Hi Jerome,
Can this f> 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