Fig. 233 shows a hearth or furnace, partly in cross section, for heating the edge of a plate for flanging. It is a brick-walled basin re-inforced around the sides with iron plates and covered with perforated plate sections as shown.

The furnace is operated as follows: Supposing a curved edge of a large plate is to be flanged, draw a chalk line BC about 5 feet long, representing the curve of the flange. The perforations along this line and those within 3 inches on each side of it are left open as shown, but the remainder, over the entire surface of the plates, are stopped by dropping boiler rivets into them. Finely broken soft coal free from sulphur and clinker is dampened and packed in a layer about 8 or 10 inches deep over the rivet-covered perforations, leaving the open perforations about BC uncovered. A fire of high-grade soft coal or preferably coke, is built along BC over the open perforations, and the air blast is turned into the hollow space beneath the plates. The air is free to escape only through the fire along BC, hence the fire is easily confined to this space. When a plate edge is placed over this strip of fire, it is covered by several blocks of wood and old pieces of sheet iron which confine the heat and facilitate heating the plate.

When a plate is flanged along two edges meeting at a corner, the corner flanging must be done at nearly a welding heat. The corner is rounded off before flanging to remove the excess of metal.