Tarnish is a common form of surface corrosion that affects many metals, resulting in a thin layer of discoloration. It forms when the metal reacts chemically with compounds in the surrounding environment, primarily sulfur and oxygen in the air. This chemical change creates a film on the metal’s outermost layer. Understanding what tarnish looks like on different materials helps in identifying and treating the discoloration.
General Appearance and Texture
Tarnish first manifests as a subtle alteration of the metal’s original brilliance, replacing the reflective surface with a dull, matte finish. The initial stage often appears as a hazy film or slight cloudiness that gradually spreads across the object. It typically begins in hard-to-reach areas or in small, patchy spots before covering a wider area.
The texture of tarnish remains smooth to the touch, which distinguishes it from other forms of corrosion. Because tarnish is a self-limiting process, it forms an extremely thin layer that seals the underlying metal from further reaction. This surface film is non-reflective and obscures the bright luster of the metal beneath it.
Tarnish does not typically pit the metal surface or flake away, adhering tightly as a uniform layer of discoloration. As the process continues, the color deepens, transforming from a faint haze to a more pronounced shade. This darkening is solely a surface change and does not compromise the structural integrity of the metal object.
How Tarnish Varies by Metal Type
The color and progression of tarnish depend heavily on the specific metal and the chemical compounds it reacts with. This variation is due to the different chemical compounds formed on the surface. For example, sulfur compounds often lead to dark sulfides on certain metals.
Silver is particularly susceptible to tarnishing, which typically starts as a faint yellowish or golden hue. This discoloration quickly progresses to dark brown, then deep gray, and eventually a solid black film of silver sulfide. The copper alloy often present in sterling silver contributes to this dark discoloration due to the formation of copper sulfide.
Copper, brass, and bronze, which are copper alloys, tend to develop tarnish in warm, earthy tones. The discoloration usually presents as a darkening film that is reddish-brown or a deep, rich brown. Depending on environmental exposure, copper can also develop a black tarnish over time.
Pure gold is highly resistant to this chemical process, but gold alloys can show some discoloration. This slight, dull, dark film is not gold tarnish but a reaction involving the base metals, such as copper or silver, mixed into the alloy. White or rose gold jewelry may show minor darkening due to the tarnishing of their non-gold components.
Tarnish vs. Rust and Patina
Tarnish is often confused with other types of surface changes, but its visual and structural characteristics set it apart from rust and patina. Rust is chemically defined as iron oxide and is an aggressive form of corrosion that occurs on iron or iron alloys like steel. Visually, rust is typically a reddish-orange color with a coarse, flaky, or powdery texture.
Unlike tarnish, rust is destructive, eating into the metal structure and causing pits and cracks. Tarnish, in contrast, is non-destructive, forming a smooth, thin layer, typically black or gray, that remains on the surface. The chemical products of tarnish, such as metal sulfides, are structurally different from the iron oxides that form rust.
Patina is a specific type of surface layer closely related to tarnish, often described as protective. While tarnish is generally considered an unwanted dulling, patina is typically uniform and stable, such as the blue-green coloration on aged copper structures. This color, known as verdigris, is usually a copper carbonate compound and forms slowly over time. Patina is welcomed for the antique character it lends to the metal, whereas tarnish is the initial, uneven darkening that precedes the stable patina.