A blade is the flat part of a tool, weapon, or machine (such as a fan) that normally has a cutting edge and/or pointed end typically made of a flaking stone, such as flint, or metal, most recently steel. A blade is intentionally used to cut, stab, slice, throw, thrust, position and/or place (an example of this is razor wire), shoot (an example of this is the ballistic knife), scrape (an example of this is an ink eraser) or strike an animate or inanimate object.

Materials for production

Material for weapon blades has to be carefully selected to achieve a balance between hardness and toughness and their ratio to each is dependent upon the intended use of a blade. In antiquity, the main metal used was copper, then of bronze and later iron. Perhaps the most well known is pattern welding, a technique used for katanas (samurai swords) and blades made to resemble damascus steel blades. This was a very labor-intensive technique - and thus such swords were very expensive.

Various techniques may also be employed to make the blade stronger or harder. Copper and bronze can be "work-hardened" by simply hitting the blade with a hammer while it is cold. Blades made of steel with a high enough carbon content (greater than 0.2%) can be heat-treated by heating the steel up to a critical point (most simple carbon alloys become non-magnetic slightly below that point), then quenching it with forced air, oil, or water depending on the steel. Quenching puts an enormous amount of stress on the metal, and often a sword would break into pieces during that step. If the sword survived heat-treating, it would be tempered by heating it to a relatively low temperature for an extended period of time. The tempering process would make it slightly softer, but also tougher and "springier", and thus less likely to break or chip during everyday usage.

Case hardening is a process of increasing the carbon content at the surface of very low carbon steel. It is done by placing the object to be hardened in a sealed container along with carbon-containing material; in antiquity, this material was usually horn or hide. The container would then be heated until it was glowing red, and held at that temperature for a while, based on the size of the part being hardened, allowing carbon to penetrate the steel by a few thousandths of a centimeter. At that point, the object would be dumped out of the container into a water bath to quench it, resulting in a very hard surface, but completely unhardened core. There is very little evidence of this having ever been done to swords except, perhaps, the very earliest of iron blades. Due to the inherent weakness of a sword's cutting edge, coupled with the high-impact stresses of combat, such a thin hardened surface over a soft core would provide very little advantage in terms of edge-holding, other than mild wear resistance.

Another important aspect of many blades are so-called "fullers". Despite popular belief, fullers were not "blood grooves" that facilitated quicker bleeding of the victim and easier removal after insertion. Rather fullers helped to make a blade stronger and more durable at the core by giving it an I beam cross section, thus reducing the amount of steel needed to keep the spine stiff. This was very important in ancient times when high quality steel was more labor intensive to make, smiths would scrape the fuller with a U shaped tool before hardening and reuse the scraps. Modern day fullers are made by positioning a heated blade over a bottom fuller, setting a like sized top fuller on the top side of the sword, and hitting the top fuller with a hammer.