@johnabs @freemo Gents - you can form an a thin layer of magnetite by heating or by electro-chemical means and it has the blue colour. due to interference. The relevant bit is that it is a barrier to oxygen diffusion so the underlying metal doesn't oxidise any further in air. Oxides on aluminium and stainless ( Cr oxide) act in the same way without needing any intervention ("self-passivation") but not on iron, which forms increasing amounts of (hydrated) oxides ("rust") if left alone.

@SteelFolk @freemo Thanks for the clarification! I couldn't recall if it was exactly the same thing (magnetite vs standard rust) anymore 😅

I used to do a lot of surface chemistry before a big career switch, so I couldn't recall the exact details, but I was mostly commenting on the structural coloring caused by the technique (thus, the technique's apparent namesake), even though the passivation is the intended application 😁

@johnabs @freemo You'll remember that there are nearly always layers of different oxides due the differing oxygen activities. It's also affected by iron activity too if it interacts with alloy elements, so pure iron rusts like like hell, which was why car bodies did at door bottoms when they were made with 'rimming' steel aimed at superior surface quality.

@SteelFolk @johnabs

Yup and thats cause the natural type of rust on iron (red rust) is brittle.. black rust created intentionally however is stronger without the cracking and chipping.

@freemo @SteelFolk Yeah, but my point was that despite the black color of the magnetite, as long as the film is thin enough, the color of the part can be iridescent blue, same thing with other structural colors: proteins that are normally drab brown can still be all sorts of colors as long as you make the light interfere in certain ways, which is neat IMO 😁

@johnabs @SteelFolk yes but the color blue in this case isnt from light interference like it is in the other examples. Hemetite is just a very dark blue color and looks blue when relatively transparent but black when thicker.

@freemo @SteelFolk If you watch that video, you'll see a different effect than what you're referring to which is apparent from the iridescence of the coating, additionally, the film changes color to gold first, then blue, based on temperature, which is indicative of the thin film effect, as noted here, https://www.tandfonline.com/doi/full/10.1080/00202967.2022.2154495, in the section on Thermal Tinting.

@johnabs

I honestly cant find any source that addresses if it is the thin film effect or not.. several reasons I dont think it is.

1) If it were thin film you should see a rainbow of colors develop, rather we see either a reddish color (its powder color) or a shade of purplish blue.. but you wont see other colors, like green, develop

2) hemetite that is thin enough to see through, but still far too thick for thin film, will still have a bluish quality to it

3) The thin film effects I know of are with transparent oxides like aluminum and titanium. Hematite oxides are not transparent.

All that said, hell if i know, maybe it is thin film and there is some explanation I dont know of where it could only create blue interference.

@SteelFolk

@freemo @SteelFolk

1. You do see the multicolor effect in the event the film is not uniformly thick, which is what you can observe in that article on the thermal treatment. However, a uniformly thick film can only reflect a specific wavelength (here's an example for titanium anodizing https://youtu.be/O2RaIJhZ81I?t=755): this is visible from the first part of the article I linked in which they deposited AlTiN on steel and achieved similar coloration to blueing, and similar evolution of the colors based on the films' thicknesses in nanometers. Also note that a large factor in the final color's iridescence is the surface finish: rougher finishes will look less iridescent.

2. [Hematite](https://en.wikipedia.org/wiki/Hematite?useskin=vector) and [magnetite](https://en.wikipedia.org/wiki/Magnetite?useskin=vector) are not blue even in low concentrations or when powdered finely, only black or red, or brown depending on the crystalline structure. If this were the explanation, you would not expect to see gold and "straw" colors appear in the beginning of the hot blueing process, only blues that get progressively darker.

3. The material does not need to be completely transparent, as this layer has to be thinner than the wavelength of light it's reflecting (thus on the nanometer range). Based on the fact that light is wavelike, it can penetrate into thin films of materials that are normally opaque at larger scales. As an example, it occurs on those structural proteins I mentioned which aren't transparent either.

If I'm wrong, I'm pretty sure @SteelFolk can probably correct me, but I'm fairly certain it's the same process as what's going on in the Titanium and Aluminum versions.

Oh, and thanks for the fun discussion, it's been a while since I've been able to talk about this stuff :D

@johnabs @SteelFolk

Im less certain of my stance the more we talk.

