the problem with air bubbles that tend to adhere to the wax surface
can be resolved by first “annealing” the wax by putting it for a few
hours in slightly warm water. Afterwards brush a light soapwater
onto it, there exist also special (expensive) liquids for this
purpose. As for the investment: do not mix with distilled water,
because some brands do not react as they should. However, some tap
water sources are sometimes not suitable too, but if the investment
gets hard in a normal way, there should not be a problem.
In my experience, vacuum casting is not my choice for very large
pieces. I have much better results by casting as the ancients did:
just by gravity. You have to have sufficient vents at the right
places, but this works just perfectly. The temperature of the mold
should be as low as possible. For casting pieces this large, I bake
the molds as usual first to a 720 C, but let them cool down to 300
C. This can take many hours before the inside of the mold also
reaches this temperature. The temperature of the silver can stay
normal, lets say about 50-100 C above melting point. Heating too
high causes to much attraction of oxygen and since you have a large
amount of silver, it stays quite long on this temperature, so your
casting can be carried out relatively calm. Just before casting,
putting in a small piece of zinc helps to eliminate excessive oxygen
from the cast. Melting below argon gas is also an option, but
perhaps not available.
Cracking of the core is the most difficult problem but can be
resolved by making a stainless steel reinforcement structure in the
core, or by putting fine silver (alloyed silver oxidizes because of
the copper content) through the wax model en thus holding the core.
I woudl recommend at least 1,5 mm diameter for each pin and would
put at least 10 of them in place to hold. A different solution can be
of casting upside down, the core stands on itself and you will have
much less problems of falling down pieces from cracking cores.
Also, the warming up of the molds is very important. This has to be
done as slowly as possible, certainly up to 500B0C (higher can go
quicker) and without opening the furnace now and then to check
things. These are often facts which cause cracking.
Conservation Department of Metals
Royal Academy of Fine Arts, Antwerp