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Knife sharpening is the process of making a knife or similar tool sharp by grinding against a hard, rough surface, typically a stone, [1] or a flexible surface with hard particles, such as sandpaper. Additionally, a leather razor strop, or strop, is often used to straighten and polish an edge.
The smaller the angle between the blade and the stone, the sharper the knife will be, but the less side force is needed to bend the edge over or chip it off. The angle between the blade and the stone is the edge angle – the angle from the vertical to one of the knife edges, and equals the angle at which the blade is held. The total angle from one side to the other is called the included angle – on a symmetric double-ground edge (a wedge shape), the angle from one edge to the other is thus twice the edge angle. Typical edge angles are about 20° (making the included angle 40° on a double-ground edge). [2] The edge angle for very sharp knives can be as little as 10 degrees (for a 20° included angle). Knives that require a tough edge (such as those that chop) may sharpen at 25° or more.
Different knives are sharpened differently according to grind (edge geometry) and application. For example, surgical scalpels are extremely sharp but fragile, and are generally disposed of, rather than sharpened, after use. Straight razors used for shaving must cut with minimal pressure, and thus must be very sharp with a small angle and often a hollow grind. Typically these are stropped daily or more often. Kitchen knives are less sharp, and generally cut by slicing rather than just pressing, and are steeled daily. At the other extreme, an axe for chopping wood will be less sharp still, and is primarily used to split wood by chopping, not by slicing, and may be reground but will not be sharpened daily. In general, but not always, the harder the material to be cut, the higher (duller) the angle of the edge.
The composition of the stone affects the sharpness of the blade (a finer grain, usually, though not always, produces sharper blades), as does the composition of the blade (some metals take and keep an edge better than others). For example, Western kitchen knives are usually made of softer steel and take an edge angle of 20–22°, while East Asian kitchen knives are traditionally of harder steel and take an edge angle of 15–18°. The Western-style kitchen knives are generally in the range of 52–58 on the Rockwell scale, which denotes the relative hardness of a material.
Knife sharpening proceeds in several stages, in order from coarsest (most destructive) to finest (most delicate). These may be referred to either by the effect or by the tool. Naming by effect, the stages are:
Named by tools, the same three stages are:
The word "honing" is ambiguous, and may refer to either fine sharpening (step 1.2) or straightening (step 2).
The finest level of sharpening is done most frequently, while the coarser levels are done progressively more rarely, and sharpening methods differ between blades and applications.
For example, a straight razor used for shaving is stropped before each use, and may be stropped part-way through use, while it will be fine sharpened on a stone a few times per year, and re-ground on a rough stone after several years.
By contrast, a kitchen knife is steeled before or after each use (and may be steeled during heavy use, as by butchers), and sharpened on a stone a few times per year.
Blades are damaged primarily by buckling – compressive force, from being pressed into a hard object, such as bone, ice, or a hard cutting board – and by bending, from sideways pressure. Both of these tend to roll the edge of a blade, due to the metal's ductile nature.
Blades may also be damaged by being corroded by acid (as when cutting lemons or tomatoes) or by high temperatures and corrosive chemicals in a dishwasher.
If a knife is used as a scraper, a pry-bar, or encounters hard particles in softer materials, there may be a sideways load at the tip, causing bending damage. [3]
Blade damage is avoided by:
Blade sharpness can be checked in multiple ways.
Visually, a very sharp knife has an edge that is too small to see with the eye; it may even be hard or impossible to focus in a microscope. The shape near the edge can be highlighted by rotating the knife and watching changes in reflection. Nicks and rolled edges can also be seen, as the rolled edge provides a reflective surface, while a properly straightened edge will be invisible when viewed head-on. [4]
By touch, a blade can be checked by running a thumb across the blade (perpendicular to the edge – not along, which will cut it). A sharp blade will have a distinct edge, like a corner, and may sing slightly from vibration, while a dull blade will have a round edge and the thumb will slip over it.
