Unravaling Wrought Iron & Cast Iron

Unravaling Wrought Iron & Cast Iron

Most people refer to IRON as STEEL and STEEL as IRON. However these are two very different products. Iron, or iron ore, is the fourth most abundant element on the planet and can be found and mined in the earth to make various steels, wrought iron, cast iron, high/low alloy steels, etc. Iron ore is the base element to make all grades of iron and steels, and is also an essential mineral which helps make hemoglobin in the human body.

Raw iron can be processed and refined in a huge number of ways to produce the various grades of steels and alloys that can be found practically everywhere, from the smallest smartphones to the largest skyscrapers.

For this article, we are focusing on two specific iron products: wrought iron and cast iron (we will also touch briefly on mild steel). The different methods by which these iron products are made give them unique physical advantages that are suitable for particular applications. Let’s take a dive into what sets wrought iron and cast iron apart, and why they have become historically relevant materials.

What Is Wrought Iron?

The term “wrought iron” is used a lot nowadays, often incorrectly, to refer to historical metal pieces, regardless of the actual metal composition. When any custom-shaped metal piece that has been painted black is referred to as wrought iron, this is an inaccurate statement that fails to take into consideration the technical definition of what wrought iron is. However, there were undoubtedly actual wrought iron pieces that were created throughout history, given that the wrought iron process was discovered as early as 3500 BC.

Examples of early ironwork date back to ancient Egypt and Mesopotamia. Around the 8th century BC, early civilizations such as the Hittites and the Mycenean Greeks began equipping their armies with iron swords. The vast availability of the raw material equipped entire armies with iron weapons. Knowledge about the use of iron spread from the Middle East to Greece and the Aegean region by 1000 BC. It had reached western and central Europe by 600 BC, and by the 5th century BC iron swords had replaced bronze all over Europe.

Before the Middle Ages, wrought iron was used primarily for weapons and tools, however the medieval period brought with it a multitude of uses for wrought iron. It began to be used to cover doors and buildings to protect against the attacks of raiders. But, more prominently, wrought iron work began to appear for decorative pieces

Wrought iron derives its name from the process by which it is made. Raw iron is literally “wrought” (an archaic version of the term “worked”) while still molten hot by hammering and folding. Because of how malleable and ductile low-carbon iron is when heated, this process can be done multiple times to work the iron into specific shapes. The more heating cycles a piece of wrought iron undergoes, the stronger it becomes.

Wrought iron is composed of iron and up to 2% of the byproducts of smelted iron, also known as “slag.” The term slag refers to the glass-like material that is produced when iron separates from iron ore at high temperatures. It is a mixture of silicon dioxide and metal oxides made up of the waste removed from the raw ore material. Wrought iron typically has a visible fibrous structure due to the silicates from slag. In addition to this siliceous slag, wrought iron contains less than 0.08% carbon. Since carbon makes iron stronger by changing its structure, the very low carbon content in wrought iron actually makes it more ductile, therefore easier to shape.

Before the Middle Ages, wrought iron was used primarily for weapons and tools, however the medieval period brought with it a multitude of uses for wrought iron. It began to be used to cover doors and buildings to protect against the attacks of raiders. But, more prominently, wrought iron work began to appear for decorative pieces

Wrought iron derives its name from the process by which it is made. Raw iron is literally “wrought” (an archaic version of the term “worked”) while still molten hot by hammering and folding. Because of how malleable and ductile low-carbon iron is when heated, this process can be done multiple times to work the iron into specific shapes. The more heating cycles a piece of wrought iron undergoes, the stronger it becomes.

Wrought iron is composed of iron and up to 2% of the byproducts of smelted iron, also known as “slag.” The term slag refers to the glass-like material that is produced when iron separates from iron ore at high temperatures. It is a mixture of silicon dioxide and metal oxides made up of the waste removed from the raw ore material. Wrought iron typically has a visible fibrous structure due to the silicates from slag. In addition to this siliceous slag, wrought iron contains less than 0.08% carbon. Since carbon makes iron stronger by changing its structure, the very low carbon content in wrought iron actually makes it more ductile, therefore easier to shape.

The low carbon content in wrought iron contributes to its high malleability but it does not preclude it from becoming a stronger metal. As we mentioned above, when wrought iron is continually worked during heat cycles it increases strength and structural integrity.

If you’ve ever seen a medieval movie where a blacksmith hammers a piece of iron to produce weapons, then you have a good idea of how wrought iron is made. A medieval setting is also appropriate for wrought iron, as the process has mostly gone out of fashion in favor of cheaper and more physically desirable alternatives such as mild steel. Products that used to be made using wrought iron, such as garden fences and gates, are now often made using a casting process.

Pros and Cons of Wrought Iron

PROS

1. Stronger and More Ductile Than Cast Iron

The low carbon content of wrought iron gives it extra ductility, allowing it to deform to a greater degree before failing. The characteristic has made wrought iron the material of choice for many of its old-fashioned applications such as bridges, railroad tracks, and the boiler used in steam powered boats. Wrought iron is highly resistant to fatigue, making it excellent for equipment subjected to a lot of stress.

2. Holds Coating Well

Wrought iron naturally has a rough exterior surface which makes it suitable for holding platings and coatings. Compared to cast iron or steel, a wrought iron surface can hold coatings that are up to 40% thicker. This is great for protecting wrought iron against corrosion and other deleterious effects of weather. Because of the excellent adherence of coating treatment to wrought iron, it is often used for outdoor applications like gates, guardrails, and fences.

