Here's the disclaimer. I have been casting dozens of flasks weekly
since 1971, but am constantly challenged by the Orchid experts. My
Students in our casting class experience this many times. There are
a few reasons in my observations. I don't have all your details about
process, but here are a few thoughts.
Incomplete burnout. Peak temperature should be 1350 f. No grey color
in the investment upon quenching will show you this. Larger loads
need more time at hottest temperature. This assures the flask will
breathe when the molten metal goes in. The air must travel out.
Some foreign matter gets cast into the flask. If the metal looks
good in the melt (real shiny smooth & clean surface) cast as quickly
as possible. A pinch of borax flux is plenty on most melts. 20 mule
team borax works fine and is cheap. (Laundry detergent & almost pure
borax) Stir when molten to mix metal as well as skim the crud off the
Make sure any old investment is removed from a re-melt. Deox metals
work the best and cast a second and third time with little problems.
Make sure you have a decent spru button. Not enough metal will have
the tail end of the pour go into the flask. This can be a problem.
Sequence is- melt, flux, stir & cast as quickly as you can. The
metal when stirred should feel fluid. Vac casting or spin casting? If
vac cast pour the metal through the flame as it goes into the flask.
Vac flasks will cast better if there is vent at the inner flask edge
(spur wax works fine).
Use a neutral flame for melting. Not reducing or oxidizing. Propane
oxygen is the best fuel combination with a larger torch. Bigger than
the bench torch. Not the Little Torch.
Old investment? Bottom of the barrel is usually the worst casting
unless you mix the unused investment once in a while.
Be safe with proper gloves, goggles for melting and vent/mask for
any investment contact.
Mpls Comm & Tech College