Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. Cast Iron was first invented in China (see also: Du Shi), and poured into molds to make weapons and figurines. Historically, its earliest uses included cannon and shot. In England, the ironmasters of the Weald continued producing these until the 1760s, and this was the main function of the iron industry there after the Restoration, though probably only a minor part of the industry there earlier.

Cast iron pots were made at many English blast furnaces at that period. In 1707, Abraham Darby patented a method of making pots (and kettles) thinner and hence cheaper than his rivals could. This meant that his Coalbrookdale Furnaces became dominant as suppliers of pots, an activity in which they were joined in the 1720s and 1730s by a small number of other coke-fired blast furnaces.

The development of the steam engine by Thomas Newcomen provided a further market for cast iron, since this was considerably cheaper than the brass of which the engine cylinders were originally made. A great exponent of cast iron was John Wilkinson, who amongst other things cast the cylinders for many of James Watt's improved steam engines until the establishment of the Soho Foundry in 1795.

Cast iron bridges

The Eglinton Tournament Bridge, North Ayrshire, Scotland, built from cast iron.

The Iron Bridge over the River Severn at Coalbrookdale, England

The major use of cast iron for structural purposes began in the late 1770s when Abraham Darby III built the Iron Bridge, although short beams had been used prior to the bridge, such as in the blast furnaces at Coalbrookdale. This was followed by others, including Thomas Paine, who patented one; cast iron bridges became common as the Industrial Revolution gathered pace. Thomas Telford adopted the material for his bridge upstream at Buildwas, and then for a canal trough aqueduct at Longdon-on-Tern on the Shrewsbury Canal.

The Pontcysyllte Aqueduct, Llangollen, Wales, viewed from the ground

It was followed by the Chirk Aqueduct and the Pontcysyllte Aqueduct, both of which remain in use following recent restorations. Cast iron beam bridges were used widely by the early railways, such as the Water street bridge at the Manchester terminus of the Liverpool and Manchester Railway. However, problems arose when such a bridge collapsed shortly after opening in 1846. The Dee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often in wrought iron. The bridge had been under-designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. The centres of the beams were put into bending, with the lower edge in tension, where cast iron is very weak. The best way of using cast iron was by using arches, so that all the material is in compression, where it is very strong. Nevertheless, cast iron continued to be used for structural support, until the Tay Rail Bridge disaster of 1879 created a crisis of confidence in the material. Crucial lugs for holding tie bars and struts had been cast integral with the columns, and they failed during the early stages of the accident. In addition, the bolt holes were also cast and not drilled, so that all the tension from the tie bars was placed on a corner, rather than being spread over the length of the hole. The replacement bridge was built in wrought iron and steel. Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents.