goldsilverpro said:
The colors are dyes that are absorbed into the porous anodized (aluminum oxide) layer. The gold looking color most probably isn't gold. I've only seen real gold plate on aluminum a few times in my life.
http://www.caswellplating.com/kits/anodizedye.htm
A long time ago, in a refinery I worked at, we had about 1000# of titanium sheet that had a very gold looking color on it. I hadn't heard of anodized titanium, at the time, and was convinced that it was gold plating. I wasted several hours screwing around with it until someone told me it was dyed anodizing.
Au plating on Aluminum looks surprisingly less "golden" than anodizing that is gold in color.. I've seen two types of Au plating on aluminum.. either it is going to be REALLY yellow like the stuff you see on connector pins, etc.. like this..
[img:200:165]http://www.vandenhul.com/UserFiles/Image/Accessories/cartridgeclips.jpg[/img]
or in the case of reflective optics, it will be dark, almost like highly polished yellow brass, but darker.. like so..
[img:243:169]http://www.lasergold.com/images/image_4.jpg[/img]
The best source for Au plating on Aluminum is going to be mirrors for optics firms (ahem.. I work in the field).. They use Au plated mirrors for IR aplications.. IR reflects nicely off gold, but honestly, if you find mirrors like that, you may even make out better reselling it as an optical mirror.. don't be fooled by "scratches" or "digs/pits", they're still useful believe it or not.. I work in a precision optics firm and some of the mirrors we use on our test stand are surprisingly old and have seen much abuse, but they work for our purposes.. don't just discard because they're not "pristine"
FYI.. (they can be made of Aluminum, Silicon Carbide, Beryllium/Copper (be careful with that stuff), Titanium.. etc)