The operation of making watch hairsprings requires special skill. In making by hand, flat wire is fastened at one end to the arbor of a winder not unlike a mainspring winding tool and wound up quite tight, and kept flat by a brass guide on each side like a bobbin. When wound singly and released, the spring will open out a trifle only, and the finished spring is a " close-coiled "one. But when two or three wires are wound up one over the other, the results are more open in the coils. The best hairsprings are afterwards fire-hardened and tempered, but common ones are left soft. They are hardened by being heated to redness in a box specially made to exclude the air, and then plunged into oil or water. They are tempered by being heated on a metal plate until a slip of bright steel placed beside them turns to a full blue. They are then polished by means of rouge and oil on a peg or wood polisher (this is very delicate work), and afterwards "blued" by heat on a metal plate over a lamp.flame. These fire-hardened hairsprings are expensive, but are always used in the best watches.