Fig. 18 shows two sections of a double puddling furnace, built double to save space, building material and heat. This furnace is built of common brick, and lined with refractory brick. It is held together by vertical iron braces spaced along the sides and connected across top and bottom by tie rods.

A particular feature of the puddling furnace is its hearth. Beside forming a basin for holding the molten contents of the furnace, the materials of the hearth have an essential chemical purpose in assisting the changes brought about by the process.

The foundation of the hearth is three cast-iron plates resting on masonry supports. Hollow cast-iron air-chambers, AA, rest on these plates and extend through the sides of the furnace, opening into the air. These strengthen the ends of the hearth and assist greatly in relieving the fire and flue bridges of the intense heat.

The hearth is made of furnace slag and iron ore. This slag is composed mainly of iron oxide and silica and is called cinder. The hearth is prepared by fusing large and small lumps of ore and cinder upon the plates to a depth of about 6 inches. The sides of the hearth basin are also formed of lumps of iron oxide, fused together by the heat of the furnace. Hoop iron is burned to an oxide over this mass to give it an even surface of pure oxide.

Fig. 18. - Double Puddling Furnace.

The materials of which the hearth and its sides are prepared are designated as the fettling of the furnace. They are gradually consumed as a part of the operation of the furnace and must be renewed occasionally.

The hearth materials are kept from melting during the operation of the furnace by the radiation of heat from the plates on which it is built.