Porosity is a casters’ mistress-- you have to treat her nicely, but
make sure she keeps her place. Oooh, did I say that (very un PC)!
The most effective means I know of to control porosity are to make
sure your sprues are the right size and placed correctly, to use
good metal, and to use your torch properly, avoiding prolonged or
over- heating and over-oygenating your flame, regardless of whether
you use a centrifuge or vacuum.
From what you describe, sounds more like flame/temp or metals issues
since you say the porosity is evenly distributed. Shrinkage porosity
will occur around sprues or in thick areas that are next to thinner
I know this will probably stir some people up, but oxy-acetylene is
a very hot and rather dirty flame for precious metals. Preferred for
the melt you describe would be propane or natural gas with oxygen–
cleaner burning gases, plenty hot. Hydrogen and oxygen is also
excellent for some applications, very clean, hot, but is trickier to
use and can be more dangerous-- uberflammable.
Very important-- Be sure you adjust your flame to a reducing
atmosphere (more fuel than oxygen). Preheating your crucible is also
a good practice.
My second suggestion would be to get away from just remelting old
metals and ALWAYS add 50% fresh metal, i.e. from the refiner, or
that you have alloyed from fresh metals yourself. The problem with
making your own alloys is quality control-- I would rather pay the
extra two cents and be assured of predictable results. Alloys are
more sensitive than you realize-- remelting old buttons without
adding fresh just perpetuates the problems and allows error to
Imhop, you would be wise to exchange all your old trees and metal
for brand new metal and start all over. You might try one of the
special silver casting alloys with built-in oxidizers and flow
enhancers-- most refiners have their own proprietary mixes, but we
find they are great.
Is your burnout cycle holding 1350 at the top end for at least one
hour? Stray gases from incomplete burnout can cause porosity.
Are you steam dewaxing? That can help the casting process greatly if
you’re using softer waxes (not hard carving waxes).
Doesn’t sound to me like your issues are related that much to
investing or to flask temp-- if you’re getting complete castings,
your flask temp is fine. Not familiar with your particular
investment, but if it’s not acting right with the manufacturers
recommended water/ powder ratios, you may have a bad batch-- this is
a common problem with investment. But that wouldn’t have much to do
with your porosity issues.