Electroforming is a metal forming process that forms parts through electrodeposition or electroplating on a model, known in the industry as a mandrel. Conductive (metallic) mandrels are pre-treated chemically to allow subsequent separation of the finished electroform, non-conductive (glass, silicon, plastic) mandrels require the deposition of a conductive layer. Conductive layers can be deposited chemically, or using vacuum deposition techniques. The outer surface of the mandrel forms the inner surface of the form. A thick layer of electroplating is applied until the plate itself is strong enough to be self-supporting. The mandrel is most often separated intact or dissolved away after forming, but occasionally (as in the case in decorative electroforming) left in place. The surface of the finished part that was in intimate contact with the mandrel is rendered in fine detail with respect to the original, and is not subject to the shrinkage that would normally be experienced in a foundry cast metal object, or the tool marks of a milled part. The side of the part that was in contact with the plating solution is less well defined, and that loss of definition increases with thickness of plating. In extreme cases, where a thickness of several millimetres is required, there is preferential build-up of material on sharp outside edges and corners. This tendency can be reduced by a process known as periodic reverse, where the plating current is reversed for short periods during electroforming and the excess is preferentially etched away. The finished plated form can either be the finished part, or can be used in a subsequent process to produce a positive of the original mandrel shape.