How iron is made
What is Iron?
Iron is a metallic element (periodic table symbol: Fe) with a melting point of 1150 degrees celsius upwards.
It is the fourth most widely distributed element on the earth’s crust but is found as iron ore rather than as a useable metal. Iron ore comes in a variety of forms and looks like rock. It is a mixture of iron, oxygen and other elements, mixed in with sands and clays.
Image, right: ore being mined from bell pits at Wilsontown.
To make a useable metal, an ironworks has to get rid of the unwanted components of this mixture and keep as much of the iron as possible, producing a purer metal.
The iron-making process develops
Iron-making reached Britain from Europe and the Middle East around 450 BC. At this time the process only needed iron ore, charcoal and clay. Water power was used to power bellows and hammers from the 13th century, and the blast furnace was introduced in the 15th century in Belgium.
Charcoal was used where there was timber in ready supply but for the areas without lots of woodland, coal was a possible fuel for the blast furnaces. Coal usually contains sulphur, and this means that iron made with coal as the fuel will also contain sulphur. This is fine for cast iron but no use for wrought iron.
Coke is a purer form of carbon and has no sulphur. It's made by part-burning coal. Coke was used in blast furnaces for the first time in 1708, and Wilsontown Ironworks used coke as the fuel for its blast furnaces from the very beginning.
The recipe for making iron
The men working in the charging house at the blast furnaces would have been perhaps the most skilled workers at the Ironworks. They would not be local and maybe not even Scottish. It was their job to ensure that the correct amounts of the various ingredients were added at the right time to the blast furnaces - large ovens used to cook up the iron mixture.
The blast furnaces would ideally be working 24 hours a day, seven days a week. They were only blown out when repair and maintenance was required.
The ingredients to make useable iron were limestone, ironstone, coke and air. The coke is the fuel and the ironstone provides the iron ore. Ironstone is first roasted in calcining kilns, located next to the blast furnaces, to remove impurities. The air, or blast, was blown into the blast furnace by powerful steam engines. (They used cold air until 1828 when it was discovered at Wilsontown that hot blast produced better quality iron).
Air is required to keep the mixture burning inside the furnace. Limestone is added because it combines with the impurities in the iron ore, it acts as a flux. This mixture of limestone and impurities is called slag, this was not wanted and was removed from the blast furnaces by opening a tap which the slag would flow out from once it had floated to the top of the mixture.Diagram of the furnace at Wilsontown
Once the mixture had been heated for the required period and the slag tapped off, a tap would be opened at the bottom of the blast furnace for the molten iron to flow out from. It would be allowed to flow into prepared sand beds called pig beds where it would set. This is how they made pig iron, so called because the arrangement of the pig beds was said to resemble a sow with feeding piglets.
Inside a blast furnace
After pig iron was made at the blast furnaces some of it would then be taken to the refineries then the forge and the rolling mill.
There were two blast-refineries at Wilsontown where pig iron was first subjected to a blast of air to burn out some of the impurities before puddling. The refineries would have been close to where the iron emerged from the blast furnace and conveniently placed for iron to be taken across the culvert to the forge for puddling. After refining the metal was cooled in a cistern or water trough.
Detail from the forge at Wilsontown forge is a building where metal is heated and shaped.
This building was first constructed in 1790/1 following plans drawn up by John Rennie who worked as an Engineer for Boulton and Watt at the time. It stood across the culvert from the blast furnaces. At its heart stood an engine house with 2 steam engines in 1810, one single-power and one double-power. One of these engines was second hand, it had been bought from the King and Queen Foundry at Rotherhithe owned at time by Gardner, Manser & Co. It probably started working in the forge at Wilsontown in August 1791.
The forge originally contained helve hammers, chaferies and fineries for refining iron, but this method became unprofitable. So after John Wilson Snr & Sons took over they installed new equipment that included a steam engine, puddling furnaces and 2 new hammers, they possibly altered the building as well.
In 1810 the engines now provided the power to work 3 shingling hammers and one drawing hammer. Also in the forge at this time were 10 puddling furnaces, 2 balling furnaces and one chafery.
Iron would be brought to the forge from the refineries and reworked in the puddling furnaces to burn out (mainly) carbon. Following this the now crude malleable iron was reheated in a balling furnace or chafery before being hammered (shingled) to expel scale and finally drawn out into blooms for taking to the rolling mills. This might require a number of ‘heats’. For this reason refining, puddling, hammering and finally rolling needed to be close together.
The hammers were of two types – helve and tilt. For shingling these were of the helve type and worked by cams off the axle which lifted the nose (hammer end) directly with the fulcrum secured at the other end. This worked more slowly than the tilt hammer, but gave a heavier stroke. The tilt hammer was used for drawing and gave a more rapid but gentler stroke. In this type the fulcrum lay in the middle; by depressing the tail, the hammer end at the opposite end was lifted by the leverage exerted through the fulcrum.
Movement of iron within the forge was by wheelbarrows, one four-wheeled and three two-wheels carriages along cast-iron ‘barrow runs’. There were 14 water boxes (boshes) around the forge for cooling working tools.
The Rolling Mill
Details from the rolling mill at Wilsontown rolling mill is a factory for shaping metal by passing it between pairs of rolls.
The rolling mill lay beside the forge to the north. Before being taken into the rolling mill the blooms from the forge were weighed. The rolling mill appears to have been built around1802-4 and was described as open-sided with stone pillars at suitable intervals and arched between. Because of the large area covered the roof was supported internally on cast-iron pillars and probably had wrought-iron roof trusses that were tied into the walls by the wall-plates. Internal slender cast-iron pillars would have allowed the movement of materials within; like the forge the floor was laid with cast-iron plates to prevent damage when iron was move about.
The mill was powered by a single-acting steam engine with a massive 24-ft diameter flywheel. The mill engine was placed centrally so that the three pairs of merchant bar rolls could lie one side of the main drive with the three pairs devoted to plain (boiler plate, sheet and hoop) work on the other. On this side there was also a pair of bolt rolls with cutters worked off an eccentric. A lathe for turning the rolls back to true (rolls tended to wear unevenly) was also driven off the engine.
An Inventory from 1813 lists equipment such as 100 Tongs and Hooks which would have been used for drawing the iron between passes. Between each pass iron would have needed reheating and there were furnaces for this purpose. After rolling, finished iron was assembled on 2 bundling benches which weighed over ½ ton each, but there was also a long wooden bench for the same purpose: possibly this was for boiler plates which might have scratched more easily.
What did they make at Wilsontown?
There is no customer list available for Wilsontown Ironworks but we know of one or two customers, e.g. Richard Crawshay, who bought bar iron. The Wilsons sold their iron through iron merchants who had large warehouses, like James Pillans in Leith. We know that iron from Wilsontown went to warehouses in Leith, Glasgow and London.
- Pig iron – sold to other Foundries
- Cast iron products such as shot (4-18 pounders) for cannons, axles, pipes
- Bar iron / Wrought iron / Merchant Bar
- Blooms – bought by other Ironworks / Foundries
- Boiler and other plates
- Hoop iron – eg for barrels
- Sheet iron
- Iron rods
- Ballast for ships
- Special items for local sale
- Nailrods - sold to an Ironmonger in Edinburgh