This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The property of welding possessed by wrought iron is due to its continuing soft and more or less pasty through a considerable range of temperature below its melting point. When at a white heat, it is so pasty that if 2 pieces be firmly pressed together and freed from oxide or other impurity they unite intimately and firmly. The flux used to remove the oxide is generally sand, sometimes salt.