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I have recently been “brewing” what is commonly called “iron acetate” for ebonizing wood, and I'd like to understand the chemistry of the iron/acetic acid reaction, which should be fairly simple, but turns out to be more complex than I thought.

One previous question relates to the tannin/mordant reaction, a later stage in the process; another asks for feedback on a recipe for iron (II) acetate, which only adds to my confusion; while a third asks about decomposition of iron (III) acetate.

Overview

The basic idea is that you put fine steel wool in white vinegar, the iron reacts with the acetic acid to form an iron (II?, III?) acetate solution; you then then paint this on the wood where the iron acetate reacts with naturally occurring tannins (or introduced by way of previous application of a tannin-rich solution, such as black tea) in the wood, yielding iron tannates, which are dark-colored, thus staining the wood.

Problem

There are many websites, blogs, and articles online describing the ebonizing process and how to make “iron acetate” with varying degrees of utility and accuracy. None of them contain a detailed and accurate description of the chemistry, which I would like to understand.

From the existing online articles, there is a great deal of conflicting information (even to the point of direct contradictions). It's so bad that it is not at all clear to me what the goal of producing the solution is: do we want iron (II) acetate or iron (III) acetate? Does it matter?

Procedure

I put one US quart of warm distilled white household vinegar in a mason jar (5%, ~0.83 mol CH3COOH).

I added two 0000 wire wool pads (~23g, 0.41 mol Fe), unrolled, cut into strips, washed in acetone (to remove protective oil) and dried.

I placed the mason jar inside my vanilla Weber barbecue grill in the hot sun, reasoning that the somewhat elevated temperature might speed up the reaction.

I lightly agitated the contents a couple of times per day with a bamboo skewer, squeezing out bubbles trapped in the wire wool to keep it from floating on the liberated gas (presumably hydrogen).

Throughout this process, the liquid remained clear, although some reddish crud was deposited on the mouth of the mason jar.

As some of the solution evaporated, it was topped up with vinegar. This was done twice or three times over a week. Total additions were at most 4 fl.oz. (100 ml).

After a week or so (maybe 8 days), all of the steel wool appeared to have been consumed, and there was a dark grey or black sludge in the bottom of the mason jar, which could just have been unreacted fragments of steel wool.

There was some (predominantly) charcoal grey “foam” on top, but with a large bright orange-brown spot.

I filtered the liquid through a coffee filter into a transparent plastic container (a washed-out 40 oz. Lysol bottle).

During filtering, the liquid appeared dark, with a distinct green tinge.

The last few ounces were filtering slowly (lots of crud in the filter), so I left it for a couple of hours.

On returning, the solution did not appear green, but had a definite purple tinge (my g/f even said “Why is it purple?”).

Over the last few days, the solution has lost its purplish color and stratified — there are a few millimeters of dark brown precipitate at the bottom (probably not as much as it seems due to the concave bottom of the Lysol bottle), the bulk of the liquid is almost clear orangey brown, there is a narrow but distinct dark band about 35mm from the top, then above that, it is cloudier and slightly lighter-colored than the bulk. Unfortunately, the narrow band and top stratum are too subtle to be seen in the photographs.

Precipitate in filtered solution

On top, there is an orangey-brown deposit around the edge and crystalline flakes floating on the surface.

Surface of filtered solution after a few days

The filter paper has dried, leaving a fine reddish-brown powder.

Dry filter paper

Reddish-brown powder from filter paper

Other Observations

The colorless liquid in the mason jar, prior to filtering, was much more effective than expected. From all online descriptions, iron acetate is not terribly effective on low-tannin woods like pine, yet only a few hours after putting the steel wool into the mason jar, a drop of the solution placed on an untreated pine strip produced a very noticeable graying effect (much like I'd expected from the final result); the next day, the solution produced a mid gray/brown; and after a few days, a dark brown.

Solution on untreated pine

On pine treated with tea, a jet black color was produced by all but the weakest solution, which was dropped on while the pine was still damp with tea (hey, I'm impatient).

Solution on pine painted with tea

Assumptions

Ferrous (iron II) acetate is soluble in water. It is a white crystalline solid. Its tetrahydrate is light green.

Ferric (iron III) acetate is insoluble in water, but soluble in ethanol. It is an orangey brown solid.

