Wrought iron is a popular material used for decorative and functional purposes in a variety of settings. Its dark, blackish appearance leads many people to assume that wrought iron is always black. However, the answer to whether wrought iron is truly black or not is a bit more complex.
What Is Wrought Iron?
Wrought iron refers to iron that is heated and worked by a blacksmith or other artisan to shape it into various objects. It differs from cast iron, which is formed by pouring molten iron into a mold. Wrought iron has a fibrous internal structure due to being heated and hammered or bent into shape.
Traditionally, wrought iron was made directly from iron ore. The ore was heated in a furnace to extract the iron, then the iron was worked by a blacksmith while still malleable to create various tools, furniture, railings, and more. Modern wrought iron is often made from scrap iron or pig iron that is remelted and refined before working.
The Composition of Wrought Iron
Wrought iron has a low carbon content, usually less than 0.08% by weight. It also contains very low levels of other impurities like sulfur, phosphorus, silicon, and manganese. This composition gives wrought iron good working properties while also making it resistant to rust and corrosion.
Here is a table showing the typical composition of wrought iron:
| Element | Weight % |
|---|---|
| Iron | 98.82 – 99.26% |
| Carbon | 0 – 0.08% |
| Manganese | 0 – 0.03% |
| Phosphorus | 0 – 0.04% |
| Silicon | 0 – 0.1% |
| Sulfur | 0 – 0.05% |
As the table shows, wrought iron is mostly pure iron with very little carbon or other elements. This gives it its soft, fibrous structure and malleability when heated and worked.
The Color of Raw Wrought Iron
Freshly worked wrought iron directly from the forge has a dark silvery gray appearance. This is the natural color of pure iron metal. However, the color rapidly begins to change as the wrought iron interacts with oxygen in the air.
At normal ambient temperatures, wrought iron slowly oxidizes, forming a thin film of black iron oxide, or magnetite (Fe3O4), on its surface. This black oxide coating thickens over time, darkening the surface of the iron. After just a few days, wrought iron will develop a matte black coloration.
So while raw, pure wrought iron is actually a light gray, the black oxide coating it develops gives it its familiar dark black shade.
Preserving the Natural Color of Wrought Iron
Because the black color of wrought iron comes from surface oxidation, the underlying steel can be preserved in its original gray tone through chemical conversion treatments or coatings. One common method is to treat wrought iron with phosphoric acid. This converts the surface to black iron phosphate, stopping further oxidation.
Applying a clear acrylic lacquer or wax coating immediately after phosphating can also protect the metal and maintain its lighter as-worked appearance. However, most blacksmiths allow wrought iron to oxidize naturally to the familiar black color.
Differences in Composition Affect Color
While pure wrought iron turns black from surface oxidation, differences in composition can affect the final color. Higher carbon wrought iron alloys will be darker after oxidizing compared to pure low carbon iron. Other alloying elements like manganese also influence the final tone.
Cast iron, which has a higher carbon content around 2% – 4%, turns a darker gray-black when oxidized compared to wrought iron. Meanwhile, small amounts of nickel (0.5% – 1%) in iron alloys impart a paler gray color.
So while almost all wrought iron develops a black surface color, the exact shade can vary based on the specific composition and manufacturing process.
How Blacksmiths Use Heat to Alter Colors
Skilled blacksmiths can use heat to alter the colors and textures of wrought iron surfaces. When wrought iron is heated, thin oxide layers form producing a range of colors depending on temperature. This process is called fire scaling.
Here are the colors that can be produced at different heats:
| Temperature | Oxide Color |
|---|---|
| 300°F – 600°F | Pale yellow |
| 600°F – 800°F | Golden brown |
| 800°F – 1000°F | Purple |
| 1000°F – 1200°F | Deep blue |
| Over 1400°F | Black |
By carefully controlling the temperature, a blacksmith can create multi-colored finishes on iron. However, these thin heat oxides offer no protection from further oxidation. So without a sealant, the colors are temporary and return to black as the iron oxidizes.
What Gives Black Wrought Iron Its Color?
When wrought iron appears deep black rather than gray, the dark color comes from multiple factors:
- Thick magnetite (Fe3O4) surface oxidation
- Presence of tiny iron carbide particles
- Conversion coating treatments like phosphating
- Applied sealants and finishes
The combination of iron oxides, iron carbides, conversion coatings, and black finishes creates the signature black look of wrought iron. While not the natural color, it has come to be the expected appearance of iron decorative objects and furniture.
Does Paint or Powder Coating Change the Color?
Since bare wrought iron left in air oxidizes to black, many iron pieces receive an additional paint or powder coating. This is done for decorative purposes and to provide further protection from corrosion.
Painted or powder coated wrought iron can be any color depending on the pigment used. Black is a common choice as it is harmonious with the natural black oxidation of the iron. But other colors like green, white, or bronze are also popular for iron furniture, fences, railings and art.
The underlying iron still oxidizes to black, but the coating layer conceals this. Scratches or chips in the coating expose the black iron surface below.
What About Rusted Wrought Iron?
When kept dry, wrought iron maintains its black oxide coating and resists further corrosion. But exposure to moisture can lead to red iron oxide (rust) formation, damaging the surface.
Light surface rust may only partially damage the black oxide layer, resulting in a mottled black and orange appearance. As rust penetrates deeper over time, it can completely replace the black with reddish-brown rust colors.
Heavily rusted wrought iron loses its original black finish. Restoration requires removing the rust and re-oxidizing or refinishing the surface to regain the black color.
Conclusion
While associated with the color black, wrought iron begins as a grayish metal. It’s the natural oxidation processes that give iron its familiar black hue over time. Skilled craftsmen can encourage different temporary colors through careful heat treatments. But the stable, long-lasting color of weathered wrought iron will always be a classic deep black.
