Joint construction is the family of techniques used in the bench trade to build hinges, articulated links and other movable connections in jewellery. A correctly executed joint allows a bracelet, brooch pin, locket or watch case to flex, open or close repeatedly without distortion, while remaining strong enough to take the stresses of daily wear.

Anatomy of a hinge

The simplest jewellery hinge is the knuckle hinge. Two or more short tubes, called knuckles, are soldered alternately to the two parts to be joined. A wire pin, the rivet, is passed through all the knuckles to lock them together while still allowing rotation. The number of knuckles is almost always odd, with a centre knuckle on one side and two flanking knuckles on the other, because an odd count distributes load symmetrically and resists side-play. The knuckle wall thickness, the fit between pin and knuckle bore, and the squareness of the end faces all determine how smoothly the joint moves and how long it lasts.

Common joints in the trade

• Brooch pin joint: a small two-knuckle hinge that holds the steel pin in tension against a catch.
• Locket hinge: usually a five-knuckle or seven-knuckle assembly running the full length of one edge, sometimes hidden under a bezel.
• Bracelet link joint: paired tubes brazed into the body of each link and crossed by a long pin or screw.
• Concealed box hinge: a hinge built inside a box clasp so that no knuckles show on the outside.
• Watch case lug joint: a precision hinge holding the strap pin, machined to tolerances closer to those of horology than of conventional jewellery.

Materials and execution

The traditional bench rule is to make the knuckles and the pin from compatible alloys with similar wear characteristics. Eighteen-carat yellow gold knuckles with a hard yellow gold or platinum pin are typical for fine work. The knuckles are filed flat and square on a joint file, then cleaned and fluxed before soldering with a hard solder so that softer assembly steps further down the bench order do not weaken the joint. After soldering the bore is reamed to a uniform diameter, the pin is cut to length, and one or both ends are riveted, threaded or peined to retain the pin.

Failure modes

Most hinge failures seen in repair work fall into three categories: knuckles that have split along the seam because the tube was rolled rather than seamlessly drawn, pins that have worn the bore oval because of mismatched alloys, and joints in which solder has flowed into the bore during manufacture and seized the action. Good joint construction therefore depends as much on material selection and bench order as on geometry.