In a practical sense, the sharpness of an edge is a subjective to the intended use of a knife. For example, a knife that shaves very well may not be able to cut a squishy tomato - is it still a sharp knife? A knife that will go right through the squishy tomato may not shave at all - is it a sharp knife? Both are sharp, but only for their intended use.
Intended Use - Before you sharpen your knife, you should know how you plan to use it. Is it a utility or pocket knife? Or is it a kitchen , field dressing or carving knife. The way you sharpen your knife may create an edge that is very sharp for some applications and not sharp at all for others. It is important to choose an appropriate edge geometry and level of polish for your intended use.
If you will be working in the kitchen or in the field, you will most likely be slice-cutting. If you plan on shaving or carving, then you will be push-cutting.
Slice-cutting applications generally require a moderately polished edge with an included angle of between 34 and 44 degrees. That means you will be sharpening each side of the edge between 17 and 22 degrees.
Push-cutting applications generally require a highly polished edge and an included angle of between 26 and 34 degrees (13 - 17 degrees per side.)
Sharpening Basics - Definitions
The following sharpening terms are provided through the Merriam-Webster Dictionary, Wikipedia and from a variety of articles and conversations with knife sharpening enthusiasts.
Abrasives - Any material that is used to scratch or remove metal
Arkansas Stone - These stones are mined in Arkansas and are commonly used for knife sharpening and honing. 'Novaculite (also called Arkansas Stone) is a sedimentary rock composed mostly of microcrystalline quartz and is a recrystallized variety of chert. It is dense, hard, white to grayish-black in color, translucent on thin edges, and has a dull to waxy luster. It typically breaks with a smooth conchoidal (shell-like) fracture. The word novaculite is derived from the Latin word novacula, meaning razor stone. Novaculite is widely distributed in the Ouachita Mountains of Arkansas.' The above quote comes from the Arkansas State Website.
Bevel - (From Wikipedia) 'A beveled edge refers to an edge of a structure that is not perpendicular (but instead often at 45 degrees) to the faces of the piece.' The term bevel when used in knife sharpening refers to the edge of the angled portion of the knife blade between the face of the knife and the edge of the knife as show in the diagram below: Burr - (From Wikipedia) 'In engineering, a burr refers to the raised edge on a metal part. It may be present in the form of a fine wire on the edge of a freshly sharpened tool or as a raised portion on a surface, after being struck a blow from an equally hard, or heavy object.' A burr, also called a wire edge, is created as a result of sharpening metal. The burr forms on the edge of the knife where the planes of the bevels (or, in the case of one-sided knives or chisels, the plane of one face and the bevel) intersect. The diagram below shows the burr projecting from the edge of the knife:
Chisel Ground - (From Wikipedia)'As on a chisel, only one side is ground (often at an edge angle of about 20 – 30°) whilst the other remains flat all the way to the spine. As many Japanese culinary knives tend to be chisel ground they are often sharper than a typical double beveled Western culinary knife. (A chisel grind has only a single edge angle. If a double bevel has the same edge angle as a chisel grind, it still has two edges and thus has twice the included angle.) Knives which are chisel ground come in left and right handed varieties, depending upon which side is ground.' A chisel ground edge is depicted in the image below:
Convex Ground - Also known as bullet tip or gothic arch. (From Wikipedia)'Rather than tapering with straight lines to the edge, the taper is curved, though in the opposite manner to a hollow grind. Such a shape keeps a lot of metal behind the edge making for a stronger edge while still allowing a good degree of sharpness. This grind can be used on axes and is sometimes called an axe grind. As the angle of the taper is constantly changing this type of grind requires some degree of skill to reproduce on a flat stone. Convex blades usually need to be made from thicker stock than other blades.' A convex ground edge is depicted in the image below:
Diamond Stone - An abrasive used in sharpening. Diamond stones are most commonly created by electroplating industrial diamonds onto various metals such as nickel. Diamond stones come in varying degrees courseness. The standards for diamond stone courseness vary and do not always compare directly with courseness of other sharpening abrasives.
Double Bevel - (From Wikipedia) 'A back bevel, similar to a sabre or flat grind, is put on the blade behind the edge bevel (the bevel which is the foremost cutting surface). This back bevel keeps the section of blade behind the edge thinner which improves cutting ability. Being less acute at the edge than a single bevel, sharpness is sacrificed for resilience: such a grind is much less prone to chipping or rolling than a single bevel blade. In practice, double bevels are common in a variety of edge angles and back bevel angles.' A double bevel is depicted in the image below:
Edge - The intersection of bevels or faces on a knife.
