All metals have a property called hardness, which is the property of the metal that resists bending. Soft metals are pliable and easy to bend while hard metals are stiff and hard to bend. The hardness of metals can be changed by heat-treating the metal in a process called annealing or by bending the wire in a process called work hardening.
Since it is made of metal, wire has this same hardness property. Most modern manufacturers of jewelry wire make the wire with a defined hardness, generally a hardness of 0, 1, 2, 3, or 4. Historically, these numbers were associated with the number of times that the wire was pulled through a draw plate, becoming harder or stiffer each time it was drawn through the drawplate. A hardness of 0 meant that the wire had been drawn through only once and was as soft and as pliable as possible. A hardness of 4 meant that the wire had been drawn through five or more times and the wire was as stiff and as hard as possible. Today, the designations 0, 1, 2, 3, and 4 no longer correlate to the number of times that the wire has been drawn through a drawplate because now the hardness of a wire can be changed by heat-treating it. Most jewelry wire that is sold now is designated dead soft, half-hard, or hard, where dead soft is wire that is manufactured with a hardness of 0, half-hard is wire manufactured with a hardness of 2, and fully hardened wire is wire with a hardness of 4.
Dead soft wire is extremely soft and pliable. It can be easily bent and is excellent for making rounded shapes such as spirals. It is also excellent for wrapping wire around beads to make them look as though they are encased. The disadvantage of using soft wire is that the finished piece can be bent out of shape if not properly handled.
Half-hard wire is slightly stiffer than dead soft wire. Half-hard wire is excellent for making tight, angular bends, for making loops in wire, and for wrapping wire around itself. However, it is not very useful for making spirals. Finished pieces made with half-hard wire are usually more permanent than pieces made with soft wire.
Hard wire is very stiff and tends to spring back after being bent. This can make it harder to work with when using a jig. Hard wire will not make a spiral. The advantage to hard wire is that components made out of hard wire, while difficult to make, are very permanent.
As in many things, no single wire is perfect for all applications. Soft wire is easy to bend and shape, but the finished product may be bent out of shape if squeezed. Hard wire is difficult to bend but makes permanent shapes. Half-hard wire is a compromise between the two. The ideal wire will be easy to bend until in its final shape but then very stiff, but since this wire does not yet exist, when making wire wrapped jewelry, the artist or crafts person may harden the wire as one step in making the jewelry. Hardening the wire can be accomplished by hammering or by manipulating the wire in a process called work hardening.