PHYSICAL ELEMENTS IN CHILLING CASTINGS. 259
would to a great extent be a guide for their next "heat,'' providing the pig metal to be used was exactly the same. Tn melting iron in an '4 air furnace " there is a chance to change its composition from what a " chill test " might prove it, before the metal would be tapped or poured into a mould; but with cupola work such a practice is not permissible. Small cupolas may, in some cases, be used to test pig metal before it is used in regular cupola mixtures, but analyses are generally a cleaner and preferable plan. It is only where analyses cannot be obtained or relied on that testing pig metal in small cupolas, before being used in regular mixture, is a plan which it may, in some cases, be well to adopt.
The above treatment of this subject is not to be taken as decrying the plan of taking '' chill tests'' of mixture in any or all cases, as such course is advisable under all circumstances, since it enables a founder having experience to form a close estimate of what he has obtained in his castings and assists him to know whether a change in the chemical properties would be advisable for any following heats. Chilled work will always crystallize in planes at right angles to the chilling surface of the i-ron mould used for chilling the casting. A standard chill which the author has devised for testing the "chill' 'of iron can be seen in Chapter LXIX.
The factors most constant in testing the chill of an iron are heat and friction. Heat is the best factor for testing the durability of such castings as rolls, and friction those like car wheels. It is not to be taken for granted, as held by many, that * * white '' or chilled iron has no degree of hardness or that the depth of a chill determines the hardness, for this is not true. We may have two castings of exactly the same depth.... Shop scrap ...... 15 x ro x 1.50 i. So 22.50 iS.oo