Solid surface is a man-made material usually composed of a combination of alumina trihydrate (ATH), acrylic or polyester resins and pigments. Solid surface is a non-porous low-maintenance material used for surfaces such as countertops. It can mimic the appearance of granite, marble, stone, and other naturally occurring materials, and can be joined nearly invisibly by a trained craftsman. Typically manufactured in sheet form for fabrication into finished countertops, solid surface can also be cast into a variety of shapes, including sinks, shower pans and bathtubs. Sheet goods can also be heated and bent into three-dimensional shapes using a process called thermoforming, which adds to the versatility of the product. Color and design flexibility are key factors when choosing engineered composites over natural stone.