CONS

1. Very Labor-Intensive Manufacturing Process

The main reason why so little wrought iron is made today is that it takes a lot of manual work to get even a single wrought iron piece done. Between the repeated cycles of heating, hammering, folding, and cooling it can take several days to weeks to finish a piece made of wrought iron. Not only is this costly, but it also means that the slow process can no longer keep up with the fast-paced demands of modern consumers.

What Is Cast Iron?

The difference between wrought iron and cast iron can be easily defined by two factors: what they are made of and how they are made.

To start off, cast iron is an alloy, still made mostly of iron but with a carbon content greater than 2% by weight. Cast iron also typically contains up to 3% silicon and other elements that influence its color such as graphite or carbide impurities. The differences in composition of cast iron gives it a lower melting temperature, higher brittleness and resistance to deformation, and excellent castability, hence the name.

Since the exact composition of the alloy made for cast iron can be easily controlled, several variants of cast iron have been developed based on exactly what elements and compounds have been added to the pig iron. This has resulted in four types of cast iron:

  • White cast iron: This contains carbide which results in good wear resistance, high compression, strength, and hardness.
  • Gray cast iron: This contains graphite in its microstructure. It is easy to machine and is resistant to wear.
  • Ductile cast iron: This is a form of gray iron which contains small amounts of caesium and magnesium. These elements result in high strength and ductility.
  • Malleable cast iron: When cast iron undergoes heat treatment to improve its ductility it forms a malleable cast iron.

Pros and Cons of Cast Iron

PROS

1. Harder and Does not Deform

One of the characteristics that distinguishes cast iron from wrought iron is that it is much harder, less ductile, and does not easily deform under stress.

2. Excellent for Cookware

Cast iron remains a very popular material for various types of cookware. It has excellent heat retention, good resistance against mold growth, and these superior pans are vastly preferred by many professional cooks, though they require special maintenance to prevent corrosion.

3. Easier Manufacturing Process

Making cast iron is simply faster and cheaper than wrought iron, making it an economical choice.

CONS

1. Poor Impact Resistance

Because of the increased content of carbon in cast iron, it loses out on the ductility of wrought iron. This makes cast iron harder but more brittle, not able to deform to endure impact or sustained stress. While this property may be desirable in certain applications, it also means that cast iron is more prone to cracking than steel or wrought iron.

2. Poor Machinability

Related to the brittleness of cast iron, machining a cast iron part using a lathe or drill can be a very risky process because of how easily the material can crack. This severely limits the finishing options for a cast iron part after the casting step.

Historical Uses of Wrought Iron

As we have seen earlier, wrought iron went beyond tools and weapons and extended to decorative purposes.

The implementation of the puddling process (in which a puddle of congealing red-hot iron was worked manually using long rods) in late 18th century Europe as a way to produce wrought iron allowed the material to be produced in larger scales. The new manufacturing method kept pace with the zeitgeist of the Industrial Revolution spurring the use of wrought iron throughout Europe and eventually the United States.

The production of wrought iron hit its peak in the 1860s as the popularity of ironclad warships rose. It then skyrocketed when the railroad emerged as the modern system of public transportation in Europe and the United States.

Wrought iron found its way to the trusses of bridges, railroad tracks, structures used to erect buildings, and the panels that made the body of ships. It’s difficult to imagine how much manual labor went into making these structures out of wrought iron, but that was the reality of an era where automated manufacturing methods did not exist.

Wrought iron became known for its incredible durability and therefore was also used to manufacture a vast array of household tools, utensils, and decorative metalwork. These products made with wrought iron had very long life expectancies with minimal maintenance required as long as they were properly shielded from corrosive substances.

Modern Day Uses of Wrought Iron

Although wrought iron was the most prevalently used form of malleable iron during the 19th century, this status did not last. In the 20th century the Bessemer steel making process could make vast quantities of steel in a single pass thus ending mass wrought iron production. The production of steel, specifically mild steel (iron with low carbon, low cost, high tensile strength, corrosion resistance, and ductility) became comparatively much cheaper and faster to produce in larger scales than the costly and time-consuming process of blacksmithing wrought iron products.

Wrought iron is still used today, but to a much smaller extent than it was in the past. Most of the true forms of wrought iron in the world today are found in structures built in the 19th century and have successfully withstood the test of time (the Eiffel Tower for example). In fact, many modern products that we might consider to be wrought iron today are actually made of mild steel. Finding true wrought iron products today is considered a rarity and its production has become a skilled art form performed by only a select few capable individuals.

Cast Iron Finds a New Application

Ductile cast iron, with its superior fatigue strength and wear resistance, is used for the crank shafts of vehicles, automotive suspension systems, door hinges, heavy duty gears, and hydraulic components.

Where to Purchase Wrought Iron?

Since commercial wrought iron ceased to be produced commercially in the 20th century, its availability is very limited. A lot of wrought iron products available for purchase on the market today are actually repurposed wrought iron products that were originally manufactured in the 19th century by blacksmiths. Antique items like fences and staircase railings can be purchased and restored. Newly wrought iron products, however, are now considered artisanal items.

Final Words

Wrought iron and cast iron are two of the most relevant materials of eras gone by. Although the use of wrought iron has declined sharply because of a preference for more automated processes, cast iron is still hanging around, although it’s now only used for a few niche applications

Even if these two materials are no longer as relevant today as they were in times passed, there’s no denying that they played huge roles in the evolution of construction, transportation, and technology.