Kathy, one thing that might be causing your problem is an excessive
amount of borax flux. You might try cutting back on in. Too much
flux will leave balls of borax between the metal and the investment,
causing holes and many other problems.
Another is related to using “old” silver or gold. Never include
soldered metal in your castings. The solder will bubble and cause
pits. You can cut out the solder joints from your scrap and still
use the scrap for casting grain. Just make sure there is no solder
left on the metal you are going to cast.
To find the solder on your scrap, use an oxidizing flame and heat
the metal, using no flux or dip, until it gets to the blue color.
The solder joints should jump out at this point and be very easy to
see. Just make sure that you cut them out completely before using
the metal for casting.
Don at Campbell Gemstones