Box Iron Slugs.
By the 19th century a type of iron had been developed called the box iron. The iron was made of iron and normally had a wooden handle. The box iron was hollow on the inside. The box iron was heated by a piece of hot iron known as a slug. The slug was heated up on the kitchen range and then placed inside the hollow box iron.
It was custom to keep two or more of these slugs heating on the range so as the iron in the box iron cooled down it could be quickly replaced by another hot one.
The box irons shown here have a sliding door at the back. The hot slug was put into the iron through this door. Some box irons have a top that can be lifted off instead.