Materials

Stripped down version.

0-140V variac
Bridge rectifier 250V 5 amp
Wires
Glass jar
Stainless steel plate
Titanium hook
Trisodium Phosphate

Construction

Stripped down version.

Connect the bridge rectifier to the variac by running wires from the variac output terminals to the AC input posts of the bridge rectifier. The AC posts are designated with ~ on them. They are located at opposite corners of the bridge rectifier. Since AC is the input it is not important which wire goes to which post on the rectifier. The opposite posts of the bridge rectifier are marked with a + and a -. These are the positive and negative outputs of DC voltage. The negative post will be connected to the cathode and the positive will be connected to the anode. The Cathode is a piece of either titanium or stainless steel that is submerged in a liquid. The anode is the positive side of the of the DC current and is connected to the piece being colored. There is also an anodizing medium that has to be used when anodizing, many people use Trisodium Phosphate as the medium but other things will work also. Coca Cola has been used as an anodizing medium with good results. I like Trisodium Phosphate because it is clear and therefore you can see your piece changing colors. You can stop when you get the color you want. Trisodium Phosphate can be fund in the hardware store in the department that sells industrial cleaners.

You simply attach your piece to be colored to a titanium hook by hanging the part on the hook. Attach the positive DC lead to the hook. Attach the negative lead to the cathode that is submerged in the TSP. Turn on the variac and set it to zero volts. Hang your piece into the TSP and then slowly turn up the voltage on the variac. You will see the titanium part start to change colors. When you get to a color you like, just turn the variac back to zero and shut it off. Take out the piece and dry it off. Before anodizing you must be sure that your part is polished and CLEAN or you will get blotches of color that is not attractive and will need to be redone. See the drawing below for a diagram of this system.

You may want to expand this system a little by adding a volt meter so that you can quickly get to the voltages you want. You may also want to add an amp meter. You don’t need either one, but they can be nice to have. It is not shown in the drawing but there is also a 6amp fuse in line between the wall plug and the variac on the hot lead.

The next diagram shows the system with the amp meter and the volt meter installed. Although not shown in the diagram, I mounted everything inside a box to make it look nice.

How It Works

The AC current comes out of the variac and is converted to DC current by the bridge rectifier. The DC current then passed through the anode to the part and then through the TSP to the cathode and back to the bridge rectifier, completing the circuit. When the voltage is applied it causes an oxide to be formed on the part. The oxide is clear but blocks light. The light enters through the oxide and is reflected off of the part. The oxide then blocks some colors of light from passing through it. The color that does get through is the color you see. The thicker the oxide, the different colors that can pass through. Therefore, by varying the oxide thickness you vary the color obtained. The way to vary the oxide thickness is to apply different voltages to the part. Once you get a color, you can change that color by increasing the voltage and therefore making the oxide thicker. However, you can’t turn it down and go back to a previous color. If you want the previous color, you will have to remove the part and start over by sanding and polishing the part.
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