Polymer-modified plaster (Jesmonite)

Advantages A good ‘alpha’ plaster can be mixed with acrylic polymer liquid in place of (or occasionally in addition to) water which makes casts much stronger and even ‘weatherproof’ for outside sculpture. The resulting mix can also be used in place of resin with glassfibre matting or other reinforcement to build or cast very durable shells. The mix generally has a longer working time than plaster/water (i.e. can be 20-30mins as opposed to 10-15mins) and it can enable finer, more detailed castings. ‘Jesmonite’ is one popular brand, usually sold as a system, but acrylic polymer liquid can also be bought on its own, i.e. from Tiranti, for use with any alpha plaster.

Not so good More expensive than using the plaster on it’s own (i.e. Jesmonite ‘kit’ comprising 2.5kg plaster plus 1kg liquid is c. £28.30 at 4D, the same price as 25kg Crystacal R plaster!). Mixing needs to be very thorough (power-assisted mixing recommended for large amounts). Mix much more prone to air bubbles (leave to stand a little) .. these can be a pain! – tip: used compressed air to blow on the first layer once poured to pop air bubbles.

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