A ring mandrel is a long tapered metal shaft that is marked with ring sizes. A ring can be placed on a mandrel to identify the ring's size and a ring's size can be altered by using the mandrel as a metalsmithing tool. When a ring is up to 1/2 size too small and the ring does not have tension-mounted stones, a jeweller will often make a ring slightly larger by stretching the ring on a mandrel.
Anneal your ring by heating the metal to between 1,37.7 and 648 degrees C. Set the ring on a fire brick and use a torch to bring the ring up to temperature. Pick up the ring with copper tongs and immerse the ring in a dousing pot or pickle solution. Annealing softens and rearranges the crystals in the metal, allowing the metal to be worked.
Lightly oil a steel ring mandrel. Slide the ring onto the mandrel until it is level and snug. Position and clamp the mandrel in a vice so that the taper of the mandrel is pointing upward, and the ring is level.
Tap the ring down using a mallet or block and hammer. (A block is a small block of wood that is often contoured to fit the curve of the mandrel.) Make small taps all the way around the circumference of the ring so that it is stretched evenly. Go around the circumference twice. Remove the ring, turn it over and position it snug and level again. The ring must be turned because a mandrel is tapered. To stretch the ring evenly, the taper must be countered by tapping against the original angle of the taper.
Tap until the ring aligns with the correct new size on the mandrel. No ring should be stretched more than 1/2 a ring size as the ring is more likely to break. Do not use the stretching process for rings with tension-mounted stones.
Some gold rings are soft enough that annealing may not be necessary. However, if a ring is work hardened, it is more likely to break during mandrel work so it is usually a good policy to anneal the ring.