DIY Steel Blackening with Vinegar – Why the Uneven Finish?

Hi All!
Trying to blacken steel parts with vinegar, l resulted in uneven surface colouring.
Can't figure out what it was that may have done wrong.
Is there someone here who has succeeded blackening parts with this method?

Hey Delta!

Yep, a few folk here have experimented with vinegar blackening / forced patina methods over the years, uneven colouring is usually down to one or more things from what i've seen:

Surface prep, any oil, fingerprints, polishing compound, or mill scale left on the steel can stop the reaction taking evenly, always remember, the cleaner the metal, the better the finish. Acetone or brake cleaner is a solid choice you can use beforehand and will help a lot.

Steel composition, different steels react very differently Mild steel tends to work better than some alloy steels or stainless.
Oxide layer inconsistency, if parts have been heated, welded, or previously oxidised unevenly, the vinegar can react "patchily".
Solution strength / contamination, old vinegar or reused solution can give inconsistent results, always use fresh juice.
Temperature, slightly warming the part or solution can help the reaction become more even, nothing crazy just take any chill out the metal. I cant remember if this one works for Anodising too !?!

A lot of people doing the “vinegar blackening” method are actually using vinegar and hydrogen peroxide with salt, which tends to create a more aggressive oxidation layer before oiling and/or waxing the finish.

One thing I’ve noticed myself is the finish often looks terrible until you wipe the loose oxide off, repeat the process a few times and then seal it with oil or wax. That’s usually when it suddenly starts looking much more uniform and deeper black rather than patchy grey/brown.

Tell us what type of steel your using, what prep you did and maybe a photo of the finish you got.

Be interesting to see what methods others here have had success with :beer:

Thanks for the informative response Lee.
You know, for me, the vinegar approach was a way to escape the harse chemicals trap eventhough the NaOH + NaNO3 method gives me top results every single time.
Yes, i understand that for two main reasons these parts came with uneven oxide color.
First and foremost must have been poor cleaning as you very rightful say with second the spring steel alloy elements contained in it.
This last one i can not control.
So the parts i am trying to darken are made in a progressive die and the blanks are 0.3mm thin spring steel.
But...trying to mark some of these parts today by etching with FeCl3 which was the left over from a PC board work of the past.
I came across a pleasant surprise.
I accidentally found a new oxidizing method.
That of FeCl3. And this approach is speedy. It blackens the parts almost instanly. I mean several seconds after immersion.
I tried to upload images of these parts yesterday but i failled to locate the upload link or button in this page or elsewhere in the forun. I was looking for the "add an attatchment" icon
but it wasn't there.

Cheers!