. 15 to 25 degrees is a good all-around angle. Slicing knives should have sharper angles, down to ten degrees. Chopping knives need blunter angles, out to thirty degrees.

Using knives

"Always cut away from yourself" is the basic adage to keep in mind while using a knife. By extension, assume that the knife is going to slip, and look where the blade would go. In Boy Scout parlance, an area within the radius of the arm and blade length combined is called the "blood circle". When checking the blood circle it is best to hold the knife by the blade, otherwise you defeat the purpose.

Knives offered to another person should always be offered handle first.

A sharp knife is often claimed to be a safer knife. Dull knives lead to excessive use of force to cut materials, increasing the chance that the blade may slip and the force will be transferred to an unintended destination such as the user or another person or object. Also, a dull or damaged knife will inflict a worse wound than a relatively 'clean' cut from a sharp knife. Conversely it can be argued that what is dangerous is not knowing how sharp a knife is and thus how much force to use.

A knife should be kept clean, dry and sharp. Steel blades rust easily, but oiling will prevent pitting due to oxidation and tarnish. Most knives are not intended as pry bars or screwdrivers. Either use is likely to break off the tip of the blade, or to bend or break the knife beyond repair. Most high-quality knives are also tempered very hard, so that they will retain an edge longer. However, this may also make them brittle.

Sharpening

Knives are sharpened by grinding against a hard surface, typically stone. The smaller the angle between the blade and stone, the sharper the knife will be, but the faster it will dull. A guide is very helpful. Very sharp knives sharpen at 10 degrees. Typical knives sharpen at 15 degrees. Knives that require a tough edge (such as those that chop) sharpen at 20 degrees. For an extremely durable edge (such as a chisel or drawknife), sharpen to 30 degrees. In general, the harder the material to be cut the higher the angle of the edge. The composition of the stone affects the sharpness of the blade (finer grain produces sharper blades), as does the composition of the blade (some metals take/keep an edge better than others).

Examples of sharpening tools are the clamp-style systems, which use a clamp with several holes with pre-defined angles. The stone is mounted on a rod and is pulled through these holes, so that the angle remains consistent. Another variant is the crock stick setup, where two sticks are put into a plastic or wooden base to form a V shape. When you pull a knife up the V, the angle is held for you, as long as you hold the blade perpendicular to the base.

Remove a wire edge (burr) if one forms during sharpening. Use a slighly steeper angle with very light pressure to do so. If not removed, it will break off in use, and the knife will instantly become dull. An alternate method of removing a wire edge is stroking from side to side on a very fine stone, using light strokes. This will flip the burr back and forth as it is ground off.

To feel for a wire edge, move your thumb lightly across the edge. It should come off with no resistance. If you feel a little bit of pull at the edge or the nail is sightly abraded, you may have a wire burr.

Honing stones (also called whetstones) come with coarse and fine grits and can be hard or soft describing whether the grit comes free. Arkansas is a traditional source for honing stones, which are traditionally (though a poor practice) used with water or honing oil. India is another traditional source for stones.

Ceramic hones are also common, especially for fine grit size.

Water stones (both artificial and natural) come in very fine grits. They are stored in water, and develop a layer of slurry which dulls the edge if you hone the blade as if honing into the stone. Generally, these are more costly than oilstones. Oil is not to be used on these.

Oil is sometimes used to lift the metal dust, called swarf, off the stone. This is generally bad to do during polishing. There are better ways than oil to clean a hone.

Coated hones, which have an abrasive, sometimes diamonds, on a base of plastic or metal are another kind of hone. Rather expensive are sharpening blocks made with corundum.

Stropping a knife is sometimes a finishing step. This is traditionally done with a leather strap impregnated with abrasive compounds, but can be done on paper, cardstock, or even cloth in a pinch. It will not cut the edge significantly, but produces a very sharp edge with very little metal loss. It is useful when a knife is still sharp, but has lost that 'scary sharp' edge from use.

Other times the final step is done with a steel. This fine process can effect alignment of the edge. Realigning the edge goes a long way in keeping the knife sharp, as often times, a rolled edge will make an otherwise sharp knife dull.

Mechanical consideration of the strength of sharpened edges and rolled-over knife edges

If a knife is used as a scraper or encounters hard particles in softer materials or is used asymmetrically, there may be a sideways load near the tip. In this case the knife should resist bending or breaking. Making some simplifying assumptions about the forces and the knife edge's ability to resist them may shed some light on ideal sharpening.

