A unitary structure formed of two or more sheets of glass between each of which is interposed a sheet of plastic, usually polyvinyl butyral. In usual manufacture, two clean and dry sheets of plate glass and a sheet of plastic are preliminarily assembled as a sandwich under slight pressure in order to produce a void-free bond. The laminate is then pressed at 75–225 lb/in.² (0.5–1.5 megapascals) under heat at 239–302°F (115–150°C) long enough to unite. For use in surface vehicles the finished laminated glass is approximately ¼ in. (6 mm) thick; for aircraft it is thicker.