This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
Cast Iron for building purposes, possesses many advantages for strength, economy and adaptability to ornament and decoration. Unlike wrought iron and steel, it is not subject to rapid oxidation, and whatever tendency it may have in that direction, paint properly applied is a great preventive. In preparing iron for castings, the "foundry pig" is remelted in the "cupola," the bed of which is covered with shavings, then from 1600 to 1800 pounds of hard coal are added, then a "draft" consisting of from 1000 to 1100 pounds of pig iron and scrap; when several draughts have been fed, coke is added at regular intervals.
In charging iron, wood and coal all together, the iron when melted, being the heaviest, works through to the bottom, the purest iron being at the lowest point.
The metal for the finer castings, such as leaves for capitals, fascias, cornices, stair risers and ornamental stair strings, is poured first. Then metal for the heavy castings, such as columns, lintels, etc.
Castings for architectural work are mainly done in what is called green sand, which contains a small percentage of clay and oxide of iron and is more or less porous.
Cores are used for forming vacancies in castings where the pattern cannot be formed to draw from sand, and are made of white sand from the seashore, commonly called dry sand, being mixed with flour, sour beer, etc., forming a paste, and baked hard in the oven.
Cast iron is crushed by a force of 90,000 pounds per square inch, and will bear without permanent alteration 18,000 pounds per square inch, with an ultimate tensile strength of from 15,000 to 20,000 pounds; but it has an irregular elasticity, and castings may have initial strains through unequal cooling, or they may be thinner on one side than the other, or they may be weak through concealed holes, "cold shuts" or cinder. Therefore castings should be thoroughly tested with the hammer, and columns supporting heavy weights should be drilled (for inspection) on four sides if square, and two places if round.
The tenacity of cast iron being about one third (1/8) that of wrought iron, should not be subjected to more than one sixth (1/6) of the breaking strain, or say 3000 pounds per square inch.
The weight of cast iron per cubic foot averages 450 pounds.