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Chemical crystal ID?

When I clean wrought iron or steel pieces I pickle the ferrous metal
in a solution of hydrogen peroxide and vinegar. After leaving the
ferrous metal in the pickle for a while the solution turns to a
cobalt blue color. Recently I left an open jar of this solution on a
table for a few weeks and it evaporated. Today I looked in the
bottom of the jar and it was covered with small 1 to 2 mm crystals, a
deep blue green in color. From the color I would suspect that they
were some sort of copper containing substance but since there was no
copper involved, do any of you chemical wizards out there have have
an idea of just what these crystals might be?

Jerry in Kodiak

do any of you chemical wizards out there have have an idea of just
what these crystals might be? 

I don’t know what the reaction is with acetic acid and peroxide -
it’s fairly complex, I know that. The question is, are you simply
pickling bare steel, or is this after welding or something? That is,
does it have a flux on it? The nearest I can come up with is that
you’ve made Prussian Blue - Iron Ferrocyanide. The problem with that
is that it needs nitrogen (in solution), and nothing you say contains
it. Prussian blue is not too far away in color from the windows bar
across the top of explorer - deeper maybe, but about the same hue.

The question is, are you simply pickling bare steel, or is this
after welding or something? That is, does it have a flux on it? 

There could have been some boric acid flux as well as gold. I think
I’ll experiment a little bit and see if it still does it with steel

Jerry in Kodiak

Take a deep breath, sit down, get comfy. I’ll say that I’m a good
amateur chemist - any pros want to speak up, I’ll certainly defer to
that. And that I don’t actually know this off the top of my head,
it’s research. And the easy answer is I don’t know what your blue
crystals are. You have acetic acid and hydrogen peroxide - those are
your reactive agents. Those put together under certain conditions
make peracetic acid, which is an organic peroxide. Peracetic acid is
linked to polymerization, benazene, alkynes (acetylene), and alkenes
(methyl ethyl ketones). In other words, that organic peroxide steps
across the line from simple inorganic chemistry into much more. What
this means is pretty simple: if you put iron into acetic acid,
you’ll get iron acetate, which is red. If you add boron (boric acid)
which has a green flame test, well, that’s what’s called a metalloid

  • it can go AC or DC depending on the circumstances. Plus it’s
    trivalent. You get into boranes, adducts, perborates (from/with H2O2)
    Then you throw in hydrogen peroxide, which is just aching to get rid
    of that extra oxygen (H2O2), and you have quite the soup mix - an
    organic acid with a peroxide with a trivalent metalloid and iron,
    which is chemically versatile to begin with. It’s well beyond my
    ability to sort out, but I’d say you’re looking at a more complex
    molecule than something like “iron acetate” - more like “iron
    dihydrogen tetraborate”, which I just made up, that’s not a real
    thing, most likely. I will say that acetates tend towards the blues
    and greens, though. Otherwise that should make it all very clear,
    right? HAHAHAHA…

Jerry, I’ve written a bit on this, but after sleeping on it again…
You say there’s no copper involved. If there’s copper - brass from
brazing? then all of it is very simple - it’s copper acetate, which
is a very beautiful cobalt blue color. It’s the fact that you say
there’s none that’s curious…

John, Thanks so much for all the effort you have put in to answering
my question of the blue crystals. I’m overwhelmed! As for the
possible introduction of copper into the pickle, since I was
soldering gold to the wrought iron/steel perhaps the copper in the
alloy leached into the pickle? The color certainly is a rich cobalt


I learned this the hard way when cleaning some gold wires I mined
one time out of breckenridge, Colorado

gold, in the presence of iron, and free chlorine, will form gold
chloride (AuCl) responsible for the rich, cobalt blue color you see
in medieval stained glass.

this is probably what you have

Mark Zirinsky

Hi Jerry… URMVQW! I enjoy chemistry - I’m only SO good at it,
though. Your problem is to me an interesting puzzle - a little brain
exercise. And just saying, “It’s that!” from your solution is beyond
me, as I said. Now you’ve injected gold (alloy, actually - the gold
won’t do anything) and solder. Again, there’s a huge neon pointer
saying “Copper, Copper”, but you say there’s none. In that case,
it’s really simple - it’s some copper thing - acetate or something. I
went to just for an example - I just randomly
picked something called “Arthurite”, which has the formula:


That’s iron, copper, arsenic, oxide, phosphate, sulfate, basic and
hydrated, all rolled into one. Just an example of how mother nature
can make things that one wouldn’t conceive of. Maybe your crystals
are simple molecules, like copper acetate - I just wonder if you
have a more complex thing going on though.

I am profoundly embarrassed to conclude that whatever was in the jar
was probably not what I thought it was. :-[ I tried to duplicate the
effect a couple of days ago by mixing up a half and half mix of
hydrogen peroxide and vinegar and added iron… As John indicated
earlier it should have turned red. It turned red. I added gold and
copper. still red, but it cleaned the copper nicely. I can’t believe
I added Chlorox to the first mix since I don’t keep it in the studio.
The only other thing I keep there is a jug of ammonia. I guess I
could have absent mindedly used that instead. If that’s not
apossibility I’m fresh out of ideas.


I added Chlorox to the first mix since I don't keep it in the
studio. The only other thing I keep there is a jug of ammonia 

Jerry, the only problem with the fact that you’re driving yourself
nuts over a trivial thing is that I do it, too, sometimes!! LOL…
There’s an old stage trick of a guy who get concentrated HCL, pours
it onto some metal, it pops and smokes and fumes, the crowd goes,
“ooh, aaah”. Then he gets NaOH (lye solution), does another demo,
the crowd goes “ooh, aaah”. Then he pours the two together, stirs it,
and drinks it down. “OOOH, AAAH!” And of course what he’s drinking is
salt water. That’s what can happen when you just start mixing things
together. Meaning that mixing acid, peroxide, ammonia and chlorine
can shorten your life - and give unintended results. You say you
don’t have chlorine bleach around, but you know that mixing it and
ammonia is a very, very bad thing to do, right? Phosgene gas…
Anyway, if you had put ammonia into the mix, you would have nothing.
The acetic acid and the ammonia base would immediately combine,
you’d have ammonium acetate (which is white) and water at a
corresponding neutral Ph. Mixing an acid and a base doesn’t double
your kick, it gives you a salt and water. In your experiment, you put
iron into acetic acid, which reacted. Likely it also depleted the
acid, since vinegar’s only 5%. If you want to do it, put everything
in at once, or even better, put each metal in it’s own container, and
then try two together, with and without peroxide, etc. Controlled