The effects of impurities can be ignored.

The distilled vinegar did not have much dissolved oxygen.

Speculation

In the mason jar, the iron in the wire wool reacted with the acetic acid, mostly yielding ferrous acetate tetrahydrate (and hydrogen, which just bubbled off), which accounted for the green tinge of the solution observed during filtration. There was also some ferric acetate created in the presence of the limited dissolved oxygen and that available at the surface; this precipitated out into the gray sludge, which also contained unreacted fragments of wire wool. The orange spot in the surface foam was, most likely, ferric acetate and (?) ferric oxide hydrates.

In the Lysol bottle: on filtration, atmospheric oxygen was introduced to the solution, which enabled some ferrous acetate to transition to ferric acetate (pathway?), as this which is what has precipitated out. This ferrous->ferric reaction (?) is continuing at the surface. Some of the precipitate has been deposited around the edge of the bottle and some has crystallized to form the flakes on the surface.

In the filter paper, the sludge was residual unreacted wire wool, ferric acetate, and ferric oxide hydrates, dampened with ferrous acetate and acetic acid solution. These have reacted and dried out to a mixture of ferric acetate and ferric oxide hydrates, accounting for the orangey brown color.

Questions

  1. Is the above speculation broadly correct?

If the end result of putting wire wool in vinegar ultimately becomes ferric acetate (insoluble precipitate, soluble in ethanol), why don't the instructions ever tell you to filter off the precipitate and dissolve it in denatured alcohol? Surely that would make more sense. If that is the goal, why doesn't the usual recipe call for some hydrogen peroxide?

OTOH, if the desired result (for ebonizing) is ferrous acetate, why don't the instructions tell you to avoid introducing oxygen? Tell you to brew it with an airlock? Warn you that the solution will ultimately “go bad” when the ferrous acetate becomes ferric acetate and precipitates out? Tell you to keep it in an airtight container, use it quickly, etc.

  1. What caused the purple tinge?

Immediately after filtration, the solution appeared purplish. Was this just the mixture of an orangey color and a greeny color as the greenish ferric acetate tetrahydrate turned to orangey ferrous acetate or was there something else going on?

  1. If an aqueous solution of ferrous acetate becomes ferric acetate, which precipitates out, what is the most likely reaction/pathway (under these conditions)?

I'm sure I could work something out, but I'm not sure it'd be correct :)

  1. Are there any simple diagnostic tests I could do?

There may be very simple (backyard/household chemical) tests that would enable me to confirm/refute parts of my speculation or estimate relative concentrations. I'm thinking along the lines of “a drop of that in two drops of ammonia will turn a banana blue” rather than “put 10 ml in a test tube with Ludicrously Expensive's reagent in an argon atmosphere”.

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Ok, without going through all of this experimentally I'll shoot from the hip knowing some chemistry.

(1) With this coloration you don't want a paint that sits on top of the wood.

(2) The desire is to get the iron to wick into the cellulose fibers.

(3) An iron-acetate complex must wick better than a iron ion (or else you could use iron chloride).

The gist here that an iron ion isn't floating around in a water solution as a "free" ion. It really has water molecules which are "bonded" to the atom to form a "complex." The acetate complex must wick better, probably because it is overall uncharged. So you basically can't wick $\ce{Fe^{2+}}$ into the wood and leave $\ce{Cl^{-}}$ in the solution.

(4) If the pH of the solution gets too low, then iron hydroxides form.

(5) The overall reaction of the acetic acid to dissolve the steel wool is: $$\ce{Fe} + \ce{H^+}\ce{OAc^+} \ce{->} \ce{Fe^{2+}}\ce{OAc^{-}_2} + \ce{H_2}$$

The point in (5) is that the reaction consumes $\ce{H^+}$ and the pH will increase (i.e. the solution is less acidic).

I suspect that if the pH gets too low then nasty things happen. I think you want to use pickling vinegar with is 10% acetic acid, not household vinegar which is only 3%. For consistency you also don't want to leave excess steel wool in the solution forever. I'd guess figure on about a 75% reaction. So figure out much steel wool would react with 75% of all of the acetic acid and use that much steel wool. That will insure that you have excess acetic acid which will dry off the ppt.

The staining solution is probably unstable, so just mix what you want to use within some "short" period of time.

