I love the colors of anodized titanium, and I really enjoy making chainmail out of titanium rings I've anodized myself.
Anodizing is a process which uses electricity to produce a coating on the surface of a metal. Titanium is interesting because the process naturally produces different colours without adding extra dyes or pigments.
This instructable will show you how to color small pieces of titanium using readily available materials such as 9V batteries and Coke or baking soda. (It does not cover "painting" designs on large pieces of titanium.)
The results are not very predictable, but part of the fun is trying different things and seeing the results.
Step 1: Things You Will Need
Plain titanium rings. I got mine from http://www.theringlord.com/. Get quantities and sizes appropriate for the project you have in mind. I use larger rings as hooks to hang the small ones on during the anodizing process. You will need either some large titanium rings or a piece of titanium wire bent into a hook shape.
9V batteries. Three is about the minimum number to get nice colours. After taking the pictures for this instructable with three batteries, I tried using nine batteries. This produces a larger range of colours (and a more dramatic chemical reaction), and I highly recommend it. (It's also more dangerous, so be careful.)
Battery snaps. (See picture.) You'll need these (or some kind of battery holder) to connect the batteries together. You can get them at a store like RadioShack or anywhere that sells components for electronics projects.
An electrolyte solution. Lots of anodizing instructions mention Coke and Pepsi, and these do work. However, different soda formulas work differently (Coke Zero doesn't work at all). I've also had good results with grapefruit juice. For this instructable, I tried two different things: white vinegar and baking soda dissolved in water. Both worked, but the same number of batteries produced completely different colours. You may want to experiment with a few different things and see what works best for you. (Anything acidic is worth a try.) Keep in mind, though, that it will be easier to see what's happening if you use baking soda than if you use cola (because the cola is dark).
An empty margarine tub or similar container to hold your liquid.
Some scrap metal. This doesn't have to be titanium, but it needs to have an equal surface area to the rings you are anodizing. I used a large coil of thin steel wire, which worked for me.
Wire strippers and a small pair of pliers will make it easier to connect the wires. Also, if you have a multimeter you'll be able to check your circuit to make sure your connections are good.
Some tape to hold connections in place may help. You could also use test leads with alligator clips to connect the positive and negative ends of the circuit to the titanium and the scrap metal. (I didn't happen to have any, but it would be easier.)
You should really wear rubber gloves - I didn't, but I also shocked myself. Twice.
Step 2: Connect Your Circuit
The batteries need to be wired together in series. That means, connect the positive (red) wire from one battery to the negative (black) wire from the next battery, and then connect that battery's red wire to the next batteries black wire, and so on so that the batteries are hooked together in a chain. One end of the chain of batteries has the positive wire unconnected, and the other end has the negative wire unconnected.
You can make a connection just by twisting the bare part of the wires together. This will be easier if you strip an inch or so of each wire. It's probably safer to connect the battery snaps together before snapping them onto the batteries.
You can use just one battery if you want, but you won't get much colour from it. I have usually found that I can get nice colors in the blue-purple range using 3 or 4 batteries. With 9 batteries, I was able to get yellows and pale greens, bright pinks, and even (sometimes) turquoise blues by removing the rings at different points in the process.
Connect the scrap metal to the negative end of the circuit. (That's the black wire.)
You can tape it to make it more secure if you want.
Close a large titanium ring and wind the bare part of the positive (red) wire around it securely.
Open a second large ring. In the pictures, I have it taped to the closed ring. I did find that the tape started falling off once it got wet, so later I started just hanging the open ring on the closed one. The rings you want to anodize will hang on this second ring.
Step 3: Anodize!
Put your electrolyte into your margarine tub. If you're using baking soda, shake a pile of it into the tub, then add warm water and stir. You need to make it deep enough to completely submerge your scrap metal and the rings you want to anodize.
Hang some small rings on the large open ring. The small rings don't need to be completely closed, but they need to be close enough to closed that they won't fall off. If you use too many, some of them won't get a good connection. 6 or so is probably good to start with.
If you're not taping the large rings together, it's best to put the small rings on the open large ring first, then hang the open ring on the one connected to the batteries.
Lower both the scrap wire and the titanium rings into the liquid. You'll need the liquid to be deep enough that both ends can be completely submerged.
Try to submerge the small rings but not the larger hook rings. Keep the battery lead out of the liquid.
Move the rings close to the scrap wire (maybe 1/4 inch apart) but don't let them touch.
You may have to wait a few seconds. Then small fizzy bubbles will appear around both the scrap metal and the titanium.
If you're using a clear liquid, you will be able to see the titanium change colour after the bubbles appear on it.
With 9 batteries, I got a lot of audible fizzing and in some cases (mostly when I submerged the whole open ring) steam.
If you are using 2,3, or 4 batteries, you will probably see your rings turn a darker colour and then stop. If you use more batteries, the process may quickly change through a series of colors, or it may pause at one color, then progress to the next after a few seconds. Pull the rings out when you see a color you like, or when the process seems to have stopped. If you don't get a color you like, try using a different liquid or adding another battery. Also check that your connections are good. The process is very variable - the same number of batteries with the same electrolyte may produce different results on different batches of rings.
Your rings will be sticky when you're done. (If you used grapefruit juice, they will also smell funny.) Wash them, but don't lose them down the drain. I used a towel to cover the drain in the sink, then ran water over the rings and scrubbed them between the folds of the towel. You could also use a strainer.
Step 4: Tips
As I mentioned in the previous steps, this process is very variable and can be affected by many factors.
You will have difficulty getting good colors if you don't have a good connection. I noticed that the two large rings tend to accumulate residue after being dipped several times. This residue interferes with the connection. If the first few batches of rings change colour but later ones don't, or change slowly, try changing the large rings.
I find that using different electrolytes results in different colours. With white vinegar, 3 batteries turned the rings a dark bronze colour. Using baking soda, the same 3 batteries made a nice denim blue.
The sequence of colours for titanium is (more or less) bronze, purple, blue, yellow, pink, turquoise as you increase the voltage. The labels on the pictures list the number of batteries that produced each color.
When you connect batteries in series, the voltage adds, so two 9V batteries produces 18V, three is 27V, and so on. (When I used nine 9V batteries, it should have given me 81V, but actually my multimeter showed 85V.) You can find charts that show the colors for each voltage, but as I said the process varies a lot so don't rely on charts too much.
Using nine batteries, the anodizing process seemed to naturally produce pink. I did get some light blues and turquoises by leaving the rings in longer, but the change from pink to turquoise was very slow and required some patience. As long as there are a few bubbles on the titanium rings, there is some chance that they will progress to the next colour. However, there are no guarantees and you should expect to ruin a few rings.
I suspect that ten or eleven batteries would make turquoise easily, and I may try that next time. I also suspect that six or seven batteries would naturally produce yellow.
Theoretically, you should be able to take a yellow ring and anodize it again to make pink, but I noticed that the process seemed to get stuck if I took the rings out and then put them back in. I had better luck taking the rings out when they turned a color I liked, which is why using a clear liquid is good.
Step 5: References
Here's some places to look for more information on anodizing titanium:
Anodizing instructions, with color chart: http://mrtitanium.com/anodizing.htm
This instructable also has good information, aimed at "painting" a bicycle: https://www.instructables.com/id/Anodize-Titanium/