Not all rings can be hammered up in size, and it’s never a good idea to hammer a ring up a large amount as hammering is a type of stretching and it will thin the metal of the shank. But for just sizing a ring up a half size, if the ring’s shank is heavy (thick) to begin with, hammering is a great method.
Slide the ring onto a steel mandrel and double check the current size. Then take the ring off, flip it over and check the size again. Both size measurements should be the same.
Before beginning to hammer the ring you need to clean it. Once you are sure the ring is clean, you can begin the repair. When stretching a ring, it is easier to do if you anneal the ring first, but you can also complete the repair without annealing. Put the ring on the mandrel and secure it to your bench to allow you the use of both your hands.
To hammer the ring up, you will need a goldsmithing hammer with a slightly domed face. You should use small even blows to planish, or hammer flat, the outside of the shank. Periodically take the ring off the mandrel and flip it so the other side is on the thickest part of the mandrel, this will ensure that you maintain evenness in your stretching.
While striking the shank, gently pull the ring up the mandrel. This will keep the ring round and under pressure allowing you to do a good job of stretching the ring. When the shank approaches the desired size, stop and check your work, and then continue until you reach the desired size.
Check both sides of the ring to ensure that they are both at the desired size. Then finish the ring by cleaning and polishing it.