I have been experimenting with gravity casing into carved gypsum
plaster molds. This is part of an ongoing research project into how
certain medieval fine metalwork may have been done. I can make my
molds work in a number of ways using modern materials and techniques
that would not have been available in the 7th century, so for the
sake of demonstration I am trying to learn how to make this method
work using just the most basic direct methods.
This is not lost wax I am talking about, but directly carved plaster.
Investment, although it is gypsum plaster based, doesn’t work very
well for carving. There is plenty of archaeological evidence for clay
molds during this period, but clay does not carve well enough to
accomplish certain techniques. Plaster will make exactly the mold of
the correct shape and level of detail. The problem is getting it to
fill with metal.
It takes a very long time to drive off the water from the molds in
order to get them dry enough that they do not release steam when
silver or bronze is poured. Baked hotter than 500 degrees F (260
C)and the material cracks. A 3/4 inch thick mold takes more than 10
hours in the oven to dry. Cast in a centrifuge it casts just fine,
but gravity cast it seems that the material just is not that
permeable to let the gases out and give a good fill.
It appears that there is an entire group of modern plasters
available for making cope and drag sorts of molds, so somebody must
be actually doing this. Additives like talc or pumice increase
permeability but ruin the carvability of the material. I am thinking
I should try thinner molds and maybe more permeable materials for the
backup plate, but I would appreciate hearing advise from anyone who
has experience with casting into plaster molds.