Please note: I wrote the bulk of this article some time ago for another site that I used to own. It's not really up to TG standards as it has no images to illustrate it. This is an issue that I hope to rectify as and when I have time to take suitable photographs but in the meantime I offer it here in a text only format:
In his article on Latex Moulds, Andrew Nelson showed us how to create a simple mould using latex. While this will work well for casting small items, when casting large items, the moulds will tend to be distorted by the weight of the casting material and an external support should be made.
It has been suggested that this should be done before the removal of the master from the mould however the author prefers to remove and check the quality of the mould before proceeding. The master must of course be reinserted into the mould around which the support is to be made.
The authors preferred method is to use plaster reinforced with jute scrim although glassfibre can also be employed. There is no need to use a release agent with these materials and the latex. However, an obvious requirement of this support is that it should be possible to remove it and it will therefore need to be made in two or more pieces.
Begin by deciding how many pieces the support should split into and where it is appropriate for the splits to occur. For simplicity, we will assume that a 2 piece support case is appropriate. Modelling clay or plasticine can now be rolled out and cut into strips before being applied as shuttering along the line where the support case it to split. Plaster and scrim is now applied to one half of the mould op to the clay wall and left to dry.
When dry, the clay wall is removed. Several indentations should be made in the newly exposed edge to enable the pieces of the finished support case to be matched up easily. Obviously these indentations should not be so deep or so angular that they will cause problems in separating the pieces and the edge of a small coin can be usefully employed to make appropriate dome shaped indentations.
Although it was not necessary to use a release agent between the latex and the plaster it is essential that it be used between the two halves of plaster. For this purpose the edge of casing can be painted with varnish or shellac to make it non-porous. Alternatively a coating of vaseline or washing up liquid can be applied.
The second half of the casing can now be created with more plaster and scrim.
When the whole of the casing is dry it can be carefully removed and the master removed from the inner latex mould.
Note that when casting, it is usually appropriate to put some of the casting material into the latex mould and tease it in the usual way (to release bubbles from the casting material), BEFORE it is encased in the support structure. The latex mould is then encased in the support casting with string or sticky tape bound around the outside to hold the casing together while the casting process is completed.