Piano wire, or "music wire", is a specialized type of wire made to become springs or (as the name implies) to be used as piano strings. It is made from tempered high-carbon steel, also known as spring steel, which replaced iron as the material starting in 1834.
Piano wire has a very high tensile strength to cope with the heavy demands placed upon piano strings; accordingly, piano wire is also used for a number of other purposes, including springs, surgical uses, and in special effects.
The oldest record of wire being made for musical instruments is from Augsburg in 1351; this probably predates the harpsichord and may have been wire for a psaltery. Earlier wire, used in harpsichords, was of brass or iron.
Starting around 1800, the piano began to be built ever more ambitiously, with sturdier (eventually, iron) framing and greater string tension. This led to innovations in making tougher piano wire. In 1834, the Webster & Horsfal firm of Birmingham brought out a form of piano wire made from cast steel; according to Dolge it was "so superior to the iron wire that the English firm soon had a monopoly." But a better steel wire was soon created in 1840 by the Viennese firm of Martin Miller, and a period of innovation and intense competition ensued, with rival brands of piano wire being tested against one another at international competitions, leading ultimately to the modern form of piano wire.
The technological developments also benefited from demands of consistency from other special wire products like telegraph and barbed wire. Innovative piano makers kept pace with these advances by augmenting metal framing in their instruments and increasing tension of their strings.
General-purpose, high-carbon steel, drawn music wire (such as ASTM A228) is manufactured in both inch and metric music wire gauges (m.w.g.) in diameters as small as 0.15 mm up to 4.8 mm (0.006 to 0.192 inch). A small number of companies produce the tough, high tensile polished wire intended for limited music instrument markets, which is manufactured from steel of a specific composition by cold drawing. Unlike many other forms of wire, piano wire has no twist and is not formed from bundles of smaller-diameter wires.
Piano strings are among the most demanding of all applications of steel. Placed under high tension, they are subject to repeated blows, they are stretched and slackened during tuning and are still expected to last for decades. Similar challenges arise in plucked instruments, along with the additional demand of being bent when plucked.
Piano wire is sold by weight and packaged in tight coils. It springs back to a gentle curve but can be straightened using a series of opposed rollers. It requires careful handling for safety and appearance, since it can be marred by perspiration, and it requires special cutters, as the hardened steel will otherwise quickly dull the cutter.
Piano wire is also used in the fabrication of springs, fishing lures, special effects in the movie industry, scaffold cross-bracing, orthodontic and pharyngial surgery, and for the cutting of cheese and soap. It is also commonly used in hobby applications such as model railroading, both control line and radio-controlled aircraft, and knitting. At least in urban legend, it is employed by assassins as a garrote.