Bode, I have made copies of ancient Greek and Roman coins using a method similar to what you describe. To make the mold we used a two part substance called RTV (Room Temperature Vulcanizing ) Silicone. These molds have a shelf life of approx. 25 years (depending on frequency of use). According to a conservator friend of mine, RTV is the least harmful fo the Silicones and should prove relatively harmless to the objects. Since the molds last for a long time, the originals only need to be in contact once. A real concern here is the fact that RTV will make scarely accurate molds. Some copies I have made have been mistaken for the real thing. While I am always quick to set the record straight, it is prudent to control access to and use of these copies. Hope this helps. Patricia McElyea Curator of Collections Arkansas State University Museum [log in to unmask]