Subject: Blackening Aluminum

From: [email-masked] (J. Kimberlin)
Date: 12 Aug 1995 20:11:13 -0700

There have been several postings about blackening aluminum recently.  I 
wrote an article for *Live Steam* magazine back about 1977 on the subject 
and have modified it a little since then.  I do use it myself and thought 
that it would be of interest to the newsgroup.  So:

                                BLACKENING ALUMINUM
                                         by
                                  JEROME KIMBERLIN

              There are several ways to color aluminum black and among them
         are black anodizing and paint.  You could rub dirt into the
         aluminum surface, I suppose, but of all the methods, I think
         chemical coloring is the superior method.  It is certainly
         cheaper, faster, and home use allows the model engineer greater
         flexibility in the timing of his decoration of models in progress.
              Surface preparation of parts to be colored black is all
         important as any irregularities are not covered by this finish.
         Paint does build up and fill in scratches and other voids.
         Castings, however, should look like castings if the prototype used
         castings, so surface finish is always adjustable to the builders
         idea.  The point here is to emphasize that this blackening
         technique will not cover up mistakes.
              You will need three chemicals.  These are: Nitric Acid,
         Copper Nitrate, and Potassium Permanganate.  You will also need
         some good quality water - either distilled or deionized.  I will
         give the dimensions of the mixture in both metric and English
         units so that both types of measures are accommodated:
              Take:    water         3 quarts          750ml
              Add      Acid          1/2 oz              5ml
              Add      Copper        3 oz               25gm
              Add      Permanganate  1 oz               10gm
              Add      Water to make 1 gal              1 liter

              Obviously you will have to make up more or less solution to
         fill the container you will use to color aluminum parts and the
         parts to be colored should be completely covered by the solution.
         You should use a glass or plastic container.  A metal container
         will poison the solution prematurely.
              At 75 degrees F (24 C) temperature, the blackening process
         will take about 15 minutes using a fresh solution.  If it takes
         longer it means the solution is deficient in one of the compo
         nents.  Usually, copper nitrate and nitric acid need be added.
              Aluminum is a strange metal to most of us.  While we cannot
         see it, the surface of a newly machined or cleaned piece of
         aluminum combines with oxygen in the air to form a self protecting
         coating of aluminum oxide.  This happens within minutes.  If this
         surface continues to grow (get thicker) the blackening solution
         described here will not work satisfactorily.  Thus, the piece to
         be colored should be cleaned just before immersing into the
         coloring solution.  In my experience, glass bead blasting is a
         superior way to clean the aluminum surface and the choice of bead
         size determines surface finish.  Once the bead blasting has been
         accomplished, the beads can be washed off with hot water and the
         aluminum piece immersed in the blackening solution.  I recommend
         that the time between blasting (cleaning) and immersion in the
         blackening solution be less than two hours.  I once waited five
         hours and was disappointed in the results.  Once the blackening
         process has been completed, wash off the workpiece with tap water,
         drain and spray with WD-40 or other water displacing oil.

              There are a number of ways to clean aluminum satisfactorily.
         It is possible to simply sand the surface clean, or scrub it clean
         with an abrasive.  One can also chem clean aluminum by degreasing
         the workpiece then dipping it into lye (Draino, for instance) for
         a few minutes or seconds as required, then rinsing.  The shape of
         the workpiece and the model engineer's facilities often dictate
         what method of surface preparation will be used.
              Model engineers wishing to use this solution to blacken
         aluminum castings or other parts should be aware that the chemical
         components may be hazardous.  While the solution itself is not
         particularly dangerous it can make your hands purple, so use
         rubber or plastic gloves.  Potassium Permanganate is classified as
         an oxidizer even though dilute solutions of it are used throughout
         the world to sterilize vegetables used in salads, etc.  Concen
         trated nitric acid is just plain bad.  The technique for using it
         is to pour out a little in a glass container and then use an eye
         dropper to transfer the liquid to a measuring container when the
         volume wanted is small, such as that described here.  Nitric acid
         also turns your hands yellow, hurts, and removes fingerprints.  A
         good way to avoid eye damage is to wear a face shield such as the
         one you should be wearing when working in front of your grinder.

I think I wrote this in Wordperfect 5.1 for DOS.  I hope those interested 
do not have too much trouble interpreting...

JerryK