Resistance to thickness-loss or creep is important to prevent the art object from shifting within its box in transit. Packing materials should also create insulation or dead air in order to maintain a constant relative humidity. Three categories of plastic materials are discussed: polystyrene, polyethylene and polyurethane. Among the polystyrene products are: Styrofoam, Pelaspan Pac, Alta-Pak, Olo-Flex, and Dylite slabs, the latter having been used for the cushioning of Michelangelo's Pieta. Special polyethylene packing materials are: Foam envelopes, sheets and tubing of flexible polyethylene netting, Tear Pack or bags in multiple rolls, and Dimple-Pack sheets for carpeting storage bins and wrapping fragile objects. Polyester urethane and polyether urethane foams involve different formulations and produce different characteristics and properties. Urethane foam may be cut into blocks or sheets. There is a sandwich-type construction in which the foam is laminated with plywood and there are Kudl-Pak boxes with fixed convoluted cushioning and removable polyurethane pads. Clean Towel has been found excellent for wrapping objects and Flo-Pak filling which consists of hollow cylindrical plasticcoated paper tubes. Tufflex is a wood product with backings of cotton net or Kraft paper and is offered in wide rolls or cut to size sheets.