And to point 1 i see only shades of red and blue, red being henetites streak color and blue being its crystaline color. The reason im skeptical is cecause witb thin film i usually expect the full color spectrum including greens.

@freemo @johnabs I can add that Hemetite is reddy-brown when it's in piles of 1000s on tonnes. Magnetite looks black, I've never seen the blue colour on the rock itself, so I supposed it was interference when you heat up a bit of steel. I doubt anyone has looked at it in detail!

@SteelFolk

Ok so I think there is some confusion here... Hemetite is red "true color" and dark-blue to grey "apparent color" (that is, its color in a crystalline form). The true color is also called its powder color or steak color. In other words, when its solid it will look silvery gray to dark-blue... when crushed to a powder it will look red.

Magnetite is a some what different substance. Hematite is pure black ruse, or Iron III oxide, the red rust you are used to seeing on iron is Iron II oxide. Magnetite is a mix of Iron II oxide, Iron III oxide, and Iron in a specific crystalline form

Magnetite is magnetic, other forms of iron rust are not. Hemetite has a metallic like luster where magnetite does not. Magnetite can also form several crystalline forms including cubic forms (which are very rare).

Magnetites streak color and apparent color are the same, black.

@johnabs

@freemo @SteelFolk Please don't take this the wrong way, but could you source that claim on the hematite's true color? I've looked everywhere and cant find confirmation.

And even if we take that the color is as you say, the likelyhood of growing unadulterated crystalline hematite that would have this color in such a thin film, while adjacent to metals with their own crystal structures that would impact the hematite crystals seems low and doesn't explain the gold/straw color during the hot blueing process either.

@johnabs @SteelFolk

Yea, as I stated true color is also called streak color or powder color. You will find it more often refered to as streak color.

See here for a source: https://geology.ecu.edu/geol1501/mineral/hematite/

@freemo @SteelFolk Right, and looking at this source, I don't see any mention of blue, hence my reservation.

@freemo @SteelFolk edit, my apologies, I meant the apparent color in the original comment

@johnabs @SteelFolk You asked about its true color (reddish) not its apparent color (dark blue).

Searching for the blue tint is going to be hard since most hits are non scientific. Also you have to make sure you are looking for the apparent color and not real color.

You can find a source here: https://www.webmineral.com/data/Hematite.shtml

Under "Reflected color" it describes hematite as "White to gray white, with a bluish tint, with deep blood-red internal reflections."

@freemo @johnabs It fascinates me to see haematite jewellery looking so metallic. I understand its name is derived from a similarity with blood. When you see a dusty ore mine you can see why, especially when it's wet, but it is more orange like a robin's breast really.

@freemo That's good. Much iron and steel terminology causes confusion - even 'iron' is loose, so if you say steel is refined iron, chemists everywhere object.

@SteelFolk steel is usually iron doped with carbon.

@freemo We spend most of the time removing carbon from iron. And on average a steel's Mn content is 5X that of its C content. C is insoluble in iron at service temps. Most steel makers don't know those points!

@SteelFolk Yes, too much carbon is bad of course. Steel is carbon doped but we are usually talking like 1% carbon or less. In fact more than something like 0.5% carbon content is considered high carbon steel if memory serves.

@freemo @SteelFolk @johnabs

Regarding color, my understanding (without looking anything up) is that structural color is a general term that refers to any color that results from an interference pattern from multiple waves emanating from a material.

The term "thin film interference" is a specific case of structural color which occurs when two layered media of differing refractive index allow light to both reflect and pass through the upper thin layer and reflect from the bottom layer (the interface of two differing media) resulting in two reflected waves that interfere and create a color. (examples, oil on water or soap bubbles)

The reason you get rainbow colors with thin film interference is because either the thin layer varies in thickness or also because of the angle of incidence -- anything viewed off of 90° is going to be a longer distance between the upper and lower layer which results in a different frequency of interference. This is why the colors change as you change the viewing angle.

@freemo Iron made in a blast furnace is saturated with C and is about 4.5%.This is a crude cast iron and we reduce it to 0.06% to make mild steel. Above around 0.8%C it forms Fe3C (Cementite) and looses toughness but is hard, so used for tools which shouldn't need to resist impacts. These are rough guides and are equilibrium based. Once you add other alloys or do heat treatments, everything is different.