A blade's sharpness may be tested by checking if it "bites"—begins to cut by being drawn across an object without pressure. Specialized sticks exist to check bite, though one can also use a soft ballpoint pen, such as the common white Bic Stic. A thumbnail may be used [5] at the risk of a cut, or the edge of a sheet of paper. For kitchen knives, various vegetables may be used to check bite, notably carrots, tomatoes, or cucumbers. In testing in this way, any nicks are felt as obstacles. A blade polished to a finer edge (greater than 3000 grit or so), although technically sharper, can have a harder time initiating cuts on soft vegetables or fruits with a skin, such as tomatoes or bell peppers, due to the lack of micro-serrations on the apex of the knife. These micro-serrations can decrease the effort needed to initiate cuts by tearing the food on a microscopic level.
"Biting" sharpness is considered ideal for kitchen knives, but sharper blades are desired for shaving and surgical scalpels, which must cut without side-to-side slicing of the blade, and duller but tougher blades are more suitable for chiseling and chopping wood.
Grinding is generally done with some type of sharpening stone. Sharpening stones come in coarse and fine grits and can be described as hard or soft based on whether the grit comes free of the stone with use. Many sources of naturally occurring stones exist around the world; some types known to the ancient world are no longer used, due to exhaustion of former resources or the ready availability of superior alternatives. Arkansas, USA is one source for honing stones, which are traditionally used with water or honing oil. India is another traditional source for stones. Ceramic hones are also common, especially for fine grit size. Japanese water stones (both artificial and natural) come in very fine grits. Before use, they are soaked in water, then flushed with water occasionally to reduce energy loss to friction, and to keep material from clogging the stone's pores. [6] [7] The mixture of water and abraded stone and knife material is known as slurry, which can assist with the polishing of the knife edge and help sharpen the blade. Generally, these are more costly than oilstones. Coated hones, which have an abrasive, sometimes diamonds, on a base of plastic or metal, are also available.
Diamond stones can be useful in the sharpening process. Diamond is the hardest naturally occurring substance known and as such can be used to sharpen almost any material. [8] (Coarse diamond sharpening stones can be used for flattening waterstones. [8] [9] ) Alternatively, tungsten carbide blades can be used in knife sharpening.
Steeling helps maintain sharpness. This process realigns the edge, correcting for dulling causes such as a rolled edge. A sharpening steel is a type of hardened cylindrical rod used similarly to honing stones. For example, a butcher steel is a round file with the teeth running the long way, while a packer steel (used in the meat packer's industry) is a smooth, polished steel rod designed for straightening the turned edge of a knife, [10] and is also useful for burnishing a newly finished edge. Because steels have a small diameter they exert high local pressure, and therefore affect the knife metal when used with very little force. They are intended for mild steel knives that are steeled several times a day, but are not well suited for today's tougher and harder blade steels. Steels harder than 60 HRC can be prone to chipping when steeled, so stropping may be a more suitable option for maintain the integrity of the apex. Diamond steels are now available that have an industrial diamond coating and can remove blade metal as well as straighten, therefore used correctly they can re-profile a knife instead of just honing.
Stropping a knife is a finishing step. This is often done with a leather strap, either clean or impregnated with abrasive compounds (e.g. chromium(III) oxide or diamond), but can be done on paper, cardstock, cloth, or even bare skin in a pinch. It removes little or no metal material, but produces a very sharp edge by either straightening or very slightly reshaping the edge. Stropping may bring a somewhat sharp blade to "like new" condition.
