anodizing

I anodized the Al housing for the new light this week.  For some reason I found it difficult to find any concrete numbers on the various parameters involved, so here is what I learned:

try to use constant current instead of constant voltage (i.e. do not connect to a car battery).  When I monitored the current starting at the generally recommended 12V, it was in the 6 amp range for my part and rapidly trailed off.  Since the process that is occurring is the creation of an Al2O3 layer which is etched by the sulfuric acid (which creates pores into which dye can diffuse), the balance between acid strength and current is critical.  My part had ~ 160 dm^2 and I was shooting for 2 amps in ~10% Sulfuric acid.  Therefore, a massive peak current of 6A which trails to nothing probably does not deposi a very even layer of anodization.  The other thing I discovered is that it is best to use a new Al electrode every time, and make sure it is connected very well to your part.  I had two test runs where the current dropped to 0, and I assumed it was because the oxide layer was so thick that there was no more conductance.  I now think that it was simply the reused Al wire getting oxidized to the point where it could not contact the part any longer.  I cleaned the wire in NaOH every time, but that is probably not enough.