Anodizing is a process where a coating is built up on the metal either by heating, chemical, or electrolytic means. In the case of titanium, the layer is an oxide of titanium. Coloring can be done by heating, but without much control of either color or uniform appearance. The most common method is to form an oxide layer by electrolytic means, similar to the process used in electroplating. The work to be colored is attached to the positive connection of a power supply (the anode), and usually another piece of titanium is connected to the negative side (the cathode) of the supply. Both are submerged in a mildly conductive solution, such as phosphoric acid (cola soft drinks), TSP - TriSodium Phosphate (dishwasher detergent). When power is applied to the contacts, a uniform layer of titanium oxide forms on the anode.  As the voltage increases, the thickness of the layer also increases. Certain colors will appear at specific voltage levels. The "change" from one color to another is not sharply defined, but rather shades gradually through a limited spectrum.

Titanium anodizing colors

The apparent color imparted to the metal is caused by interference between certain wavelengths of light reflecting off the metal and oxide coated surface. Light passing through the oxide layer, then reflecting off of the metal, must travel farther than light reflecting directly off the surface of the oxide. If one wave pattern is out of synch with the other, they will cancel each other out, making that particular color "darker" or not visible at all. If the thickness is such that a specific wavelength of light following one path closely synchronizes with that of the other path, then the wave strength (amplitude) will be increased, and that particular color would appear brighter. When the wave patterns cancel each other, it is called destructive interference, and when they match, it is constructive interference. It is possible that the thickness will create a combination of effects at the same time.  At about 110-120 VDC, the anodized titanium takes on a purple appearance, but with green highlights or reflections.

Color intensity decreases if the wavelenghs cancel out Color intensity increases if the wavelenghs match closely

The basic setup for anodizing titanium consists of:

bulleta container - large enough to hold the work to be anodized
bulletan electrolytic solution (cola soft drink, TSP or TSP-PF cleaning solution, etc.)
bulleta variable power supply - ideally this should be able to be adjusted from 0-150vDC and can sustain 10-15 Amps of current.
bulleta cathode - usually a strip of scrap titanium
bulletan anode - this is the titanium piece to be anodized
A simple anodizing setup

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