Recently, while working on a table, I needed some drive screws. Drive screws are actually rivets that have a loose spiral threading allowing them to be tapped in blind with a hammer. They can be difficult to source sometimes, but I eventually found some and ordered two bags, each of a different size rivet. The problem happened when I realized I had only ordered bright zinc plated rivets.
I love the look of raw unpainted steel. It’s subtle coloring variation is hard to mimic with paint. Well worn steel is especially nice, its red rust giving way to a natural black oxide coating in turn giving way to the shine of freshly ground and polished edges and joints. It is this coloration I try to preserve in my work. Zinc plating is pretty much the opposite as while it serves a purpose in corrosion prevention, that is not what I am looking for at this time.
So the solution is cold blacking. And it worked great. I used Birchwood Casey brand Presto Blackener gel.
Here is the process. I actually remembered to take photos as I went too.
I figured the bag the rivets came in would serve perfectly well as a marinade container.
I used maybe a cap full and it was probably still too much. The reaction is fast. You can actually watch the color transition. I sealed the bag and shook it around, mixing the gel around the hundred or so rivets in the bag. After maybe 60 seconds, they looked well blackened.
As I said before I probably used too much. Luckily I had a second bag of smaller drive screws and my OCD is not strong enough to prevent me from mixing the two sizes to make use of the extra gel.
Didn’t spill a single river while pour the second bag into the first and using my other hand to take the photos.
This shows the great difference in color between the blackened and the zinc. Keep in mind this is a chemical reaction and not a paint. The thickness of the coating is measured in single digit microns, meaning it is essentially on dimensional.
After mixing in the second bag of drive screws and mixing everything well. I flushed the bag with tap water to stop the reaction and remove any risidual gel before I laid them out on a red shop rag to air dry.
M. R. Nuss Design Co. 
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