LATEX MOULDS FOR PLASTER OF PARIS FREE
Ensure the masters are clean, dry and free of grease or oil. Latex may be used to make moulds from masters (originals) made from various materials such as plaster, clay, glass and concrete. If in doubt test a small area to see if it releases satisfactorily. Avoid petroleum-based products, solvents and oils as they are not compatible with latex and will cause its degradation over time. Wax, glazed ceramics or glass do not need sealing or application of a release agent. You can seal with several coats of paste wax or by spraying with lacquer, or shellac, followed by a coat of release agent. This does not apply to all porous items particularly wood, which due to its fibrous nature can cause problems unless you seal the surface. Latex also peels off the model quite easily. Plaster is porous and does not need a sealer or release agent. Wherever possible make your models in plaster.
Latex has a better tear strength than all synthetic rubbers and is preferred by the concrete and plaster shop industry when used as a sock mould or one that peels off a casting, just like you pull a sock off your foot. This forms a very tough blanket mould with great elasticity. As a white brush-on liquid, after multiple applications it builds up to a mould thickness of 1 to 3mm that air vulcanizes to a light amber colour. Ideal for casting plaster or concrete but can also be used for limited casting of some resins. Your mould when completed will be quite thin and if above say 15cm in height will require a back-up mould to support its shape when casting. Latex cannot be poured or applied in thick sections, it will not cure. The colour changes to an amber rubber colour.
To speed things up you can use a fan driven radiator or hair dryer to dry the water in the latex. About an hour or so between coats would be normal.
A simple mould of a chess set piece about 15cm high would normally take about 8 to 10 coats, each coat applied, then let to dry in a warm area until the white colour disappears before applying the next coat. The number of coats can vary from 8 to 20, or more depending on the size and purpose of the mould. The strongest moulds are made by brushing on, or dipping the item in, thin layers of latex. It can be pulled over relatively large undercuts when releasing the model without tearing. Latex makes the thinnest, most elastic and strongest mould of any type of mould making rubber.
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