Short answers, pulled from the story.
Cast iron is a class of iron-carbon alloys with a carbon content of more than 2 percent and silicon content around 1 to 3 percent. Carbon ranges from 1.8 to 4 percent by weight, and iron alloys with lower carbon content are known as steel.
White cast iron has its carbon combined into the hard, brittle compound cementite, which lets cracks pass straight through. Grey cast iron has graphite flakes that deflect cracks, while ductile cast iron has spherical graphite nodules that stop a crack from progressing.
Cast iron was invented in China in the 8th century BC. The earliest cast-iron artifacts date to that time and were discovered by archaeologists in what is now Luhe County, Jiangsu, in China, during the Warring States period.
Cast iron is used in pipes, machines, and automotive parts such as cylinder heads, cylinder blocks, and gearbox cases. Historically it was used for cannons, cookware, bridges, columns in mill buildings, and textile mill frames.
Cast iron is strong in compression but weak in tension, so beam bridges that put the lower edge of the beam in tension failed. The Dee bridge collapsed in May 1847 killing five people, and the Tay Rail Bridge disaster of 1879 led thousands of cast-iron rail underbridges to be replaced by steel by 1900.
Cast iron is made from pig iron, the product of melting iron ore in a blast furnace. It can be poured directly from molten pig iron or made by re-melting pig iron, then melted in a cupola or, in modern applications, in electric induction or electric arc furnaces before casting.
The first cast-iron bridge was The Iron Bridge in Shropshire, England, built during the late 1770s by Abraham Darby III. It used arches so that all the material was placed in compression, where cast iron is very strong.