With vacuum, you don’t have to worry about the ‘oh-oh’ factor of
things going wrong with whirling metal the way you do with a
centrifuge, so it’s always easier to go heavy on a vac pour. Worst
comes to worst, you just pour any extra off into an ingot mold. So
when in doubt on a vac, round up.
That said, the way I figure (sterling) pours is this: Total weight of
wax. (model, sprues, tree, everything) All in one go. Times 11.
(Yes, sterling’s 10.4, but 11 makes the math easy, and gives me a
safety margin.) So nevermind what each ring weighs individually.
Sprue them all up, then tree them, then weigh the tree. Then
multiply that by 11.
Then multiply that final figure by 1.2. (for a 20% margin for the
button.) Then melt and pour. (after burnout, of course.)
So a 1gm ring wax ends up calling for 11gm of silver, plus another
2.2gm for the button. (Total pour=13.2gm, ss) Personally, I normally
use a centrifuge, and normally don’t throw anything less than 1/2
ozt, (15gm) so for me, I’d be throwing 15gm of sterling. (especially
on a vac. They don’t hit as hard as a centrifuge, so smaller pours
have less power behind them. So make them a bit bigger.)
I get more persnickety about it with gold, because it matters a
lot more. Sterling isn’t so expensive that the extra few dollars
are worth cutting into my insurance margin. Recutting one wax, even
once, is much more expensive than a little bit of extra button that
I reclaim on the next casting.