Around the 1950s, it also became common to use polyester and epoxy for making casts. Later these materials became the “overwhelming choice (Turner, Biological Anthropology and Ethics, 94).” Not only are polyester and epoxy more rigid plastic resins, thus not as likely to penetrate the surface foramina, but they created highly detailed casts that were also highly stable. On the other hand, epoxy casts were very expensive and they produce a chemical bi-product and heat during the curing process. This chemical bi-product can permeate the silicon mold, destroy its elasticity and over time will accelerate the degradation process of the whole cast. Lastly, the silicon cast is susceptible to shrinkage since it cures by evaporation. According to Janet Monge and Alan Mann, it is not uncommon to have 10% or more shrinkage as well as some distortion. It is possible to prevent such dimensional distortion by applying a tempering agent such as fiberglass or talc (Turner, Biological Anthropology and Ethics, 91-94).