Synthetic plastic resin in liquid form are made to solidify by the addition of a catalyst. Resin, reinforced with glass fiber, is commonly used as casting material, but may also be modeled when thickened with an inert filler such as powdered chalk. It can be cut and abraded when hard.Polyester is also a common name for the plastic polyethylene terephthalate. Its characteristics include transparency, colorlessness, and high tensile strength. In addition, it is useful in art conservation because it is very chemically inert. Commonly used in sheet or film form to make folders, encapsulations and book jackets. Its thickness is often measured in mils.