Gothic Arch - See Convex Ground above.
Hollow Ground - (From Wikipedia) 'A common grind where a convex hollow is removed from both sides of the edge. It produces a very sharp edge but being so thin the edge is more prone to rolling or damage than other grinds. It is unsuited to heavy chopping or cutting hard materials. Straight razors are hollow ground. This grind is used extensively in mass produced knives.' A hollow ground edge is depicted in the image below:
Hone - (From Merriam Webster)
Main Entry: hone
Function: transitive verb
Inflected Form(s): honed; hon·ing
Date: 1798
1 : to sharpen or smooth with a whetstone 2 : to make more acute, intense, or effective : whet
— hon·er noun
Steel - (Excerpt from Wikipedia) 'passing the blade against a hard metal "steel" (which may be made of ceramic) which plastically deforms and straightens the material of the blade's edge which may have been rolled over irregularly in use, but not enough to need complete resharpening.'
Strop - (Excerpt from Wikipedia) 'In origin the word strop is simply an alternative form of strap, but it is now mainly used in the sense of a flexible rectangular piece of leather or canvas used for sharpening a razor, also known as a razor strop.
It may be in the form of a hanging belt or a hand-held paddle.'
Waterstone - Usually Japanese, can be naturally quarried or man-made. (From Wikipedia)'The Japanese have traditionally used sharpening stones which are lubricated with water to sharpen their metal tools. (Using oil on a waterstone is deleterious to the stone.) As they have been doing this for many hundreds of years, it is obvious that the first stones were those which were found occurring naturally. The geology of Japan provided a type of stone which consists of fine silicate particles in a clay matrix. This is somewhat softer than Novaculite.
Japanese stones are also sedimentary. The most famous are typically mined in the Narutaki District just north of Kyoto.'
Whetstone - (From Merriam Webster)
Main Entry: whet·stone
Pronunciation: \'hwet-'stōn, 'wet-\
Function: noun
Date: before 12th century
: a stone for whetting edge tools
Sharpening Basics - Angle Control
If there are two words that describe why sharpening either succeeds or doesn't, those two words are Angle Control. If you are able to consitently sharpen or hone at a specified angle, chances are you are going to acheive good results.
Employing some method of controlling the angle of the abrasive relative to the knife is crucial. These days there are many, many devices available for angle control. Some are simple wedges and clamps, meant to be used with a bench stone. Others are full sharpening systems designed to limit the movement of the abrasive against the knife at a specified angle.
Selecting the right angle control device comes down to personal preference and budget. Important considerations are ease of use and versatility. If the device is a pain in the neck to use, most likely your knives are going to stay dull.
Choosing Your Edge Geometry
The edge geometry that you choose for your knife should be the result of careful consideration of how you intend to use your knife. Factors that you should keep in mind are:
The hardness of the materials you will be cutting
Whether you will be push, or slice cutting
How much lateral stress will be applied to your edge
How important the accuracy of your cuts are
Hard materials such as wood, carrots, and zucchini usually require push-cutting. Softer materials generally require slice-cutting.
Lateral Stress is very hard on the edge of your knife. Where the two bevels of your blade intersect there is a very fine wire edge. When the edge of your blade undergoes lateral stress, the wire edge is often folded over to one side, greatly reducing its cutting ability. If you will be turning your hand often as you cut, as in the case of field dressing, you may want to consider creating a convex grind or double bevel on your edge. Convex grinds are the most durable but are difficult to create. A good second choice is the double bevel. These are very durable and much easier to create than a convex grind.
If you are carving or preparing intricate food creations, you will want a high degree of accuracy. A hollow grind is your best choice for accuracy but has the drawback of not being very durable. A chisel ground edge is a good substitute. It is also very accurate and acute and is a little more durable than a hollow ground edge. If a chisel grind is not an option because the faces of your knife blade are not parallel, then the next best choice is a standard V ground edge.
What is the Correct Angle for Your Knife?
When preparing to sharpener your knives, first consult with the manufacturer for the recommened sharpening angle. If you no longer have the literature that came with the knife, a google search will often yield the desired information.
If you are unable to determine the recommended angle or wish to change the angle of your knife, consider the table below:
