This article offers instructions on how to resize platinum rings whether for a size smaller or bigger, as well as, repair bench tips and tricks.
Rings are among the platinum items that jewelers most frequently receive for repair. To make a platinum ring one size smaller, remove 2.2mm of the shank. Scribe the distance onto the shank and remove the metal with a jeweler’s saw. Gently bend the shank together, closing the gap. Then cut through the seam one more time; this aligns both sides and guarantees a tight seam.
Roll a small piece of platinum until it is about 0.25 mm thick. If a rolling mill is not available, just hammer a small piece flat by using a bench block. Wedge that small, thin piece into the cut, allowing the tension of the shank to hold it in place. The piece should extend about 1 mm around the cut. Hold the ring with the third hand (away from the seam) and weld the thin piece of platinum into the shank. (Remember to use appropriate eye protection.) This process can also be done using 1700 seamless solder.
As soon as the fusing takes place, gradually move the torch around the shank, continuing with the procedure; this creates a complete metallurgical bond. (Be sure you weld the inside of the shank as well.) Once this is completed, you can clean the ring with a file.
I do not recommend using lower temperature solder for sizing rings. As these solders are a mixture of palladium and silver and contain no platinum. Since the solders are softer, they will polish out of the seam and leave a visible indentation. They may also oxidize, leaving a dark line in the sizing area.
Repeat the above technique to enlarge a ring one size, except add 2.2 mm of sizing stock. Again, a tight fit is needed; I prefer to weld the piece in place in two operations. Enlarging or reducing a ring one size is actually easier with platinum: Heat does not travel as quickly as it does with gold, and many welds can be done near stones (with care). Be sure to hold the ring in place with tungsten tweezers, since steel tweezers may leave a dark spot that could become permanent upon reheating.