BB color fade
Q.I had a multi-colored titanium curved BB going through one of my ear pierces. I recently took it out to stretch another hole, and I disocvered that, for the most part, the color had rubbed off. Is this supposed to happen? I have a barbell, same brand, multi-colored titanium going through my tongue, and, if the color does come off, is this going to harm me? Is the color supposed to fade or is it just cheap jewelry? (better not be for the money I have paid...)
A.The color effects seen in niobium and titanium jewelry are caused by a very thin (and pretty durable) transparent oxide layer. The oxide layer is produced by anodizing the metal. The jewelry is placed in a conductive solution, made the positive electrode, and current is passed through the cell. Variation of the voltage and timing determines the thickness of the oxide layer. What causes the color is that the thickness of the layer of oxide is about the same as the wavelength of light. Reflected light of the right wavelength or color is canceled or "interfered with" and removed from the light and so the reflected light is colored. These two metals among the very few that have both transparent and durable oxides. For example, you can see these effects on polished steel and on copper. The oxide layers can be created by carefully heating either of these metals. But the oxides are soft and the colors don't last. Anyway, even though Ti and Nb oxides are durable, when they are rubbed, the coatings thin out. When this happens the reflected light color becomes warmer as more short wavelengths are dropped. Eventually the color is gone. It can be restored. If you would like to experiment - use a carbon rod for the negative electrode and sodium carbonate solution for the electrolyte. Use a variable voltage power supply - you only need 0 - 15 volts (I think) at a few ma at most. Be careful how you attach your jewelry to the positive lead as you could contaminate it with other metals or scratch it. I'm not sure about the voltage needed - it depends on how effective an insulator the metal oxide is. Anyway, I'll bet you could find out with all the internet resources available to you. Just a bit more: some color effects can be produced by heating the metal under the right conditions. I believe black niobium is produced this way. BTW, anodizing aluminum produces a clear oxide that does not produce the interference effect. However, the oxide will absorb dyes. It is anodize in a sodium hydroxide solution containing the appropriate dye. Aluminum is not appropriate for body jewelry - I included that info just for completeness...
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