I've been doing a lot of anodizing tests and trying to retrace how they likely did the type III hardcoat seen in Vietnam on the colt 603 / M16A1 and other variants.
ETA: The originals were dyed Grey. Thanks live4nov for sending of the original examples .
Here is a photo of one of my samples between a modern black lower and a reproduction of an m16a1

Nothing was done to the sample to prep the surface. This sample varied from very rough to smooth and I had a number of pieces of scrap cut this way to test so I didn't bead blast them. I'm anodizing some parts now that had surface prep and will post photos of them later.

You can test how resilient the oxide layer is by placing various chemicals on the anodized aluminum. In this case I used naval jelly.

The next test I performed was to see if any dye was used on the reproduction. I used a mixture of Pure Lye and water. Dipping a Q-Tip in the mixture and rubbing it around on the part will strip the oxide layer and the dye will soak into the q-tip. My part does not have any dye, while the other 2 lowers are dyed grey top and black bottom.

 

Milspec anodizing is sealed with Nickle Acetate, that would tend to skew your comparisons I think. I don't know what effect that would have on the appearance. I'm not sure when they started that either.

I do know the old mil items seemed much smoother of a surface than just a bare anodizing.

That could explain what I'm seeing on the Modern lower and why it didn't fade any while the others did during the naval jelly test. Hopefully I can get my hands on an original part of the era to gain more insight.
I can Nickle Acetate seal my parts, but I did do this on the scrap pieces as I've not see any significant appearance changes. I'll have to run the naval jelly test again on one of my parts that is sealed with it to see if it becomes as tough as the modern lower.
 

Interesting experiment. It might be worth picking up one of the broken 603 uppers from Royal Tiger to use as a control if you can get one that hasn't been refinished.



http://www.royaltigerimports.com/product-p/00053.htm

Awesome thanks for doing this! I have some demil XM parts I can send you and some later M16 pieces too.

So here's a question. Black lugs on the early uppers. What is that all about?

Cool that you are doing this, could the different alloys have something to do with it?  Older parts are 6061 and newer are 7075, right?

I don't think any production AR15/M16s were 6061, I could be wrong though.

I think up till 1966 or 1967 they were 6061, early NDS parts were 6061 too.  (some of their 0% lower forgings were stated to be 6061 IIRC)



I've never heard of the nickle acetate coating, do you have more info / link?

I have a question....

I've noticed that some of the early grey uppers had black lugs on the bottom (where the pivot and takedown pins went through).

Here's a pic that I borrowed from the excellent variation guide that shows what I am referring to...



Why the color differences?
It seems unlikely to me that they would anodize just the lugs a different color.

While looking at one of my modern flat top uppers yesterday, I noticed that the inside (where a dry film lubricant had been applied) is almost the same exact grey color of the old guns.

Could this also be the reason for the grey coloring of the originals?

Sandblasting coverage, grit and pressure can have a real impact the perceived color as well.

I have seen this on some of the stuff I have taken in to have anodized. If I blast it comes back dark gray, If I don't its jet black.

Thanks so much. I got the parts and have been able to determine the originals were dyed grey.



 

I helped set up an anodizing line for a machine shop & read quite a bit about Type 1-3 anodizing process on-line--the info is very easily obtained.

The dye choices for Type 3 hard coat are limited by the hard coat pores. When one looks microscopically at the aluminum oxide formed by anodizing, it looks like deep honey combs that are only 2-3nanometers in diameter. A single red blood cell is about 7 nanometers. So the deeper, narrow pores of type 3 hard coat will only allow small organic  dye molecules that are grey, black green, etc.

Nickel acetate seals the dyes & anodizing--it's not a coating, but nickel acetate can be used in an anodizing set up to nickel coat parts, resulting in a nickle finish.

https://www.google.com/search?q=anodized+aluminum+pores+pics&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#imgdii=Nk-a9pYbLEcX5M%3A%3BDPJjPl-tK5Ua9M%3A%3BDPJjPl-tK5Ua9M%3A&imgrc=DPJjPl-tK5Ua9M%3A

Thanks for the info. I do have a type-3 setup that covers all of that, just trying to replicate the original.
Variables I'm testing to get the best results include

1. blasting grit

2. oxide mil thickness

3. amps per sq ft

4. Temperature

5. Dye used

Etc

Ok, so this is pretty fracking exciting!

Were all of the parts I sent you dyed?

The thing that is most notable to me about the old grey military parts is that they are a very smooth finish vs other bare anodized parts that are a rougher, more abrasive feeling surface. Its almost as if they are coated with something over the anodizing, which is why I wondered about the nickle acetate sealant. Maybe they were using a different amperage or chemical bath or something I dunno. The bronze color Martin Marietta "LM" uppers are also smooth, just a different color.

Progress being made.
Trying to get the color matching down to a science. I take a single photo of the samples, extract a uniform area from each sample and average the color of each part and reduce it to a grayscale rgb value. Then I tweak my anodizing settings mathematically.

The following parts have been beadblasted on the front, but on the backside where there is a lot of chatter and milling marks they are nearly twice as dark in color. So you can  greatly vary the shade by what surface preparation is used ( grit of blasting material, etching, polishing .etc) The below samples are smoother than the original lower, so I ordered some different blasting grits to fine tune it. Take the middle part, if it were slightly darker like the backside of it is, it would be a perfect match. A slightly more aggressive blast medium and it should be good to go.
The bottom 2 were the first runs where I used the color shade method I referenced above, and now I believe it to be a matter of surface preparation.





The below were done with a 2mil oxide layer thickness. I've gone up to 4, but find that 2 works really well and doesn't flake off like thicker coatings. The only variable tweaked on the below was the amps per square ft. The top one had smaller pores and didn't want to take the die.










 

Very impressive research/work you're doing Tech.........and I read that you're doing this in type III ?

Yes it is type-3 hardcoat and when I'm finished should be very much like what was originally used.

THIS^^^^



 

I've been keeping a close eye on this thread.

You sir, are going to get a lot of business from us Retro Heads

You may depend upon it - I'm in for 12 when I get the CNC bit done.

12 of what? 80% receivers, or full stripped receivers? I've not applied for an FFL as of yet, but a single job of 12 would cover the application cost .etc



My intention has been to find out what settings worked for me and to provide info that others could take to small anodizing shops so that there are more options and competitive pricing. I also wanted to put the information out there so that there could be some oversight of my own processes in case I miss something important.



What is the going rate for anodizing a lower?



 

I've got a bunch of lowers as a project for Baceman, then the CNC of the 80%, and finally anodizing.  With US Anodizing charging $100 per for XM Gray - a price increase from when they started, an alternative looks good, even a road trip to Louisiana would be worth my while for somebody who is serious about recreating the correct process. The A4 and M4 clones would be black, and the US Anodizing price for that is much lower.

What temp are you running the acid solution?


The H2SO4 acid bath dissolves the aluminum oxide anodizing as it forms. Balancing voltage & temperature is important to maximize anodizing to the depth that it meets criteria to be "Type 3 hard coat." Cooling the acid slows the dissolution reaction of the newly formed aluminum oxide.

ETA--I read you're measuring depth--how are you doing that?

32deg F is the bath temperature I'm using.



I know of 2 ways to measure the oxide layer thickness.



1. Measure the width of 8 different points on the part and average the results before and after. A 2mil thick layer will increase the width by .004"



2. Determine the before and after weights of the part to see how much was added.






I began with a layer with such dense pores the dye would not take and lowered the amps per sqft until they would. Above 12 amps per sqft and it was too dense.