Why add a capacitor to the Anodizer?

August 6, 2008

I received the following question:

I have a variac and full wave rectifier but no cap.
What is the reason behind adding a capasitor to the anodizer? I know it will reduce electrical ripple but what will it mean to the anodize process or final results?

In principle, the smoother, ripple-reduced output allows more even anodizing starting at the initial surge. Whether this is truly useful, I don’t really know. My experience is almost exclusively with a smoothed DC supply. But I have a switch on my main anodizer to disconnect the capacitor for those occasions when I feel like it.

RMS vs Peak Voltage The voltage will read wrong with ripple. The anodized color depends on the peak voltage. But a rippled current shows on a meter as the rms voltage, that is somewhat lower. So the color is less predictable, and the time spent at that voltage is more critical to watch.

Also, once you reach your final voltage (or at least asymptotically close enough), the smooth DC current is stopped. But a rippling supply still produces a trickle of  current as the piece you are anodizing acts as a capacitor. If you wait long enough, you can see the color continues to rise at a fixed ripply voltage.

This latter point is more important if you mask and do a succession of lower voltages for multiple colors. With ripple, the higher voltage colors will creep as you anodize the lower voltage areas.

Another note is that AC is more dangerous than DC. Edison (General Electric) made sure that the first electric chair used the AC current promoted by his rival Tesla (Westinghouse), to popularize that point. (source) But I doubt it makes much difference in any practical sense of anodizer safety.

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How can I make some of those Fancy Titanium Colors?

March 21, 2008

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.


Uneven results from dimmer switch anodizer

March 18, 2008

Here’s another question I frequently get:

I followed your instructions on building an anodizer and I would like to say that you have made a great job illustrating it. My anodizer is the dimmer and light bulb type it delivers a maximum of 160 Volts.I prepared a solution of TSP in distilled water placed the cathode (aluminum foil) and the Ti at the anode ran the circuit. The voltage keeps rising slowly and I get shades instead of definite colors (mostly violet, golden and pale blue). I can’t hold the voltage at a definite value. What should I do to get smooth colors? I tried adjusting the voltage first then immersing the piece but the voltage after immersing is lower than what I’ve just set it to. Please help me out here and thanks in advance.
David S.

First of all, the dimmer based voltage control is going to be a bit temperamental and unstable. But I used one myself for years before replacing the dimmer with a Variac.

Aluminum should work for a cathode, but should be lightly sanded to remove the invisible insulating oxide layer that spontaneously forms. I usually use titanium, but have been told by many that stainless steel works well.

When you have a large capacitor smoothing a the choppy dimmer voltage, the top end will be a bit mushy. The lower voltages are the worst for this effect. The tan, violet and blues are at the low end of the voltage scale.

Another issue in getting smooth colors is getting the voltage everywhere simultaneously. You should have the piece to be anodized immersed in the solution before completing the circuit to the leads. That is, you need a switch to turn the leads on and off, while the anodizer is running at the voltage you want.

Cleaning and chemically etching the metal before anodizing also helps assure a uniform color, and is generally considered necessary for getting the higher voltage colors.

The voltage measured on the leads or capacitor will drop when you start anodizing, and should rise back to your preset voltage in a minute or so. The time depends on how big a piece you are anodizing, how big your cathode is, and on the efficiency of your electrolyte.

Another possible problem might be the material of your attachment to the anode piece. Only titanium or niobium should touch the electrolyte at the positive side. Never use copper wire or regular (galvanized or tinned) alligator clips to immerse your piece. The current will just go though that, and little will be applied to your piece.