I have been nursing the following thought for many years.
If vacuum and centrifugal casting equipment is not available would
the following work on large models: Before investing the model,
copper wires would be attached to the top side of the wax (side
opposite the sprue) that would extend above the top of the flask.
Invest as usual. When the investment sets the copper wires are
pulled out of the investment. The resultant holes would act as air
vents. The investment mold would now be similar to tufa stone
molds. I tried this technique several years ago on a small wax model.
I poured the metal without the use of vacuum or centrifugal
equipment. The casting was perfect except for a very short thin
wire protruding from the model. Just a thought. Lee Epperson