There are some things you should know before adding to the sculpture. Plaster will not adhere well to plaster if it is too dry. The best method of adding plaster to plaster is to firstly cut grooves into the dry plaster. This helps the adhesion of the next layer of wet plaster. Secondly, it is a good idea to wet the surface dried plaster, as this will also create adhesion.
Some sculptors also apply cold glue or white glue to the surface of the dry sculpture.
Ideally you should not wait for the plaster to be completely dry before finishing this process. Once you have the rough shape of the head, you can begin carving the final form. Carving into plaster is extremely easy when it is still slightly wet. The plaster becomes harder within a few hours and, depending on the consistency of your original mixture, can become almost rock hard over a period of days and weeks. The ideal time is to begin carving is about three hours after the plaster has begun to set, i.e. get hard.