Titanium jewelry is colored primarily through a process called anodization, which manipulates the oxide layer on the surface of the metal to create an "illusion of color."
Understanding Titanium Anodization
Titanium itself is a naturally silver-colored metal. The vibrant colors often seen on titanium jewelry aren't dyes or paints, but rather the result of light interference within a thin layer of titanium oxide (TiO₂) that is formed on the surface. This process is similar to how a prism separates white light into a spectrum of colors.
The Anodization Process
Anodization involves using an electrolytic process to grow the oxide layer. Here's a simplified breakdown:
- Preparation: The titanium piece is thoroughly cleaned to remove any contaminants.
- Electrolyte Bath: The titanium is immersed in an electrolytic solution (typically a mixture of water and a mild acid or salt).
- Electrical Current: A controlled electrical current is passed through the solution, with the titanium acting as the anode (positive electrode).
- Oxide Layer Formation: The electrical current causes oxygen ions from the electrolyte to combine with the titanium atoms on the surface, forming a thin layer of titanium oxide.
- Color Determination: The thickness of the oxide layer determines the color that is perceived. Different voltages produce different oxide thicknesses, leading to a spectrum of colors. Thicker layers interfere with light waves differently than thinner layers.
Color by Interference
The titanium oxide layer is transparent. Light entering the layer is reflected from both the top surface of the oxide and the surface of the underlying titanium metal. These reflected light waves interfere with each other.
- Constructive Interference: When the reflected light waves are in phase (their crests align), they reinforce each other, resulting in a brighter color.
- Destructive Interference: When the reflected light waves are out of phase (a crest aligns with a trough), they cancel each other out, resulting in the absence of that color.
The specific wavelengths of light that undergo constructive interference determine the color that is seen. Because the oxide layer's thickness dictates which wavelengths interfere constructively, precise control over the anodization process is essential for achieving specific colors.
Examples of Colors and Voltages
While the exact voltages can vary depending on the electrolyte and other factors, here's a general guideline:
| Color | Approximate Voltage (Volts) |
|---|---|
| Bronze | 10-15 |
| Purple | 20-25 |
| Blue | 30-35 |
| Light Blue | 40-45 |
| Gold | 50-55 |
| Pink | 60-65 |
| Green | 70-75 |
Advantages of Anodized Coloring
- Durability: The color is part of the metal's surface, making it highly resistant to scratching and fading compared to paints or dyes.
- Biocompatibility: Titanium oxide is biocompatible, making anodized titanium jewelry suitable for people with sensitive skin.
- Variety of Colors: Anodization allows for a wide range of vibrant and attractive colors.
In summary, titanium jewelry is colored by anodization, a process that creates a thin, transparent oxide layer on the surface. The thickness of this layer determines the color perceived due to light interference, resulting in durable and biocompatible colored jewelry.
