How do I anodize aluminum?
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      First, this is only meant to apply to aluminum.  Other metals, such
      as titanium, niobium, and possibly magnesium and others, can also
      be anodized.

      Aluminum oxidizes very quickly, and rapidly forms an aluminum oxide
      coating that inhibits further oxidization.  This coating is useless
      as is, in terms of protecting the metal, because it is so thin.  A
      thicker coating can be produced by immersing the part in an
      electrolytic solution and passing an electrical current through it,
      similar to electroplating.  The resulting film is nearly colorless,
      and can be easily dyed because it is very porous at the molecular
      level.  Then, by placing the part in boiling water, the film's
      pores can be sealed; the oxide changes from one form to another
      as a result.

      To be more specific, parts should be very clean and grease-free.
      Commercial plants will first clean and etch the surface in a caustic
      solution, such as lye (sodium hydroxide), followed by a thorough
      wash.  The parts are placed in an acid solution, such as 15-25%
      sulphuric acid, and connected to the positive source of a power
      supply (use only aluminum hardware to make the connection; no
      copper in the solution!).  The part(s) comprise the anode, and the
      cathode is lead (lead sheet, or the whole tank might be lead).
      Appropriate current is applied, e.g. 1.5 amps per decimeter (3.4
      square inches).  This lasts 15-25 minutes if no dying is planned,
      or 45-60 minutes for dying.

      Dying is the next step, if desired.  Since the pores are extremely
      small, many common dyes will not work.  Some wool dyes are known
      to work, or you can purchase commercial anodizing dyes from an
      industrial supplier.  Typically this involves immersion in the
      dye solution, which may have to be heated to be effective.

      Sealing is then done by putting the part in boiling water, which
      changes the film from gamma aluminum oxide a hydrated form
      called boehmite.  Boil for about 20 minutes.

      Another anodizing process involves using chromic acid.  This is
      not suitable for alloys with more than 5% copper.  The film is
      thinner, but very durable.  The very thin film is also a benefit
      when very close tolerances must be maintained.  However, because
      chromic acid is a very nasty chemical, its use on an amateur
      basis is discouraged, and its commercial use may be closely regulated.

      Anodizing is sensitive to the type of alloy.  For example, alloys
      for die casting have a lot of silicon, which makes it pour and
      mold well, but it makes anodizing almost impossible.

      By all means, find a few references and read them carefully before
      attempting anodization.  If at all possible, try it out on scrap
      pieces before attempting it on your masterpiece.  Be sure to flush
      the part completely to remove remaining acid.

      An alternative is to use a commercial firm to do the anodizing.
      Some net folks report they can be a bit rough to deal with, as
      they may be more used to dealing with large jobs and may not
      appreciate a home machinist with a single 5-inch part.  Look
      in the phone book under anodizing, plating, or electroplating.

      Some have reported using a lye solution to treat the surface,
      which yields a fairly uniform fuzzy surface, that may be
      acceptable as a final surface without further treatment.  Don't
      use a strong solution or the aluminum will disappear too fast;
      do this in a well ventilated area, as hydrogen gas is emitted,
      stay away from flames/sparks, and remember that lye will eat
      people as well as aluminum!  The resulting surface can be dyed,
      or painted with a chromate primer.  (One tablespoon of lye per
      pint of water has been suggested).

      Another alternative is a product called Aluminum Black from
      Birchwood Casey, which is often advertised in model railroading
      magazines, and may also be available via gun stores (it is in the
      Brownells catalog).  Brownells also lists a nickel plating
      solution for aluminum, though it appears non-trivial to use.