Hi, 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, using a large
ingot channel that reaches the rim of your cup (which is at the
bottom of your mold), 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 500 C (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