Those black spots on the metal. What is that?

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A disease.? Granted I beat up on that old Stanley chisel building fences, a deck and a shed and there was a neglected rust spot on it for awhile. But I cleaned it up along time ago. Now the metal at that spot has sort of eroded away a little and there's a black spot there. Doesn't look like rust.. I sanded on it today with some wet and dry sand paper some came off. What's with the black. Is this black rust?
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Jim Hall wrote:

When you polish/sharpen the chisel, it creates fine metal particles which appear black to normal vision. These may have collected in the etched spot.
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It's iron oxide.
Contrary to popular belief, rust and iron oxide are in fact not the same thing. Rust is *hydrated* iron oxide (iron oxide plus water). When iron or steel gets wet and begins to rust, it's actually a two-step process: the water accelerates the formation of iron oxide -- which is black -- and then combines with the top layer it to form rust. When you remove the rust, there's still going to be a spot of black iron oxide beneath it.
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Doug Miller wrote:

Which eventually flakes off (disintegrates? evaporates?) and leaves the dreaded pitting. Little pits on the back of a chisel can be a royal pain to lap out.
I hope the chisel was one of the newer old Stanley chisels, because the older old Stanley chisels were beautiful tools and worth a fair amount.
R
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Doug Miller wrote:

Accually there are 2 forms of iron oxide one of which appears black and inhibits further oxidation and one that is orange and promotes further oxidation. Water is not necessary.
ron
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wrote:

Ummm... actually there are *four* forms of iron oxide.
From the Handbook of Chemistry and Physics: FeO -- black crystals Fe2O3 -- red-brown to black crystals Fe3O4 -- black crystals or red-black powder Fe2O3*xH2O -- red-brown amorphous powder -- that's rust.
Water *is* necessary for rust.
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Doug Miller wrote:

So you're saying if i put a piece of metal in a closed container that was evacuated then filled with a high concentration of oxygen it would not rust?
Water can act as a catalyst for rust but so can many other chemicals. Above you seem to define rust as hydrated iron oxide. Generally when someone speaks of rust he is refering to any iron oxide.
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wrote:

That is correct. It would oxidize, but it would not rust.

I doubt that very much. Generally, when people speak of rust, in my experience, they're talking about the red stuff that forms on iron or steel that has gotten wet, never about the black oxidation that forms on iron or steel that has been exposed to air but kept dry.
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So the black stuff is just another form of oxide, like rust (red stuff) from metal getting wet and you treat it similarly..? Scotch brite if off and put a sealer, topcoat, camelia oil or other protection on afterwards. Even though it may show up in the same spot weeks or months later, you can slow the deterioration by cleaning off the black and protecting the metal. Does that sound right.? Black rust, man, I never realized oxidation was anything other than red stuff.. Thanks for info..
wrote:

and
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steel
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There's one big difference: the black iron oxide is crystalline and hard. It doesn't flake off under light pressure the way rust does, and if it remains dry, does not contribute materially to deterioration of the metal.

Ever take a look at really old tools? They're nearly black -- but still solid. IMO black iron oxide is more a cosmetic defect than anything else, and doesn't really need to be removed -- as long as you keep it dry. Moisture is the enemy of iron, even water vapor. So.... polish it off if you like, or leave it alone, but definitely put some sort of sealer over it.

Not quite right. If it's black, it isn't rust. Oxidation turns iron black or dark reddish-brown, and is not in and of itself harmful if kept dry. Oxidation plus water makes rust.
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Doug Miller (alphageek at milmac dot com)
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Thanks, Doug. That was helpful..
wrote:

from
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remains
slow
Does
solid.
doesn't
enemy
or
Oxidation
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Before the advent of modern chemical bluing, gun barrels were colored by a controlled rusting process leaving the barrel black.
wrote:

from
It
remains
slow
Does
solid.
doesn't
enemy
or
Oxidation
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"Controlled rusting"? Or controlled oxidation?
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Doug Miller (alphageek at milmac dot com)
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The process was known as cold rusting. Rust (red) would actually be formed and would be polished off leaving the black oxide. To get a uniform surface, the rusting and polishing have to be repeated a number of times. The metal needs to be polished when the rust layer is light. If left to rust to long, pitting would occur. When I was a kid, I used to do this with knife blades. It took some time to get a uniform finish but, once you did, it was quite resistant to further rusting. I thought that I had come up with quite the process. Later I learned how old the process actually was. Reinventing the wheel, as it were.
wrote:

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Interesting. Didn't know that. Thanks for the explanation.

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Doug Miller (alphageek at milmac dot com)
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CW wrote:

Generally known as "browning" in the UK. Still fairly common amongst gunsmiths, especially as legal UK shooters are now generally firing antique black powder kit.
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Very common here at one time also. My grandfather's old Damascus barreled shotguns that I grew up duck hunting with were not "blued", but "browned".
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snipped-for-privacy@milmac.com (Doug Miller) wrote in

So does that rust converter stuff remove the hydrated iron oxide leaving only the non-hydrated stuff?
Puckdropper
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Wise is the man who attempts to answer his question before asking it.

To email me directly, send a message to puckdropper (at) fastmail.fm
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Don't know, sorry. Best to direct that question to the manufacturer.
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Doug Miller (alphageek at milmac dot com)
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Jim Hall wrote:

Yes. Rust comes in orange and black (with lots of browns in-between) depending on the oxidation state of the iron (which sets the ratio between number of iron and oxygen atoms in a rust molecule) and also the hydration state. Heat it, electrolyse it, and you cna convert the reddish sorts to the blackish sorts.
If you go to Clearwell Caves, you can even buy pretty purple "ochre", which is really just purple rust (and not cheap either). It has been mined there as a pigment, since Roman times.
As to your chisel, then just ignore minor pits. Keep them clean, dry, and oiled with a non-staining oil. Ideally keep them polished too, but I certainly wouldn't lap good steel off the back of a decent old chisel, just to shift a trivial pit or two.
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