Craft shops and places such as E.bay (Direct link on links page to good stockist) are able to supply Latex as a pre-vulcanised emulsion that will air dry and can be used to make moulds in which wax, plaster or resin parts can be cast. Its always best to buy a quality product where possible.
The advantage of latex moulds is that they are very elastic and can therefore be used to cast parts with significant undercuts. This elasticity can also be a down side with large blanks, as it can result in the mould deforming under the weight of the casting material however this can be overcome with the use of additional support casing as described later.
Making a Master
The first step is of course to obtain a master from which the mould will be created. There are a range of suitable materials and the fact that the latex dries in air and does not generate heat while setting is the reason for this flexibility. The porosity of the master is a factor that will result in one of two methods being used to produce the mould. These are the 'dipping' and the 'paint on' method and will be described later.
For sculpted pieces, plaster is probably the ideal medium as its porosity draws moisture from the latex allowing the dipping method to be employed for making the mould. Clay is another possibility for dipping but should be allowed to dry out or be fired as wet clay will prevent the latex from drying. Plasticene can also be used if the paint on method is employed however the interaction between plasticene and latex will result in a mould with a reduced working life.
Non-porous materials like wax and polyester resin can be used as masters if the paint on method is employed.
The Dipping Method
This needs to be done with a porous master which is simply dipped into the liquid latex. The porosity draws moisture from the latex causing it to thicken on the surface. Unfortunately, as this happens, air bubbles are formed in the latex. To overcome this, remove the master from the latex after a few seconds and use an old brush the stipple the surface and thus burst the bubbles. As this is done the latex will turn into a paste which will prevent further air from escaping. The master can now go back into the liquid latex for 15-20 minutes by which time a sufficiently thick coating should have formed.
Remove the master from the liquid latex and leave it to dry for a few hours before removing the mould as described later.
The Paint On Method
The dipping method will not work with non-porous masters so instead the latex is painted onto its surface with an old brush. An additional problem is that the thin liquid latex will tend to run off the non-porous master and several coats will need to be applied in order to build up the required thickness with the latex being left to dry a little between coats. you can use thickener as described below to help this process along. Plus always make sure that you clean your brushes in between coats. This can be done by whipping off any excess then use a small scraping tool of some sort to tease the latex from the bristles. The quicker you do it and dont allow it to dry in the better it will come out.
An 'optional extra' is latex thickener which can be added to the latex. Addition of thickener at the correct rate (as specified on the bottle) should not affect the life of the mould however excessive use can result in a rigid and brittle mould. This also can be purchased from most good craft suppliers.
Removing the master from the mould
Before removing the mould you should apply talcum powder or washing up liquid to the outside of the mould to prevent the latex from sticking to itself. The mould can then be washed, dried and again dusted with talcum powder. The powder will prevent the mould from becoming sticky and deterirating during storage.