Basically, there are 2 different types of bronze mold positives - wax, and whatever else materials for sand casting - traditionally wood.
The wax ones are 'invested' in a plaster mix. i.e. covered in it with extra rods of wax for the 'runners and risers', then turned upside down in a kiln and baked. Where the wax was before, melts to create a hollow space into which the bronze will be poured. If the object is to be large and hollow, then iron nails are pushed through the was before the plaster core is poured in, and this is what spaces the core from the main outer body of the work.
I have to say that -technically - I ended up a better craftsman than any of the teachers with 'lost wax' bronze casting, so they often asked me to take on jobs for them. One day, our head, Ben Franklin, asked me to make a wax and bronze from a mold he had made that week,
A friend of his son had died of an overdose, and the father (a doctor) wanted to have a bronze death-mask adorning his mantle-piece... for some reason. Death masks never look good, no matter how good-looking the subject was in life.
When you take a cast for a death mask, you shield the hair, ears, neck, etc. with cloth, then you stuff cotton wool up the nostrils and smear a lot of Vaseline on the eyebrows, so that the wet plaster does not run where you don't want it to, or trap the eyebrows as it sets, and you can pull the hardened plaster away from the face of the corpse. The above photo is a mask from Napoleon's corpse.
When Ben gave me the plaster negative from which I made the was positive, I noticed that it was covered in blood, and asked him why. He said that he had (with the help of the mortuary assistant) done all the above and applied the plaster, then - after it had set - tried to remove it from the vaselined face of his friend's dead son, but it would not budge. The mortuary assistant held down the shoulders of the dead young man while Ben pulled for all his might on the edge of the plaster mask. What he did not know was that - during the post-mortem - the surgeons had cut around behind the ears of the head, in order to get into the skull from the back to preserve the face for the relatives, and the whole face started to come away from the skull with the plaster, ears and all.
The bronze turned out fine, you will be pleased to hear.
Enjoy your evening!