Hi Diane,

In Alan Revere’s book Professional Goldsmithing on page 204 there is

a formula for just this very thing. It is called “Calculating the

diameter of a disk for a hemisphere” and the formula is:

(outside diameter-wall thickness) x 3.14 x 0.9

The outside diameter is the diameter of the sphere that you need to

make and you will of course need 2 discs. I was wondering why

multiply the whole thing by .9 so I emailed Alan to ask him and he

told me that the disc will stretch about 10% from the dapping and so

he is taking that much off in the beginning. I did try and it worked

for me so I am sure that it will work for you as well.

Good luck,

Lona