For me, the practical range is in the 2-80 volt, mostly 4-30v. You
are building an increasingly thick transparent oxide layer on the
surface. This oxide layer is the thickness of various wavelengths of
light and causes an interaction with the reflection of the light to
limit the reflected light waves to specific colors. As you build
thickness the color goes silverish again and if you increase voltage
you can get into a repeat rainbow, called second, third etc. order
colors which are similar to the first round of colors.
One of the things we do not think about in changing AC to DC is that
AC is usually measured as RMS voltage (root mean square, I think), not
the peak voltage. The peak voltage may well be 15-30% greater than
the RMS voltage. When I do the anodizing, my RMS AC is in the 0-130
volt range. When you rectify this, you get 50% of the voltage, but you
get the peak voltage. One of the things I like about straight
unfiltered, unsmoothed rectified AC is the ripple of the voltage .
Most of the voltage is less than 85% of the peak voltage so you can
paint the color. If you leave the electrified brush in one area, that
area will develop the full oxide layer, but you see it come up. One
bit of advise, the wet color is NOT the same as the final dry color
because of the effect of the added thickness of the water.
Have fun. If you ever have a tough question or need supplies, Bill
Sealy (sp?) of Reactive Metals is THE expert on Ni and Ti use for
jewelry (online at http://www.reactivemetals.com). I hear he is happy
to give advise over the web or by phone. Usual disclaimer, no
relationship, just took one of his great workshops.