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.

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How to tell if a piece of metal is really titanium.

March 17, 2008

Let’s assume that you found or are given a metal piece, strip, sheet, rod or wire that you hope is titanium. Titanium can be many colors, and it might have any number of coatings. I’ve received titanium covered with rust, that transferred from a piece of steel in a wet shed. I have some titanium now that has a tenacious ceramic-like coating that conducts electricity, and is a bear to grind off. I had to get it all off of a piece to do and anodizer test.

How do you tell if it really is titanium?

First: Titanium is non-magnetic. If a magnet sticks, then no. I always carry a neodymium magnet for scrap yards and estate sales.

Are there any grade markings on it? If so, there are many that might mean titanium in its many grades and alloys. Most alloys can be anodized. I usually Google the numbers to see what variation of what grade I find in scrap yards. Many different specifications exist for chemically/commercially pure titanium, depending on sub percentage trace elements (mostly oxygen) and what certifications it has received (mil-spec, medical, etc).

File or sand off an edge to be sure you are looking at the bare metal. It should be a shiny dark silver. Compare it to a piece of (freshly scratched) aluminum; it should be much darker. If the fresh metal is a different color, then, no.

Does it anodize to color? If you can get to a bare piece of metal, just apply a moist cathode (paper towel) to the positive-attached metal. 9v should do to get a slight tan tinge. 18v (2 9v batteries) will take you to dark violet.
Note:Niobium and tantalum will color the same way. But they are heavier and softer and more expensive.

If you touch it to a typical rotary hard grinder, the sparks should be bright blue-white.

If you have a way to measure its specific gravity (ratio of weight to volume, water is 1.0 g/cc) then you have another good test.
Titanium and its alloys range closely around 4.5 g/cc.
Aluminum is noticeably lighter (2.7 g/cc),
Iron and steel are distinctly heavier (7.8 g/cc).
Tantalum is much heavier (16 g/cc).
Niobium is about the same as brass (8.5 g.cc)
Copper, bronze, and brass are up to 9 g/cc, but you’ve already eliminated them by color.

If you have samples of steel and aluminum in the same size range, the relative weight is easy to check.

Shavings of titanium (from drilling or milling) burn much as magnesium strip does. Shield your eyes if you resort to this test. btw: Magnesium is much lighter in weight (1.7 g/cc).