During the 18th century, even before the Iron Bridge at Coalbrookdale dramatically demonstrated the potential structural uses for iron, those in the iron trade had begun to explore new uses for their improved cast iron that now had the combined advantages of a material with great strength when under compression with now considerably more tensile strength as well. There seemed endless possibilities for its use as a substitute for other softer metals such as brass, copper and bronze as well as a substitute for wood in many applications.
For the first time, since the Iron Age succeeded the Bronze Age about 1200 BC, the military would not be the primary customer of the iron industry. The techniques perfected over centuries to fabricate weapons could now be used to produce domestic products of equal precision and consistency. Cannon must be cast with great skill as any fault or air pocket can cause it to explode in the gunner’s face when fired. The British Board of Ordnance was so stringent in its testing and specifications that the industry had learned to consistently produce weapons of very high quality castings. British guns and cannon were considered the best and most reliable in the world.
A foundry whose precision castings of cannon are able to reliably withstand enormous explosive pressure can also cast high pressure steam engine cylinders which must also safely and reliably contain enormous pressures.
The same kind of cross-over application in the iron industry occurred in ordinance. A critical consideration when making fire arms is to achieve precision in the bore hole which directly affects the accuracy of the fired projectile. The hole left after the casting could be drilled slightly larger with precise tolerance in the easily handled barrels of pistols and long guns than in the awkward very heavy cannon. John Wilkinson (1728-1808) discovered how to achieve the same precision while boring the much more awkward heavy cannon by rotating not the drill but the cannon itself around a fixed bit. This accuracy earned him many lucrative military contracts.
The early versions of the steam engine were not designed for high pressure, so any imprecise tolerances of the cylinder and the piston were not critical. James Watt (1736-1819), frustrated by this common fault in his efforts to build machines that could hold pressure, was directed to Wilkinson and his process for achieving very precise cannon bores (patent 1774). Together, the two men developed a process for boring steam engine cylinders and making parts with a tolerance of incredible accuracy and consistency for that time. Watt and his partner Matthew Boulton (1728-1809), who were designing and installing steam engines of all kinds for many industries in Britain and abroad, specified that all the parts must be supplied by Wilkinson until they set up their own foundry, the Soho Works, at Birmingham, in 1795.
Iron began to be substituted for wood in many different applications: from door lintels to gun carriages to decorative architectural elements to bedsteads. In the textile industry where the endemic dust and lint made fire a real hazard, “fire proof” factories were being built with iron posts and beams within brick or stone walls.
By the middle of the 18th century iron was being used to great advantage in several aspects of transportation. The wooden ways that had facilitated the movement of the heavy ore carts in the mines and in the coal and iron yards were now being replaced with iron rails which reduced the friction even more and lasted much longer. The ore carts were still pulled by ponies or even men and women in the mines and mills, but one far-sighted entrepreneur, Richard Trevithick (1723-1777), was experimenting with making a steam engine that would propel them along those iron rails. Soon the ore carts and their wheels were no longer of wood either, but of wrought, cast, and plate iron. These innovations look forward to the 19th century railroad era which lies outside both the time frame and subject matter of our story.
A large domestic and over-seas market developed toward the end of the 18th century for cast iron pipe to supply water and gas lines within the expanding cities and towns all over Europe. For a long time the British iron trade had this market to themselves due to the high reputation of their products.
Continuous improvements in the smelting and casting of iron meant that now delicate and precise iron gears and parts were being substituted for brass and bronze in all kinds of mechanical products from clocks to fine machine tools. Iron parts were cheaper, wore much better, and were just as precise. As the textile and other industries became more mechanized, their machinery and its parts were made of iron for the same reason.
As a structural material for building the boxes that carried the water and the barge boats in Thomas Telford’s new canal aqueducts to all manner of new kinds of bridge – arched, truss, cantilevered, suspension, tubular – iron was now accepted as the strongest, most economical and practical material. It was the material of choice in many structural applications for the next fifty years.