I got in on this post a little late so if this is a repeat please
forgive me.
I have no idea if it works or not but looking at the technique I
would suspect it should. If you can’t get the PDF from this link
drop me a note and I’ll send it to you.
…
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
http://www.ganoksin.com/gnkurl/1ku
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
…
Good Luck. Dan.
Dan.
Dearmond Tool