The large warts you describe are caused by investment flaking off
from the surface of the mold.
You might find those flakes of investment, which are free floating
in the mold before pouring the metal, imbedded some where in you
castings. If you file off the warts you might find the investment
flakes imbedded under the surface of the warts.
If you used the same steps for you bad castings as you used for all
the good ones I would think the problem is in the investment.
Your investment might have been contaminated by moisture in the
The investment to water ratio might not be to spec. It is easier
to mix to spec large amounts of investment that smaller amounts.
The measurement of water to investment becomes more critical when
mixing small amounts of investment. For instance, several ml of
water out of spec will have less impact on say 5 pounds of
investment then it would have on 1 pound of investment.
Small clumps of investment that were not mixed fully might have
moved against the wax in the flask. These clumps are weak because
they are not fully mixed. I pour my investment into the flasks
through a kitchen strainer. I mix with an electric mixer for 2
minutes and yet I still find small clumps of investment not fully
mixed in the strainer.
The flowing metal can hit thin sections of investment and break
away the surface.
Orchid members continually prove that lost wax casting is not a
science. For instance I pour silver into an 850 degree mold and gold
into a 1000 degree mold. Apparently the different casting
temperatures we use works for both of us. Metal temperatures could
compensate for the different mold temperatures.