> Please, tell me a little more about anodizing: how to sensitize, what is
> the address of Metalphoto, what chemicals do they use etc.
We had a discussion about this on the list about a year or so ago, so
there should be some information in the archives. In a nutshell, the
aluminum piece to be anodized is connected as the anode (positive
electrode) in an electrolytic cell. The electrolyte is typically a 10%
sulfuric acid solution and the cathode (negative electrode) can be
anything that won't dissolve in the acid. Cathodes are normally made from
lead, but another piece of aluminum would work. Copper contacts will
dissolve in the acid and contaminate the electrolyte so can't be used. The
current density needed is about 10-15 amps/(square foot). Voltage is
usually around 12V DC so a car battery charger works OK. The temperature
also affects the type of film formed. Lower temperatures give a thinner
denser oxide layer, whereas higher temperatures give thicker, more porous
layers. If the temperature gets too high the surface of the aluminum is
simply dissolved (electropolishing). Room temperature (20 Celsius) is
about right. Process times are on the order of 30 minutes.
What happens is that the process grows a porous layer of aluminum oxide on
the metal surface. By washing the acid off of the aluminum part and
dipping it in a dye bath, various colored metalic finishes can be
acheived. The surface is sealed by hydrating the oxide in a boiling water
bath. Here in the U.S. the Maglite flashlights are a good example of a
common object that has been given this treatment.
I have a reference to "dying" the oxide with a silver halide solution, but
no particulars. I don't know about the chemistry of the MetalPhoto
process either, but will try to get you their address tomorrow.
- Wayde
(allen@boulder.nist.gov)