The grade of iron which a furnace is producing is governed within certain limits by the chemical makeup of ore, fuel, and flux, but the composition of these elements of the charge is not always uniform, nor are heat conditions in the furnace always the same. These variations give rise to iron of various compositions, within certain limits, and when iron is cast into pigs, a sample from each heat is analyzed chemically, to determine the per cent of each of the five ingredients named, and the iron is graded from this analysis.

In the old way of casting iron into pigs in the sand bed, the cast had to be broken up to be removed. This gave opportunity to inspect the fracture and judge approximately the quality of the iron from its carbon. These fractures showed either grey, mottled, or white color, in the order of the uncombined carbon which the iron contained, and by these colors the iron could be graded for its suitability for different uses. The grey iron is soft and used mostly in the foundry, as it makes excellent castings and can be easily machined. The white iron, on the other extreme, is hard and brittle, difficult to cut, and is best adapted for steel or wrought iron making.

Various grades of pig iron are named from the locality or company which produces them, particularly in England.

Buyers of pig iron now purchase their iron according to its chemical composition, and not according to its trade name, nor according to inspection of fracture. The American Society for Testing Materials has recommended that in each car load of pig iron, one pig in each four tons shall be selected for sampling and analysis.

II. The Classification, Ingredients and Properties of Iron and Steel.