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 the fuel coming into direct contact with the iron, 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.
The hearth is where the iron is charged, melted and puddled. The hearth's shape is usually elliptical;  in length and  wide. If the furnace is designed to puddle white iron then the hearth depth is never more than . If the furnace is designed to boil gray iron then the average hearth depth is . Due to the great heat required to melt the charge the grate had to be cooled, lest it melt with the charge. This was done by running a constant flow 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  and is loaded with  of coal. If anthracite coal is used then the grate is  and is loaded with  of coal.
A double puddling furnace is similar to a 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 is also more economical and fuel efficient compared to a single furnace.