175. Charcoal iron is simply that which has been made in a furnace where charcoal has been used as the fuel. It is generally used as a foundry iron for special purposes.
176. Foundry iron is used for making castings by being melted and then poured into molds. For this purpose an iron that will readily fill the mold without much shrinkage in cooling is desired. Other properties of foundry iron will depend upon the character of the castings desired.
177. Grading Iron.—Iron is graded and classified according to its different properties and qualities by two methods; namely, chemical analysis and examination of fracture.
Grading by analysis, although not universally used at present, is no doubt the more perfect method, because the foreign substances contained in the metal can be accurately determined. Grading by fracture is more generally used, although it cannot be considered absolutely perfect, but when done by one who has had years of experience and has trained his eye to discover the different granular constructions and luster of the fractured parts, it is very nearly correct; unless the properties are to be known to an absolute certainty, grading by fracture is sufficiently accurate for all practical purposes.
Fig. 149.—General View of a Smelting Plant, showing Blast Furnace Stoves and the Ore Pile.
Questions for Review
What is ore? Name four grades of iron ore. What is native iron? What is the difference between it and other iron ores? What class of ore contains the largest percentage of iron? Red hematite contains less, so why is it considered more valuable than the magnetite? What amount of iron do the limonite and ferrous carbonate ores contain? What determines the value of an ore? How is ore prepared for reduction? What are the results of these preparations? What are fluxes used for? What flux is most generally used? What effect does sulphur produce in wrought-iron? What is the effect of phosphorus in wrought-iron? How is air heated before it enters the blast furnace? What is the difference between sand-molded and machine-molded pig iron? What is the objection to machine-molded pig iron? Name the different classes of pig iron and state the use of each. How is iron graded?