Cast iron has certain advantages and disadvantages as a material. It is easy to give it any desired form by molding. It resists oxidation (rust) better than either wrought iron or steel. Its compressive (crushing) strength is very high, but its tensile (stretching) strength is comparatively low. It cannot be riveted or welded by forging. It is brittle, breaking off without giving much warning, and stretching but little before giving way. It is liable to have hidden and small surface defects and air bubbles, which make its strength uncertain.

Another serious drawback in the use of cast iron is its liability to initial stresses from inequality in cooling after it is poured into the molds. That is, if one part of the casting is very thin and another very thick, the thin part cools first, and in cooling contracts; the thick part, cooling afterwards, causes stresses in the thin part, which may be sufficient to break it; or if not quite that strong the stress may be so great in the thin part that a small additional force will break it.

In the construction of machines and structures, no metal used by engineers varies so much in strength and soundness as cast iron; hence it is particularly important to know its physical properties. Cast iron may, if of inferior quality, have a tensile strength of only 5 tons per square inch, or even less. When this is the case, it has no value where strength is required; it may, however, be used for balance weights, foundation blocks, or for purposes where weight alone is of consequence. Some samples may have a strength as high as 19 tons, but the average strength is 7 tons. The strength of iron castings may be increased by the addition of vanadium. The high compressive strength of cast iron makes it desirable for use in columns and posts of buildings.