Wrought iron, oftentimes called bar iron, is made from the rough pig iron. In the process of manufacture, it is first refined in the "puddling furnace." Here it is exposed to a very great heat, and is stirred about while a strong current of air plays over it. The intense heat converts the carbon matter remaining in the iron into carbon dioxide. As this escapes, all earthy impurities rise to the surface in the form of slag, and are allowed to run off. Gradually as the iron is purified it becomes pasty or tough, and when this toughening process begins the iron is called a bloom. This takes place even though the heat of the furnace be undiminished. After a while, the iron is withdrawn and while red hot is either beaten (worked) with a forge trip hammer, which drives out the remaining slag, or is subjected to rolling. This latter process compresses its particles and makes it more tenacious. The metal is rolled while hot into bars of inferior quality. These bars are cut into short lengths and piled crossways. They are later reheated and rehammered or rolled, after which they are known as merchant bars and in this form are used for common girder work, ladders, fire bars, etc. Any desired shape may be given to the bars by means of dies. The operations of cutting, piling, etc., may be repeated several times to give the desired strength and tenacity.