Where an original model of an article is to be made prior to making working mould,
it is desirable thatthe setting time should be extended so that more time is available for modelling: t can be done making suitable retarder by additions to the plaster. in considering the dimensions of these master models, it is necessary to take into account the expansion of the plaster in setting (which can also be adjusted by chemical and the drying and firing addition). contractions of the body to be used.