In hollowing, the sheet metal is hammered into a recess in either a block of wood, cast iron, or lead. For general work it is most convenient to have a wooden hollowing block as shown in Fig. 229. Recesses can then be readily sunk into the ends to suit the shape of the work in hand. A bullet-faced hollowing hammer is used, the sheet being first wrinkled around the edge, and the courses following each other up to the centre of the plate. The plate should not be hammered too much at the middle whilst the edges are stiff, as this will tend to unduly stretch, and thus thin, the metal at that part.
"Hollowing" is usually a much quicker process than "raising," but it has a tendency to thin the sheet in the middle part more than the latter method. The stiffer metals, such as steel, iron, and zinc, are generally treated by the former process.