Assume the knife is thin and the force is applied at the very edge. Sheets of material are bent by stretching the outside or compressing the inside. Both the area taking the force and the lever arm converting force to torque are proportional to thickness, so the bending resistance is proportional to the square of the thickness. (That explains the strength per weight of aluminum, compared to steel.) If the force is applied at the edge, the bending torque is proportional to the distance from the edge. So, in this case, the ideal cross section is proportional to the square root of the distance from the edge. This is a (microscopic) parabola. This contrasts to the usual practice of trying to sharpen knives to a wedge near the edge. Perhaps this sheds light on the function of razor straps and on the practice of using two angle guides to sharpen a knife.

On the other hand, if the type of use cannot be predicted, it may be better to sharpen it to a wedge and let the first use bend the edge to an appropriate curve. A wedge shape has the property called "scale invariance". It has the same relative shape for any depth of cut.

(These thoughts were stimulated by an article posted on rec.woodworking.)

Legal considerations

Carrying knives in public is forbidden by law in many countries. Exceptions may be made for hunting knives, and for knives used for work-related purposes (e.g. chef's knives). Automatic knives (switchblades) are almost universally banned from civilian carry if not possession. Balisongs (butterfly knives) are only slightly less stigmatized, and tend to be treated as switchblades by law enforcement agencies due to their connection with gang activity. One exception is Austria, where civilian possession of automatic knives including double-edged automatic OTF ("out The front") daggers is allowed. Most Western European nations are very unfriendly toward all knives other than small pocket knives and similarly small tools, which are nonetheless not allowed on planes or in certain other venues. Even multitools like the SwissTool, Gerber multitools, and Leatherman multitools are often frowned upon, due to their having relatively large blades and/or locking ability.

Even small knives are forbidden on all commercial airliners and are among the illegal imports that may be confiscated at airports by customs staff even if packed in luggage. The knife laws of different countries vary, but are generally strict in Western countries.

In the USA

Knife laws vary tremendously. In Texas, for example, individuals may carry knives openly or concealed so long as they are single-edged, and are not daggers, switchblades, or gravity knives (balisong legality is questionable — there have been convictions). In some other States, fixed-blade knives are banned, open carry is banned, and sometimes concealed carry of anything except pocket knives is banned. Cities have ordinances further restricting these laws; in San Antonio, TX, it is a violation to carry a folding knife having a locking blade. In some metro areas such as Washington, D.C., going into office buildings or museums, or simply loitering, carrying even small 3" folding knives can be problematic. Other restricted areas in the U.S. include court buildings and federal property (the latter of which technically has a limit of 2.5 inch blades).

In the UK

Knife possession is only legislated in public places. In private almost any kind of knife may be owned with the exception of automatic knives (flick knives, balisongs, switchblades) which are under any circumstances illegal. In general, knives carried in public places are legally considered to be offensive weapons and the carrier can be charged with "possession of an offensive weapon". It is however legal to carry a knife if there is a bona fide reason to do so (e.g. chefs) or if it is part of a national costume (e.g. sgian dubh), or if it is carried for religious reasons (e.g. Sikhs). A special exception exists for penknives (pocket knives) which are legal without reason for possession. Folding knives with locking blades are not considered pen knives and are currently a legal "grey area".

Knife modifications

Knives can be sometimes be customised to the user and/or application:

• The handle can be altered in shape (for better grip) or material (to prevent electric shock or burns).
• The surface finish of the blade can be darkened or polished.

Knife superstitions

In some countries it is traditionally believed that the giving of a knife as a gift to a friend will cut or sever the relationship. To avoid such ill luck, the receiver should give a coin in return so as to "pay" for the gift. It is common to include a penny, often taped to the blade, with a knife given as a gift which the receiver is to return as "payment".

Stirring liquids or powders with a knife is considered unlucky; as the rhyme says, "Stir with a knife, stir up strife".

In some cultures giving a knife as a gift is considered a sign of respect and trust. This is especially true in Finland where various non-governmental organizations, clubs and even government agencies traditionally give a puukko (a Finnish fixed-blade hunting/outdoor knife) as a gift to trusted employers or contacts. The puukko is always presented handle first as a sign of trust and friendly intentions.

In many places in the United States it is considered bad luck to hand an open, folding blade knife to someone. This is especially true in more rural areas where carrying a pocket knife is as common as carrying a set of keys. This most likely stems from the fact that it is just not a safe thing to do.

Further reading

• Everybody's Knife Bible by Don Paul, ISBN 0938263234