Use ‘Gun Blue’ to Instantly Blacken Hardware

I use blacksmith-made hardware whenever I (or my customers) can afford it. But if you’re on a budget or have hundreds of pieces of hardware for a project, it’s time to visit the gun store.

My favorite way to blacken steel, iron or even zinc-plated hardware is to brush on a thin coat of liquid “gun blue” – typically a combination of selenious acid, nitric acid and cupric sulfate. If you brush on a thin coat it will blacken a ferrous metal almost instantly, including zinc-plated hardware. The coating is fairly durable and looks like it has been tarnished by years of age.

One coat will color the metal inconsistently, which is the way I like it. You can make the color more uniform by adding a second or third coat.

It’s nasty stuff – check out the MSDS on any of the brands. Wear nitrile gloves, safety glasses and whatever else you have on hand. It will burn your skin, especially if you have a small cut.

For large pieces of hardware, such as the corner bracket shown in the photos, I simply brush on the gun blue solution with a chip brush. To darken screws, bolts etc. I dip them in a shallow bath of the solution. Two tablespoons of gun blue can blacken almost 150 screws before the solution is exhausted.

After applying the gun blue, wash off the part in cold water and let it dry. Then I add a light coat of oil to add a little sheen and some more rust protection.

I’ve been using liquid gun blue on hardware for almost 20 years now, and the color is still the same dark blue-black today as it was when I first applied it. If you do need to touch up pieces that see a lot of abuse, they even make touch-up pens filled with gun blue instead of ink.

I know there are other chemicals and products out there that are safer than liquid gun blue, but I have yet to find any that are as fast or effective.

— Christopher Schwarz