Bronze Casting - Step 7
Making the Mold: Mother Mold, Complete

After the first half of the mother mold has set, you remove the shims. You now have half of the mold encased in plaster along the dividing line of the rubber mold. Then - again making sure all the surfaces are well vaselined up - you coat the second half of the rubber mold with plaster and allow it to set. Of course, you no longer need shims since the first half of the mold has a well defined dividing line. But you have to be careful to make sure you don't loose the shim line in the new plaster, although if you do there are ways to recover it.

When making a mother mold you need to give thought to holding the mother mold together. So in that sense the shape of the mold is important and truth to tell this is not the most sterling example of a well shaped mold. Better would have been to create a ridge along the dividing line suitable for attaching C-clamps. For larger molds, it is even common - and at times necessary - to drill holes through such a ridge and hold the halves together with bolts. That does require you use the good quality plaster.

Once the second half of the mother mold has set, you carefully - and we mean carefully - separate the two halves. For a mold of this size - not too large - you can gradually work a putty knife underneath the mold and remove it from the board (and hence the importance of having the model attached firmly but not too firmly to the surface). Then you eaaaasssssilllllly separate the two parts working from the underside of the mold, gradually loosening the rubber inner mold from the plaster and separating the plaster halves. Recommended is using thin wooden wedges to gently tap into the shim line. For this mold a small putty knife was used and gradually worked into the shim line. But if all goes well you will not break the mold, but the two parts will separate along the dividing line in such a manner that they can then be reassembled and held together with the rubber inner mold in place.

The mother mold here is larger than needed and so a bit heavier than convenient. So it is not only a mother mold, but relatively speaking, the mother of all mother molds.