Puddling (metallurgy) - Puddling Furnace

The puddling furnace is a metalmaking technology used to create wrought iron or steel from the pig iron produced in a blast furnace. The furnace is constructed to pull the hot air over the iron without it coming into direct contact with the fuel, a system generally known as a reverberatory furnace or open hearth furnace. The major advantage of this system is keeping the impurities of the fuel separated from the charge.

There were two major types of puddling furnaces used in the United States. The first is the single puddling furnace, which is based on the same design used in England and, thus, the most common. The second kind is the double puddling furnace, which was most often found on the east of the Allegheny Mountains.

The general design of a single puddling furnace is as follows. The footprint of the furnace was 3.3–3.6 meters (11–12 feet) long, 1.5–2.1 m (4.9–6.9 ft) wide (depending on hearth size) and 1.5 m (4.9 ft) tall. The outer walls were 23 centimeters (9 inches) thick and made of typical brick and then covered by cast iron plates. Wrought iron square bars, called cross binders, are run through the roof of the furnace and bolted to the cast iron plates to keep the roof from collapsing. The chimney was 9–12 m (30–39 ft) tall and 40 cm (16 in) square. There would be a small work hole allowing access to the fire, and a work door allowing access to the hearth. The average work door was 55 cm (22 in) wide by 68 cm (27 in) tall, lined with firebricks on the inside and with a small square work hole for tools.

The hearth is where the iron is charged, melted and puddled. The hearth's shape is usually elliptical; 1.5–1.8 m (4.9–5.9 ft) in length and 1–1.2 m (3.3–3.9 ft) wide. If the furnace is designed to puddle white iron then the hearth depth is never more than 50 cm (20 in). If the furnace is designed to boil gray iron then the average hearth depth is 50–75 cm (20–30 in). Due to the great heat required to melt the charge the grate had to be cooled, else it would melt with the charge. This was done by running a constant charge of cool air on it, or by throwing water on the bottom of the grate.

The fireplace, where the fuel is burned, used a cast iron grate which varied in size depending on the fuel used. If bituminous coal is used then an average grate size is 60 cm × 90 cm (2.0 ft × 3.0 ft) and is loaded with 25–30 cm (9.8–12 in) of coal. If anthracite coal is used then the grate is 1.5 m × 1.2 m (4.9 ft × 3.9 ft) and is loaded with 50–75 cm (20–30 in) of coal.

A double puddling furnace is very similar to that of the single puddling furnace, with the major difference being there are two work doors allowing two puddlers to work the furnace at the same time. The biggest advantage of this setup is that it produces twice as much wrought iron. It's also more economical and fuel efficient as compared to a single furnace.