A fuller is a rounded or beveled groove or slot in the flat side of a blade (e.g. a sword, knife, or bayonet) that is made using a blacksmithing tool called a spring swage . These grooves are often called "blood grooves" or "blood gutters" as well as fullers, although their purpose has nothing to do with blood. A fuller is often used to lighten the blade, much the way that the shape of an I-beam allows a given amount of strength to be achieved with less material. When combined with proper distal tapers, heat treatment and blade tempering, a fullered blade can be 20% to 35% lighter than a non-fullered blade without any sacrifice of strength or blade integrity. This effect lessens as the blade is reduced in length. A blade is said to be "fullered" after introduction of the groove.
The term "fuller" is from the Old English fuliere, meaning "one who fulls (pleats) cloth." It is derived from the Latin word fullo. The first recorded use of the term as a blacksmithing tool is from 1864, according to Webster's Dictionary. The term used in historical Europe is largely unknown, and due to the constantly changing nature of language, the popular term also may have varied from generation to generation. King Thrasamund of the Vandals was recorded in a letter to King Theodoric the Ostrogoth, giving thanks for a gift of swords, and refers to the fullers in the blades as simply grooves: "...their centers, hollowed out with beautiful grooves, seem to undulate with worm-like markings; for shadows of such variety you would think the metal was interwoven rather than shining with different colors." The French often use the term goutiere (gutter) or cannelure (channel). The ancient Viking term is unknown. As a verb, the old French term "gutter" meant "to cut small hollows," as in the gutter of a crossbow. The addition of "blood" to these words (i.e.: "blood channel," "blood gutter," "blood groove") was most likely a modern colloquialism, but gives the connotation of an unintended and undesirable purpose; that of directing blood toward the hands rather than lessening the weight of the blade. Therefore, in modern descriptions, fuller is often the preferred choice.
As a blacksmithing tool, a fuller is a type of swage. A swage is a tool that has a particular shape forged into its surface, which is used to imprint that shape into the metal. Swages are often tools with handles and various shaped heads, which are placed between the hammer and the workpiece, allowing the smith to work the steel into various shapes that would be impossible to make with a hammer alone. Swage blocks are often anvil-like blocks with various shapes forged in them, also for forming metal. The term "swage" is from the Old French "souage," meaning "decorative groove" or "ornamental moulding."
Fullers, in particular, are swages that have a cylindrical or beveled face, typically three to six inches long, which are used for imprinting grooves into the steel. If a groove is to be applied to both sides of the steel, two fullers may be used at the same time, sandwiching the workpiece in the middle. Often, one fuller will have a peg that holds it securely in the anvil, while the other fuller will have a handle and a flat head, for striking with a hammer. Sometimes the two fullers may be connected with a flatbar of spring steel which is bent into a wide, "C" shaped bracket, keeping the two fullers aligned with each other and freeing the smith's hands for manipulating the workpiece and hammer. A blade being fullered will generally be slowly pulled through the fullers as they are being hammered, until the entire groove is the proper depth and shape. As the fullers make the groove, they typically displace material to the side, creating ridges around the grooves. These ridges may be hammered flat, widening the blade, or they are often shaped with other swages, increasing the strength of the blade by creating thicker areas in its cross section.
Fullers have a variety of uses beyond bladesmithing. They are often used for "drawing out" steel, which consists of hammering a short block into a long bar. The fullers will often be used to notch the block or bar. If the fullers are longer than the block is wide, they will tend to elongate the block rather than raise bulges from the surface. After notching, the block is hammered smooth, elongating it further, and this process is repeated until the bar becomes the correct length and thickness. Fullers are also used for many other shaping processes in blacksmithing, from making hinges and latches to plow parts and horseshoes.
The basic design principle is that bending causes more stress in material near the edge or back of the blade than material in the middle, due to leverage. The diagram at right shows stress distribution in an ideal blade with a rectangular section, with only a small amount of normal stress present at the neutral axis. Fullers remove material from near this neutral axis, which is closer to the blade's spine if only one edge is sharpened (see photo above). This yields stiffer blades of a given weight, or lighter blades of a given stiffness. The same principle is taken to an extreme in I-beams.
In Japanese bladesmithing, fullers have a rich tradition and terminology, enough that there are separate terminologies for the top (hi, usually pronounced as bi when used as a successive word) and bottom (tome) ends of the feature. A listing follows:
- Bo-hi: A continuous straight groove of notable width, known as katana-bi on tantō. With soe-bi, a secondary narrow groove follows the inner straight length of the main one. With tsure-bi, the secondary is similar but continues beyond the straight length.
- Futasuji-hi: Two parallel grooves.
- Shobu-hi: A groove shaped like the leaf of an iris plant.
- Naginata-hi: A miniature bo-hi whose top is oriented opposite from the blade's, and usually accompanied by a soe-hi. Seen primarily on naginatas.
- Kuichigai-hi: Two thin grooves that run the top half of the blade; the bottom half is denoted by the outer groove stopping halfway while the inner one expands to fill the width.
- Koshi-hi: A short rounded-top groove found near the bottom of a blade, near to the tang.
- Kaki-toshi: The groove runs all the way down to the end of the tang.
- Kaki-nagashi: The groove tapers to a pointed end halfway down the tang.
- Kaku-dome: The groove stops as a square end within 3 cm of the tang's upper end.
- Maru-dome: Similar to the kaku, except with a rounded-end.
The Nepali kukri has a terminology of its own, including the "aunlo bal" (finger of strength/force/energy), a relatively deep and narrow fuller near the spine of the blade, which runs (at most) between the handle and the corner of the blade, and the "chirra", which may refer either to shallow fullers in the belly of the blade or a hollow grind of the edge, and of which two or three may be used on each side of the blade.
- A History of Metallography by Cyril Stanley Smith -- Mit Press 1960 Page 4
- European Weapons and Armour: From the Renaissance to the Industrial Revolution by Edwart Oakeshott -- The Lutterworth Press 1980 Page
- Nouveau dictionnaire français-anglois et anglois-françois By Louis Chambaud, Jean Baptiste René Robinet -- 1776 Page 245
- English Mechanic and World of Science by R. J. Kibblewhite -- Bradley and Co. 1892 Page 53