flask on the vacuum table with the vacuum on but not sealed
properly the item comes out bright silver but with very little
The metal is entering the flask more slowly, and being resisted by a
bit of remaining air pressure. So in essence, it doesn’t actually
fill all the way. That allows the metal surface to pull away,
slightly, from the mold wall, which allows it to remain bright, as
it’s not picking up the full matte texture of the investment. Casting
with the mold and/or metal hotter would improve the castings if this
were a consistent situation. But you then say…
If I put it on full vacuum and sealed the item comes out in fine
detail but with a coating.
That’s about how it SHOULD come out. The metal has fully “wetted” the
mold surface, and picked up all the detail, including the grain of
the investment. In doing so, it also slightly bonds to the thin
surface layer of the investment, which is your “coating”. An
ultrasonic cleaner after pickling will remove it. So would brushing,
or just about any other mechanical means of scrubbing. Part of the
reason you get as stubborn a film as you do, is the higher
temperature of your metal (fine silver) instead of sterling.
Sterling, both because it melts at a lower temperature to begin
with, but also because it solidifies more slowly and over a
temperature range, can be poured at a lower metal temperature. That
reduces the degree to which the investment adheres to the surface.
Also, even though you’re using fine silver, which may appear bright,
pickling after removing the casting from the investment will greatly
help in removing the rest of the investment. The acid weakens the
investment and it’s bond to the metal.