The first thing we tried was making up a small amount of acid solution in an ovenproof dish. To 300ml of water (pure, distilled, deionised water of the sort you might top up your car battery with) we added roughly 50ml of acid. Naturally this was all done with lots of gloves and eye protection. Remember - AAA - Always Add Acid. Whilst waiting forthe solution to cool down a bit (adding acid creates heat) I had a dig around in the garage and found a roll of roofing lead. I was just able to lift it! I cut off a square with the sharp carving knife and bent it into an L shape and sat it in my acid bath to act as the cathode. Next I found a piece of scrap aluminium round bar and stood this in the acid bath so its bottom inch was under the acid.Using a PC power supply I connected 12 volts to the bath. The positive lead was fixed to the aluminium bar, and the negative lead attached to the lead cathode.
When I turned on the power, lots of bubbles formed on the lead cathode, and a few bubbles formed on the aluminium anode. More bizarrely, a faint purple colour was seen around the aluminium. This is apparently due to manganese in thealuminium alloy.
I left the whole thing to fizz for about 45 minutes. Then I removed the aluminium and washed it under the tap. I then mixed up some red food colouring with some water and stood the bar in this for about 15 minutes. It turned pink! Finally I boiled the part in water for half an hour to seal it. I ended up with an aluminium bar with a pink bottom!
This wasrather exciting! I promptly repeated the experiment with a bit of better finished aluminium - the result was amazing! I was very happy.
Improving my anodizing
Excited by my result I decided to try and improve things I bit. For some reason I had some nagging doubts about using a PC power supply, so the first thing I did was raid the parts bin and make a simple12v power supply from a 120VA transformer and a bridge rectifier. Make sure your wires are rated for at least 5 amps, preferably 10 amps. Putting an ammeter in the circuit lets you see whats going on.
The next thing I did was get a slightly bigger tank, and mix up the full 2.5 litres of acid solution. I made a bigger cathode from the lead, andrigged up some aluminium U section to hang things in the acid with, and got some aluminium wire.
I purchased various substances to try as dye. The most successful dyes are Dylon clothes dyes. The key is to have an organic dye that is small enough to enter the microscopic pores in the unsealed anodized layer. I tried some writing inks, but that was less successful. Foodcolouring works very poorly, and then only if it is organic. You can buy commercial anodizing dyes, but for now I will stick to the clothes dye as this works well enough for me. I suspect the clothes dye could fade a bit in sunlight over time.
I also purchased some Caustic Soda granules. This must be mixed up into solution and your aluminium part can be dipped in it - this etches the surface...