Thinning does what the name suggests – it makes the knife thinner than what it was. This is to allow the newer geometry of the knife to pass through material easily. Commonly done for kitchen knives, this practice does not sharpen, but the same principle is there. This is important, since repeated sharpening will end up thickening the primary bevel and it will be harder for the knife to pass through material. Harder steels can go thinner by going at a lower angle (about 3-5 degrees), while thicker steels can be thinned about 2-3 degrees less from the usual sharpening angle. This is a small change but makes a drastic difference. [11]
A knife is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidenced by the Oldowan tools. Originally made of wood, bone, and stone, over the centuries, in step with improvements in both metallurgy and manufacturing, knife blades have been made from copper, bronze, iron, steel, ceramic, and titanium. Most modern knives have either fixed or folding blades; blade patterns and styles vary by maker and country of origin.
A blade is the portion of a tool, weapon, or machine with an edge that is designed to puncture, chop, slice or scrape surfaces or materials. Blades are typically made from materials that are harder than those they are to be used on. Historically, humans have made blades from flaking stones such as flint or obsidian, and from various metal such as copper, bronze and iron. Modern blades are often made of steel or ceramic. Blades are one of humanity's oldest tools, and continue to be used for combat, food preparation, and other purposes.
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.
A Japanese kitchen knife is a type of a knife used for food preparation. These knives come in many different varieties and are often made using traditional Japanese blacksmithing techniques. They can be made from stainless steel, or hagane, which is the same kind of steel used to make Japanese swords. Most knives are referred to as hōchō or the variation -bōchō in compound words but can have other names including -kiri. There are four general categories used to distinguish the Japanese knife designs: handle, blade grind, steel, and construction.
A blade's grind is its cross-sectional shape in a plane normal to the edge. Grind differs from blade profile, which is the blade's cross-sectional shape in the plane containing the blade's edge and the centre contour of the blade's back. The grind of a blade should not be confused with the bevel forming the sharpened edge; it more usually describes the overall cross-section of the blade, not inclusive of the beveled cutting edge which is typically of a different, less acute angle as the bevel ground onto the blade to give it a cross-sectional shape. For example, the famous Buck 110 hunting knife has a "hollow ground" blade, with concave blade faces, but the cutting edge itself is a simple, flat-ground bevel of lesser angle. It would be difficult, if not impossible, to put a "hollow grind" onto the actual cutting edge of the blade itself, which is a very narrow and small bevel.
A razor strop or simply a strop is a flexible strip of leather, canvas, denim fabric, balsa wood, or other soft material, used to straighten and polish the blade of a straight razor, a knife, or a woodworking tool such as a chisel. In many cases stropping re-aligns parts of the blade edge that have been bent out of alignment. In other cases, especially when abrasive polishing compound is used, stropping may remove a small amount of metal. Stropping can also burnish the blade.
A kitchen knife is any knife that is intended to be used in food preparation. While much of this work can be accomplished with a few general-purpose knives – notably a large chef's knife, a tough cleaver, a small paring knife and some sort of serrated blade – there are also many specialized knives that are designed for specific tasks. Kitchen knives can be made from several different materials.
In cooking, a chef's knife, also known as a cook's knife, is a cutting tool used in food preparation. The chef's knife was originally designed primarily to slice and disjoint large cuts of beef. Today it is the primary general-utility knife for most Western cooks.
The Santoku bōchō or Bunka bōchō (文化包丁) is a general-purpose kitchen knife originating in Japan. Its blade is typically between 13 and 20 cm long, and has a flat edge and a sheepsfoot blade that curves down an angle approaching 60 degrees at the point. The term Santoku may refer to the wide variety of ingredients that the knife can handle: meat, fish and vegetables, or to the tasks it can perform: slicing, chopping and dicing, either interpretation indicating a multi-use, general-purpose kitchen knife. The Santoku's blade and handle are designed to work in harmony by matching the blade's width and weight to the weight of the tang and the handle.
A straight razor is a razor with a blade that can fold into its handle. They are also called open razors and cut-throat razors. The predecessors of the modern straight razors include bronze razors, with cutting edges and fixed handles, produced by craftsmen from Ancient Egypt during the New Kingdom. Solid gold and copper razors were also found in Ancient Egyptian tombs dating back to the 4th millennium BC.
