As it is too much deviating from the subject "masks" and MM's comments overthere:

"but was also used for the blue-black, protective, anti-rust patina, that could be imparted on iron and steel (usually armour or weapons) by treating with acid. On iron this protective metallic black patina is primarily ferrous oxide, as opposed to undesirable crumbly red rust which is ferric oxide."

It started all with the term "ferrous oxide"...
In the former factory we had a process of steel proctection with "iron phosphate" which  when not applied correctly giving instead of the bluish colour, where the layer wasn't thick enough an iridescent effect...later we had zinc phosphating which was more protectif and adherent. Test the system once by dipping an iron plate in Coca Cola...

But I soon realized that it couldn't be phosphoric acid...while it was only invented recently...

Again starting with "ferrous oxide" BTW I learned in "my" time also "ferrous oxide" instead of the nowadays " Fe II oxide
https://en.wikipedia.org/wiki/Iron(II)_oxide
in comparison with "ferric oxide" Fe III oxide
https://en.wikipedia.org/wiki/Iron(III)_oxide

Seeking for steel protection with ferrous oxide I came nowhere...although I learned along the way a lot of protecting methods for steel among others for spades and all,  a method that a peasant could use with potato dipped in... I already forgot...

But as we had in the factory once a catastrophy with "blackened" "zinc plated?" parts for use outside...
And with that term "blackening" I came endly to: bluing
In the meantime I had difficulties because they spoke about the colour bluish too...and I made from that adjectif "bluishing" (making blue)...I learned today that "blue" in English can be and adjectif, a noun and a verb...
https://en.wikipedia.org/wiki/Bluing_(steel)

From the wiki:
"Rust bluing was developed between hot and cold bluing processes. It was originally used by gunsmiths in the 19th century to blue firearms prior to the development of hot bluing processes. The process was to coat the gun parts in an acid solution, let the parts rust uniformly, then immerse the parts in boiling water to convert the red oxide Fe₂O₃ to black oxide Fe₃O₄, which forms a more protective, stable coating than the red oxide. The boiling water also removes any remaining residue from the applied acid solution (often nitric acid and hydrochloric acid diluted in water). Then loose oxide was carded (scrubbed) off, using a carding brush or wheel. A carding brush is a wire brush with soft, thin (usually about 0.002 inch thick) wires. This process is repeated until the desired depth of color is achieved or the metal simply doesn't color further. This is one of the reasons rust and fume bluing are generally more rust resistant than other methods. The parts are then oiled and allowed to stand overnight. This process leaves a deep blue-black finish.Modern home hobbyist versions of this process typically use a hydrogen peroxide and salt solution, sometimes with vinegar, for the rusting step to avoid the need for more dangerous acids.^[3]







And yes MM, red oxide Fe₂O₃ to black oxide Fe₃O₄


https://en.wikipedia.org/wiki/Iron(II,III)_oxide

And yes some ferrous and some ferric...

Kind regards from Paul.

PaulRyckier wrote:

In the former factory we had a process of steel proctection with "iron phosphate" which  when not applied correctly giving instead of the bluish colour, where the layer wasn't thick enough an iridescent effect...later we had zinc phosphating which was more protectif and adherent. Test the system once by dipping an iron plate in Coca Cola...

Mention of hydrogen peroxide on the Soap thread made me think of the power and versatility of the various oxide and peroxide compounds. I’m neither a chemist nor a metallurgist but I’m fascinated by the techniques used to temper and protect assorted metals particularly steel. Being a layman in this I had once imagined that stainless steel was an absolute end-product which couldn’t be improved upon. I had also thought that stainless steel had been developed in the 1840s but was surprised to discover that it was only in the 1910s that commercial production of it began. I must have been confusing chromium steel with stainless steel all the while.

I recall hearing of a ‘flawless steel ingot’ which caused a stir when exhibited by Alfred Krupp at the London exhibition of 1851 and the Paris exhibition of 1855. I suppose the word ‘flawless’ is what caused the confusion here. I’ve never seen a picture of that ingot nor heard of its fate so it’s difficult to imagine what it looked like.

In a back courtyard of the Deutshes Museum in Munich, however, there is a huge 7-ton steel ingot wrought specifically for the museum by the Krupp steelworks in 1906. It’s not stainless steel and I’m not sure how long it has stood outdoors but it has nevertheless weathered seemingly well and, whether by accident or by design, has a blue-grey tinge to it.