The classic variety is epoxy resin, manufactured through polymerization-polyaddition or polycondensation reactions, used as a thermoset polymer for adhesives and composites. Epoxy resin is two times stronger than concrete, seamless and waterproof. Since 2000, epoxy and polyurethane resins are used in interiors as well. Synthetic casting "resin" for embedding display objects in Plexiglas/Lucite (PMMA) is simply methyl methacrylate liquid, into which a polymerization catalyst is added and mixed, causing it to "set" (polymerize). The polymerization creates a block of PMMA plastic ("acrylic glass") which holds the display object in a transparent block. These resins are also used in the artistic field (generally polyester resins), for example in the production of plastic figurines and statues as an alternative (lighter and less toxic) to metallic lead.