A sharpening jig is often used when sharpening woodworking tools. Many of the tools used in woodworking have steel blades which are sharpened to a fine edge. A cutting edge is created on the blade at the point at which two surfaces of the blade meet. To create this cutting edge a bevel is formed on the blade, usually by grinding. This bevel is subsequently refined by honing until a satisfactorily sharp edge is created.
Sharpening stones, or whetstones, are used to sharpen the edges of steel tools such as knives through grinding and honing.
Sharpening is the process of creating or refining the edge joining two non-coplanar faces into a converging apex, thereby creating an edge of appropriate shape on a tool or implement designed for cutting. Sharpening is done by removing material on an implement with an abrasive substance harder than the material of the implement, followed sometimes by processes to polish/hone the sharp surface to increase smoothness.
A ceramic knife is a knife with a ceramic blade typically made from zirconium dioxide (ZrO2; also known as zirconia), rather than the steel used for most knives. Ceramic knife blades are usually produced through the dry-pressing and firing of powdered zirconia using solid-state sintering. The blades typically score 8.5 on the Mohs scale of mineral hardness, compared to 4.5 for normal steel and 7.5 to 8 for hardened steel and 10 for diamond. The resultant blade has a hard edge that stays sharp for much longer than conventional steel blades. However, the blade is brittle, subject to chipping, and will break rather than flex if twisted. The ceramic blade is sharpened by grinding the edges with a diamond-dust-coated grinding wheel.
A cleaver is a large knife that varies in its shape but usually resembles a rectangular-bladed hatchet. It is largely used as a kitchen or butcher knife and is mostly intended for splitting up large pieces of soft bones and slashing through thick pieces of meat. The knife's broad side can also be used for crushing in food preparation and can also be used to scoop up chopped items.
A honing steel, sometimes referred to as a sharpening steel, whet steel, sharpening stick, sharpening rod, butcher's steel, and chef's steel, is a rod of steel, ceramic or diamond-coated steel used to restore keenness to dulled blade edges. They are flat, oval, or round in cross-section and up to 30 centimetres (1 ft) long. The steel and ceramic honing steels may have longitudinal ridges, whereas the diamond-coated steels are smooth but embedded with abrasive diamond particles.
Honyaki (本焼) is the name for the Japanese traditional method of metalwork construction most often seen in kitchen knives by forging a blade, with a technique most similar to the tradition of nihonto, from a single piece of high-carbon steel covered with clay to yield upon quench a soft, resilient spine, a hamon, and a hard, sharp edge. Honyaki as a term alone can refer to either mizu honyaki (water-quench) or abura honyaki. The goal is to produce a sharper, longer lasting edge than is usually achievable with the lamination method. The term has been adapted to describe high-end mono-stainless in Japan and carbon blades by non-Japanese bladesmiths that have a hamon but are made with Western steel, heat treat, equipment, finishing, and design.
Thiers Issard or Thiers Issard Sabatier is a French cutlery manufacturer; they are one of a number of companies using the Sabatier name. It exports a wide range of knives and straight razors to approximately thirty countries. They are viewed as one of the top cutlery firms in Europe.
A diamond knife is a very sharp knife in which the edge is made from diamond, invented by Humberto Fernández-Morán in 1955. Diamond knives are used for medical and scientific applications where an extremely sharp and long-lasting edge is essential. The knives are very expensive to purchase, depending on the quality and size of the knife; in addition the knives must be professionally sharpened as the edge dulls.
A fillet knife is a kitchen knife used for filleting. It gives good control and aids in filleting. It is a very flexible member of the boning knife family that is used to filet and prepare fish. Fillet knife blades are typically 15 to 28 cm long. This allows them to move easily along the backbone and under the skin of meat.
A sharp knife's edge will reflect no light because the two sides of the knife come to a razor fine edge. Conversely, a dull knife's edge will appear shiny because the two sides of the knife form a blunt rounded edge which reflects light.
