Having prepared the hearth and brought the furnace to a good heat, a charge of about 1500 lbs. of pig iron is thrown in at the working door, and with it is charged a quantity of cinder or squeezer scale.

(1) The door is closed tightly, and the heat is so regulated that the iron and the cinder become pasty and melt down together. This requires about 30 minutes, and is called the melting-down stage.

(2) After the charge has been melted and the iron and cinder well mixed, the clearing stage follows. The puddler's helper uses an iron bar with a bent end to stir the whole charge thoroughly, working through the hole in the door. The stirring brings the impurities of the iron into contact with the oxides of the hearth and of the charge, and these, assisted by any oxygen coming from the air which enters through the fire box, oxidize the remaining silicon, manganese, and a further amount of the phosphorus. During this stage, the furnace is kept very hot. A slag is formed, containing the oxidized impurities and much iron oxide.

(3) The next step is the boil, from which this whole process gets its name of "pig boiling." This lasts about 30 minutes and the operation removes carbon and the remainder of the phosphorus. During this stage the chimney damper is lowered and the working door opened to reduce combustion and produce an oxidizing flame. The charge is stirred thoroughly and constantly with the hoe or rabble (or, to use the expression of puddling, is rabbled). This vigorous action brings the carbon of the metal in contact with the iron oxides of the hearth and the charge, causing carbon and oxygen to unite, forming CO, which bubbles violently from the surface of the molten charge. This bubbling causes the lighter slag to boil over the side of the basin and flow from the furnace, carrying with it many of the oxidized impurities. Sulphur is eliminated mostly as iron pyrites in the slag boil. As the carbon burns out, the charge becomes more and more quiet.

This removal of the carbon having lowered greatly the melting temperature of the iron, small masses of plastic metal begin to collect just as butter collects in the churn. The iron is said to "come to nature" in thus collecting.

(4) In the final stage, the puddler gathers these masses into balls of about 150 lbs. each, called blooms, or puddle balls. The furnace temperature is gradually raised, and the puddle balls are brought to a welding heat. The puddler presses them sufficiently to make them hold together and they are then removed from the furnace. After their removal the excess of slag is tapped out and the furnace is ready for a new charge.

Each heat requires about 2 or 2 1/2 hours.