Hi Jerome,

Can this finish be used in long-term exterior conditions? Does it rub off with use or is the color part of the aluminum?

On Saturday, August 12, 1995 at 3:00:00 AM UTC-4, J. Kimberlin wrote:
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 a 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