Skip or John (withoutthe “c”) or anyone: I’m following, as best I
can, being about to cast my first silver in a perf machine, your
threads on casting. Sounds like a great convenience to not have
to wait for the investment to cure overnight.
The standard gold/silver casting investments actually seem to
work better when not cured as long as overnight. You don’t
actually want it to dry out, as then heat conduction into the
mold during early parts of burnout is poorer. With moist
investment, the steam generated helps force melting wax out of
the investment, rather than soaking into it, leading to less
carbonization in the investment at later stages of burnout.
Satin Cast, for example, on their instructions that come with the
investment, suggests letting the flask sit for an hour after
gloss off, minumum, before burnout. After the stuff has set
that long, you don’t get significant increases in strenght with
longer setting times, so why wait?
My question is,
can I use the platinum investments to do silver/gold
You can, of course, use platinumg investments to cast silver and
gold, but why? They are considerably more expensive, and don’t
necessarily work any better. If you’re actually burning them out
at the higher temps they are designed for, in fact, they’ll end
up less gas permiable (porous), and more likely to create fill
problems due to back pressure with a centrifuge or lack of
proper evacuation with a vacuum cast setup.
and do I
have to use a vacuum whipper or some such, or can I just use my
old portable kitchen mixer on this kind of investment and proceed
as I would for other types of investment (R&R, etc.), that is,
mix, vacuum, pour into flasks, vac again, wait for gloss over,
scrape, put directly in the kiln?
Use whatever you’ve got available. A vacuum investing machine
or mixer will work better and more easily, as it doesn’t add air
to the mix, so you’ll have an easier time getting it deaired
after mixing. But if you don’t happen to have such a thing, do
it with what you’ve got. There are an awful lot of castings,
even with platinum investments, that were simply mixed with a
kitchenaid machine, before the introduction to the jewelry
industry of the vacuum mixers. And more than a few commercial
jewelers who are doing platinum casting on a smaller scale simply
cannot justify the costs of the commercial investment mixers. I
know one guy who’s just mixing the stuff with a hand spatula
before vacuuming, and getting fine results. But he’s also got a
pretty strong wrist 
Peter Rowe