The old Gaelic harps were strung with metal strings from earliest times right through to the decline of the tradition in the 19th century.
In pre-industrial Ireland and Scotland, there are four basic types of metal that could be used to make music wire. That is, iron, copper-alloy, silver, and gold.
In his Harmonie Universelle, Mersenne also discusses strings made from lead1, but I think this is a scholarly joke, since lead is not strong enough to draw into wires.
From a technical point of view, different alloys of these four types have different properties. Their names in different languages are not necessarily very precise. Nowadays, scientific analysis can tell us exact details of the alloy composition, trace inclusions, and hardness of both ancient samples and modern wires.
Iron Soft iron wire sometimes used to be called “steel”, though nowadays steel means an industrially produced metal, much stronger than the old iron wires. Old iron wires have low carbon and high phosphorous, compared to modern steel wire. Iron is the strongest and least dense of the four old metals used for wire. Iron was drawn into wire right back into early medieval times, for making mail armour2, though it is not clear how much it was used for musical instrument strings in medieval times. By the 16th and 17th centuries iron was common in harpsichords for the treble strings3, and in more recent times iron (and from the 19th century steel) became the main music wire for all kinds of instruments.
Brass or bronze is an alloy of copper. Called in Latin aes or auricalco, Irish/Gaelic umha or phráis, all these words including the English brass and bronze could in the past refer to any copper-based alloy. Modern usage is that bronze means copper-tin alloys, while brass means copper-zinc alloys. All of the copper-alloy historical music wire that has been analysed has proved to be either yellow brass, also known as cartridge brass (copper with 22-30% zinc) or less common red brass (copper with 7-14% zinc)4. These are mostly samples of wire preserved within historical harpsichords. Brass wire was used from medieval times for stringing psalteries5 and clavichords and other keyboard instruments, as well as certain types of necked plucked instruments. Brass wire was industrially produced in Germany to supply the musical instrument trade. Bronze is a rather different matter; historical bronze alloys do not include phosphorous, which is added to modern industrial bronze to allow it to be drawn into wires. This means that historical bronze is harder to draw into wires and does not seem to have been used so much for wire-making. Brass (and bronze) are weaker and denser than iron wire, which is why brass was traditionally used for the basses of harpsichords which had iron trebles. Archaeologists are very cautious and say “copper alloy” to describe any brass or bronze type metal!
Silver Called in Latin argentum, Irish and Gaelic airgeod. Can be made in many different alloys and purities. Most common today is ‘sterling silver’, 92.5% pure, legally defined since medieval times. Silver was used in medieval times for the bass strings of psalteries6. Silver is denser and weaker than brass.
Gold Called in Latin aurum, Irish and Gaelic or. The purity of gold is described in "carats", or 24ths. So 18 carat gold is 18/24 pure, or 75%. The remainder of the alloy can be any one of a number of metals such as silver, tin, copper, etc. all of which affect the physical properties of the alloy as well as its colour (hence "yellow gold", "green gold", "red gold" etc.) There is no common formula for gold to compare with sterling silver. Gold is extremely easy to draw into wire, and gold wire jewelry is known from Ireland and elsewhere right back to the Bronze age. Gold strings are known to have been used for the basses (and sometimes the entire range) of Italian harpsichords and other keyboard instruments in the 16th century7. Gold is denser and weaker than silver.
Mixed alloys are also possible. Electrum is an alloy of silver and gold. It is rarely used today for any purpose. Gold is usually alloyed with copper; low-carat gold can have more copper than gold in it. Old gold alloys include some silver though nowadays this is usually refined out. Historically when silver was a coinage metal, it was also alloyed with copper, sometimes to less than 50% silver.
As well as monofilament wires as described above, it is possible to make compound strings. They can be made by plating or covering, so the core is one metal and the outer layer a different metal; they can be from two strands of wire twisted together as a ‘catline’, or they can be like a modern ‘wound string’ with a straight core and a second wire wound spirally around the core. I don’t know of any evidence for these being used on the old Gaelic harps - all the evidence points to the Gaelic harp strings being monofilament wire.
1. Mersenne, Harmonie Universelle, Paris 1636 Livre Troisieme, des Instrumens à chordes, p.153-4 ^
2. Dominic Tweddle, The Anglian helmet from Coppergate, The Archaeology of York: the small finds, 17/8, 1992, p.1006-1009, 1018-1026, 1060-1063, ^
3. Martha Goodway & Jay Scott Odell, The Metallurgy of 17- and 18th-Century Music Wire Pendragon Press, New York, 1987 ^
4. Martha Goodway & Jay Scott Odell, The Metallurgy of 17- and 18th-Century Music Wire Pendragon Press, New York, 1987 ^
5. C. Page, Voices and Instruments of the Middle Ages, London 1987, appendix 4. Also Nelly van Ree Bernard, The Psaltery, 1989, Knuf. page 13-14 ^
6. Medieval references to psaltery strings of brass, silver and gold wire are cited in C. Page, ‘In the direction of the beginning’, in H Schott (ed), The Historical harpsichord vol 1, Pendragon Press, 1984, p.121-122. ^
7. Patrizio Barbieri, ‘Gold- and Silver-Stringed Musical Instruments: Modern Physics vs Aristotelianism in the Scientific Revolution’ Journal of the American Musical Instrument Society vol XXXVI, 2010 ^