Another question from a visitor to my regular site:
I’m starting to anodize on my own. How do they create that color “oil slick” in the picture or the “rainbow”?
The many simple colors are all based on voltage, as described in my anodizing page.
The stripes are made by masking off areas with something waterproof, like automotive striping tape. Then anodize to a high voltage color. Then remove the tape, and anodize to a lower color. The high voltage color blocks the lower voltage colors. Voila, stripes!
The rainbow can be made in several ways. The fastest is to turn the voltage up and the contacts off, immerse the piece, then turn the contacts on and draw the piece out of the electrolyte. The color is now dependent on the immersion time rather than the voltage setting.
The oil slick is trickier. This is probably done by sponge or brush anodizing (clip the positive lead to the piece, and the negative to something absorbent soaked in electrolyte. Then very carefully apply the high voltage wet thing to the charged piece. Rubber gloves and goggles are required. If metal touches metal, then you are practicing welding. Bright sparks, damaged pieces, and possibly damaged electronics.