Many people think the term “cast iron” refers to all early iron work, or that early iron work was always “wrought.” These are misconceptions, and the basic distinction between the two is simple:
- Cast iron is iron that has been melted and poured into a mold and allowed to cool
- Wrought iron is iron that has been heated and then worked with tools
In other words, their names define how they're formed. But that's a simple distinction. Let's look a bit more in-depth into the more significant differences between them—and what this means for their overall use and application.
Many of our decorative bollards have been cast using a newer form of cast iron called "ductile iron," which offers many benefits of both cast iron and wrought iron—without the drawbacks.
What is cast iron?
Cast iron is a generic term that can refer to a range of iron alloys, but is typically associated with the most common gray iron. While cast iron may sound like the cast form of the pure element iron (FE on the periodic table), it's actually an alloy containing 2–4 percent carbon, plus additional amounts of silicon and manganese. Other impurities, such as sulfur and phosphorus are also common.
Cast iron is formed by smelting iron ore, or melting pig iron (the product of iron ore extraction), and mixing with scrap metals and other alloy ingredients. The liquid mixture is then poured into molds and allowed to cool and solidify. Due to its higher carbon content, cast iron solidifies as a heterogeneous alloy—meaning it contains multiple constituents, or materials in different phases, within its microstructure. This quality affects its physical properties as well.
Cast iron, when compared to wrought iron or steel, is very brittle and comparatively hard and non-malleable. It can't be bent, stretched or hammered into shape. Its weak tensile strength means that it will fracture before it bends or distorts. It does, however, feature good compression strength, and was used prominently in building construction before the advent of steel industries in the early 20th century.
Compared to steel, cast iron has a lower melting point, is more fluid and is less reactive with mold materials—making it ideal for casting. Casting is significantly less labor-intensive than manufacturing wrought iron, and was a prominent form of production throughout the 18th and 19th centuries. Cast iron has largely been replaced by steel in the construction industry, but it is still used for many industrial applications.
What is wrought iron?
As mentioned above, cast iron and wrought iron are often defined by how the materials are formed. But they differ also in their material composition. Wrought iron is composed primarily of iron with small amounts (1–2 percent) of added slag ("slag" being the by-product of iron ore smelting, generally consisting of a mixture of silicon, sulfur, phosphorous and aluminum oxides). During manufacture, the iron is removed from heat and worked with a hammer while still hot to achieve its intended form.
Wrought iron is often characterized by its fibrous appearance, but it's also softer and more ductile compared to cast iron. Wrought iron is highly malleable, meaning it can be heated, and re-heated, and worked into various shapes—and in fact grows stronger the more it's worked. The word "wrought" actually derives from the past participle of the word "work."
In terms of how it's used, wrought iron has a much higher tensile strength than cast iron, making it more suitable for horizontal beams in construction. In general, it is highly resistant to fatigue. It will deform without failing, unless overloaded beyond capacity or distorted from exposure to intense heat (i.e. from fire).
Wrought iron was used as early as 2000 BC in the Anatolian peninsula (now Turkey). More recently, it was widely used throughout the 19th century in building construction, but was ultimately replaced by steel in the 20th century. Today, wrought iron is used primarily for decorative applications—but even so, mild steel is often cheaper and more practical for achieving a wrought iron aesthetic.
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Ductile iron: Strength and versatility
The problem with cast iron is that, within its microstructure, carbon particles form elongated graphite flakes when cooling. Graphite is low in density and hardness but high in lubricity. What does this mean? Not only does it offer little structural advantage, it also compromises the surrounding iron matrix—creating internal stress points that are conducive to fracture.
Ductile iron is a unique form of iron developed by researchers during the 1940s. They discovered that by adding select alloys—initially a magnesium-copper alloy, and eventually cerium—they could manipulate carbon atoms to form spheroidal graphite. Within the iron microstructure, these spheroidal shapes prevent cracks from forming—which is what makes ductile iron such a versatile material.
Ductile iron can be manufactured to multiple grades to achieve high ductility and tensile strength. Austempered ductile iron, produced with additional heat treatment, has even better mechanical properties and resistance to wear. Ductile iron is also ideal for casting, as it doesn't have the shrinkage properties of steel. This makes for more precision in design and avoids the use of large feeders to offset material loss. Ductile iron is often used "as-cast," meaning it doesn't require heat treatments. This can help reduce manufacturing costs—especially over the long term.
Cast iron and wrought iron are both susceptible to corrosion when bare surfaces are exposed to oxygen in the presence of moisture. This can be challenging for outdoor environments where exposure to precipitation and humidity can be ongoing. Unlike other metals that form a protective oxidative coating, iron will—given enough time—rust and flake away entirely.
To prevent rust, iron products should be coated to prevent exposure. Paint is commonly used to coat and protect bare metal. Powder coating is another method—ideal for outdoor furnishings more susceptible to wear in high-traffic areas. Powder coatings are highly durable and won't fade, chip or crack over extended periods of time.
From Lincoln, Massachusetts to Lincoln, Nebraska to Lincoln, Ontario, North America is rich with history. Every town has a story to tell, and the buildings in these towns have played as much a part in crafting the past as the people themselves.
When restoring historical buildings with architectural iron and metalwork, quality and authenticity is critical. Large outdoor elements such as bollards, gates, fences, lamp standards and park benches play a significant role in setting an ambience faithful to historical times. Modern foundries can reproduce early metalwork by taking their design cues from drawings or the original structures themselves. Existing cast metal furnishing designs can also be suitable, cost-effective means for revitalizing surrounding areas.
- Encyclopedia Britannica. "Cast iron." Encyclopedia Britannica website.
- Encyclopedia Britannica. "Slag." Encyclopedia Britannica website.
- Encyclopedia Britannica. "Wrought iron." Encyclopedia Britannica website.
- Engineer's Handbook. "Cast Iron." Engineer's Handbook website.
- Ductile Iron Society. "Ductile Iron Data for Design Engineers." Ductile Iron Society website.
- US General Services Administration. "Historical Preservation - Technical Procedures." GSA Historic Preservation website.
- Wrought iron can still be used for decorative elements: Will668, CC BY 2.0, via Flickr