If you are making molds with plaster or casting larger pieces, then you should be embedding scrim for extra strength.
 
Plasterers Jute Hessian Scrim for use in the construction of ornate Plasterware. This adds strength to the fibrous plaster and prevents snapping and cracking out of the face. Primarily used in cornice manufacture and laid in between the first and second coat of plaster, the Jute also folds over and tucks in behind the lath to provide addition strength and structure.