1. Capable of being shaped or formed, as by hammering or pressure: a malleable metal.
2. Easily controlled or influenced: “The British [rulers] … had favoured the brother who struck them as altogether more amiable, a more malleable, more temperate man” (Paul Scott).
a. Able to adjust to changing circumstances; adaptable: a malleable leader unafraid to compromise.
b. Capable of being changed or adjusted to meet particular or varied needs: the malleable rhythms of jazz.
In the 1700s and 1800s a transformation began in Britain that created the modern world: the Industrial Revolution.
The material of the age was iron: in bridges, in machines, in the factories that housed the machines, in railway locomotives and railway tracks. Iron was everywhere.
And yet it wasn’t a new material, after all Britain’s Iron Age had begun around 800BCE. What changed was that new processes were developed to mass produce iron in the face of enormous demand.
From the late 1700s until the invention of processes to mass-produce steel in the 1860s, malleable iron was the preferred material of structural engineers.
Malleable, or Wrought Iron had been made in relatively small quantities for centuries but in the 1784 an English Navy Agent named Henry Cort patented a new way to make it. Cort’s ‘Puddling Process’, which built upon earlier work by the ironmaster Peter Onions transformed engineering and enabled new forms of bridges and other structures to be made.
Cast Iron vs Malleable Iron
Cast iron is made in a blast furnace, from which molten iron is cast in moulds. The resulting iron has a high carbon content and crystaline structure that makes it very hard but also brittle, too brittle to work with a hammer or use in structures that need to flex.
From the late 1400s onwards malleable iron was made in a two-stage process which involved first making cast iron in a blast furnace and then refining it. At first the refining was done in a finery forge in which the pig iron was laid on a bed of burning charcoal and then beaten and worked to remove impurities.
The need for a new way of making malleable iron was driven by the need for larger quantities and also by the fact that the finery forge didn’t work well with cast iron made in the new coke-fired blast furnaces. This was because cast iron made with coke had a higher sulphur content than that made with charcoal and this made the iron brittle even when heated.
The Puddling Process
Puddling was the first successful process for making malleable iron from pig iron without using charcoal. Unlike in the Finery the iron was not in direct contact with the fuel of a puddling furnace, only with the hot gases from it. This helped reduce the amount of impurities in the finished iron. The basic chemical reaction was the same as in the finery but the new method was more effective.
Before it could be used the inside of the puddling furnace had to be gently heated and coated with iron oxides to protect it. This was called ‘fettling’.
Puddling involved a great deal of skill, as well as strength. As the historian Richard Hayman puts it in his book ‘Ironmaking’ (2005), puddling “was a technique, not technology… Whereas an engine’s motion was predictable and repeatable, the product of a puddling furnace depended upon a variety of factors, not least the skill and judgement of the workman.”
This was important, particularly in civil engineering where a consistent level of quality was needed. A malleable ironworks could build a reputation on the skill of its workforce, as when the fledgling firm of David Colville & Sons won the contract to produce malleable ironwork for the second Tay Bridge following the collapse of the first.
Puddlers were generally young men as the work required a lot of physical exertion. The production of a puddling furnace was essentially governed by how much weight a man could lift with a ball of puddled iron generally weighing about 5cwt (about 250kg!). Puddlers often had to retire and find other jobs due to injury. They frequently suffered eye problems from staring into the blazing furnace.
When the ball of puddled iron was judged ready, the puddler would lift it out and onto the anvil of a ‘shingling hammer’. This was a steam hammer that repeatedly bashed the mass of iron, forcing out small pieces of slag as sparks. The result was a lump of iron with fewer impurities and flat enough to put through a rolling mill.
The image below shows a shingling hammer at work at Coatbridge’s Waverley Works in 1920, the last place in the ‘Iron Burgh’ to use the puddling process.
Once the puddled iron was flat enough it would be put through a rolling mill to thin it down to bar iron of consistent thickness and with smoother sides. The glowing-hot iron was repeatedly passed through increasingly small gaps between mill rolls. The finished size would depend on the iron’s intended use. Again, the image below shows the Waverley Works in 1920.
Illustration of a rolling mill from a 1916 textbook on making iron and steel.
Puddling Arrives in Scotland
The late 1830s saw malleable ironworks open in North Lanarkshire at Calderbank and Dundyvan. This was more than 50 years after the invention of puddling. After all, Henry Cort had held demonstrations of his new process in Edinburgh as early as 1784. However, in the early 1830s Joseph Hall came up with an improved version of the puddling process, called ‘wet puddling’. The difference was that Hall lined his furnace with roasted slag rather than cast iron. As well as speeding up the whole process, wet puddling was more compatible with Scottish pig iron.
Technological change also brought social change. Just as the pig iron industry had drawn workers to North Lanarkshire from Ireland and the Highlands of Scotland so the malleable iron industry also brought immigrants. Puddling had begun in England several decades earlier so the malleable ironworking areas of that country were sources of skilled labour.
In 1819 when the Calderbank works rolled the iron plates for Scotland’s first iron boat it did so using imported malleable iron. By the middle of the century Calderbank was one of the countries biggest producers with 60 puddling furnaces.
Steel & the Decline of Malleable Iron
Steel is an alloy of iron. It is stronger and lighter, ideal properties for engineering.
Steel had been made in small quantities for centuries. It was made in Lanarkshire too. For example, in the early 1800s Calderbank Forge made steel cutlery.
The mid-1800s saw technological changes that would make the production of steel possible on a scale that would eventually dwarf the malleable iron industry. Two competing ways of making steel, the Bessemer Process (the fore-runner of one way in which steel is widely made today) and the Siemens-Martin (or ‘open-hearth’) Process became dominant.
In Scotland it was the Siemens-Martin process that won the day. This was because Scottish pig iron (being high in phosphorous) was better-suited to this process, which was similar to the puddling process except the mixture didn’t need to be stirred by hand so furnaces could be made much bigger. Another advantage of this process is that it used scrap iron as well as pig iron.
The time when mass steel production began in Scotland with the Steel Company of Scotland’s Hallside Works, followed a few years later by David Colville & Sons’ Dalzell Works saw the market for steel expand rapidly. Steel was initially more expensive than iron but it’s advantages in shipbuilding, boilermaking and railway lines made it worth the expense and, as production increased so the price came down. A key moment came in 1876 when the Chief Naval Architect of the Royal Navy approved the use of open-hearth steel in navy vessels.
Surprisingly few malleable ironworks were converted to make steel, most notably the Dalzell Works in Motherwell. Over the late 1800s the market for malleable iron rapidly contracted and the majority closed, including Motherwell’s first ironworks at Milton.
Typical of the decline was Calderbank, one of the first Scottish ironworks to adopt puddling. It closed in 1887 but only three years later a new steelworks opened on the same site. Other ironworks survived as rolling operations for steel companies and puddling continued at Coatbridge’s Waverley Works into the 1950s, the last of a once-huge industry.