The classification of metal as natural or processed depends on how much it has been altered by human intervention. All metal elements, such as iron, gold, and aluminum, are inherently natural, having formed within the Earth’s crust over geological time. However, the strong, pure metal used for construction and manufacturing is almost always a processed or manufactured material. The transformation from a raw geological deposit to a finished product involves industrial processes that fundamentally change the material’s chemical state and physical properties.
Defining “Natural”: Elements, Compounds, and Materials
To understand a metal’s origin, it is helpful to distinguish between three scientific terms: element, compound, and material. An element is a pure substance, like iron (Fe) or gold (Au), composed of only one type of atom. A compound is formed when two or more different elements are chemically bonded together, such as iron oxide (Fe2O3). While elements and many compounds are natural, a “material” is a substance used for construction or manufacturing.
The difference between a natural and a processed material is the degree of human alteration. A natural material is found in nature and used with minimal modification, such as a rock or wood. A processed material is a raw material significantly transformed through industrial processes to enhance its performance. Most metals fall into this latter category because their naturally occurring forms are chemically bound and unusable without extensive refinement.
Metals in Their Geological Origin: Native Forms and Ores
Metals exist in the Earth in two primary geological states before human extraction. The first is native metals, which occur in a pure, uncombined metallic form. Unreactive elements like gold, platinum, and occasionally copper resist chemical reactions in the crust. A nugget of native gold, which is simply dug up and cleaned, is an example of a truly natural material.
The vast majority of useful metal elements, including iron and aluminum, are chemically locked within rock formations called metal ores. These ores are natural compounds, typically oxides, sulfides, or carbonates, where the metal is chemically bonded with other elements. For instance, iron is found mainly as hematite (Fe2O3), and aluminum is sourced from bauxite (aluminum oxide). Although the ore rock is natural, the metal element cannot be used in its pure form until these chemical bonds are broken.
The Impact of Human Processing: From Element to Material
When a metal ore is chemically transformed to isolate the pure element, its classification shifts to a processed material. The most common method is smelting, a pyrometallurgical process using intense heat and a chemical reducing agent, typically carbon. During smelting, the reducing agent bonds with the non-metal elements in the ore, leaving the molten metal behind. This chemical reaction is the first major step in manufacturing the material.
The resulting liquid metal must then undergo further steps, such as refining and purification, to remove residual impurities and unwanted byproducts called slag. For example, extracting aluminum from bauxite requires energy-intensive electrolysis. These processes ensure the metal achieves the high purity and consistency required for modern applications.
Alloying
Another step that definitively moves a metal material away from a natural classification is alloying. This process involves mixing the pure metal with other elements while molten to create a new material with superior characteristics, such as increased strength or corrosion resistance. Steel, for example, is an alloy created by mixing iron with a small percentage of carbon and often other elements. Alloying creates a manufactured material whose properties are engineered, not found in the geological record.
Categorizing Common Metals: A Spectrum of Naturalness
Materials fall along a spectrum of naturalness based on the degree of processing they undergo. At the highly natural end is native gold, found in a pure, metallic state that requires only simple physical separation to be usable. This minimal alteration means a gold nugget is the closest a metal material comes to being truly natural.
Moving along the spectrum are pure metals sourced from ores, such as the copper in electrical wiring or the aluminum in a beverage can. Although the elements are natural, extracting them from their compounds through high-heat smelting and refining is an intensive chemical transformation. The final product is a highly processed elemental material, chemically distinct from the original ore.
At the far end are engineered alloys, which are definitively manufactured materials. Steel is a man-made combination of iron and carbon that does not exist in nature. Similarly, bronze (copper and tin) and brass (copper and zinc) are manufactured materials designed for specific purposes. The metals we rely on are overwhelmingly products of industrial processing.