The other
method commonly used is called "investment" casting. The pattern
is set up in much the same way as for ceramic shell, except that more
venting is necessary due to the decreased porosity of the mold material,
which is made using gypsum plaster (plaster of Paris) as a binder for
sand, silica flour, or another refractory aggregate. There are many
proprietary investment mixtures available, or one may choose to mix
one's own. The dry ingredients are mixed with water and poured into
a container or "flask" surrounding the gated model, which
is either waxed down to a board or attached to a commercially available
rubber device which holds the pattern and flask. Jewelry flasks are
generally placed in a vacuum chamber while this mixure is still fluid,
where they are boiled at room temperature to remove air-bubbles clinging
to the models. It is helpful to de-air the investment mixture before
pouring it over the wax models, in order to reduce boiling-over when
vacuuming.