The dry solid should be stable if stored in a "sealed" container.

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    $\begingroup$ You have the direction of pH change backwards. The reaction consumes $\ce{H+}$, which means the pH will rise, not drop. At high pHs, the oxidation of iron(II) ions to iron(III) ions occurs more readily. At high pHs, the solubility of iron(III) is much lower and iron (hydr)oxides will form. But you are right when you say the solution is to use excess vinegar. That will keep the pH low. $\endgroup$ – Curt F. Jul 17 '16 at 15:32
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tl;dr. But the object of the vinegar process is to produce soluble iron(III) to react to ferric tannates. (ferrous tannates are more soluble, see iron gall ink, rust converter, etc. in Wikipedia). The easiest way to do that with household chemicals is vinegar. Tannins are a large family of polyphenolic compounds. Their color ranges from orange to brown to black to purple to almost blue (navy blue, say). If this were done industrially, I'd expect iron sulfate would be used instead of iron acetate (check out the solubility of iron(III) salts. Note that in most cases, Fe(III) is more stable (under environmental conditions) than Fe(II), so that you could use the more soluble Fe(II) salts and wait for the Fe to oxidize. Which might take minutes, hours, days, years, IDK. The one thing I'd worry about with sulfates is the fate of the sulfuric acid left behind after the fe tannate precipitates...an acetate would be a LOT gentler on the wood....so perhaps acetate is actually the best choice...IDK.

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My knowledge of chemistry is too non-existent to provide you with the precise answer to your questions but I believe that I have all the data needed to infer it. Since I’m myself interested in understanding what is going on, I’ll share them here hoping that competent people will interpret my findings. It seems I managed to get the staining substance in pure form. Let me explain how I got it.

First of all, the reaction I have is much cleaner than yours and all the others’ I’ve seen on the web. This is probably due to the fact that I use not white vinegar but what is called the “vinegar essence” and is obtainable from the most of the “Russian groceries” shops one can find throughout Europe and, I presume, everywhere where there are enough Russians. The label says it’s the 25% solution of acetic acid. At all the stages of the reaction the liquid is transparent and clear (apart from bubbles reminiscent of sparkly water); there are no layers, no muddiness. After the steel wool has completely dissolved you are left with a little bit of black sediment. As days go by the solution darkens from weak black tea colour to strong black tea colour (it is visually indistinguishable from tea) and the black crystals grow on the surface. First time I made this stain, I assumed that this was carbon from the steel.

As a side note, the others expressed some misconceptions. It is not entirely true that the end colour of wood is always the same. The more diluted solution gives a lighter grey colour which will darken further if subsequent coatings are applied until all the tannins have reacted. However, the more concentrated solution, providing the same final level of darkness, will have a brown tint. Since I was after the pure grey colour, I spent some time experimenting with proportions. The final ones were: one piece of steel wool dissolved in 250ml of 25% acetic acid — it took a couple of days to dissolve but then it sat for another week before I filtered it. The resulting concentrate was diluted with 10 parts of water. It has the pH of about 4. In my experience, the final solution does not “go bad” as others suggested. In three years it hasn’t change its appearance nor the effect. But I was close to running out of it. I decided to repeat the process, and here is when the interesting things happened.

I followed my recipe to the letter with the exception that I left the concentrate to sit for almost 4 weeks before filtering. I took a sample of the filtered concentrate, diluted it with 10 parts of water and found out that the new solution has a much lighter colour than the old one and a much weaker effect on wood. I took another sample and diluted it until it matched the colour of the old solution (to achieve this I added only 2 parts of water). Correspondingly, it produced exactly the same wood colour. It became clear that somehow I lost much of the active substance.

And then I realised that in the last batch I produced a whole lot more of the black crystals that grow on the surface and that I wrongly assumed to be carbon and discarded the first time — probably because the concentrate sat still for four weeks instead of one. Under the microscope, I saw that the crystals only appear black when they are in large enough chunks. When small, they are brown and transparent, like the beer bottle glass. I took some and put them into water to verify they are not soluble. But I was wrong and after about half an hour they completely dissolved (they also turned out to be not soluble in ethanol). The resulting solution had the same colour as my old solution (by pure chance of me putting just enough crystals in just enough water); I covered a piece of wood with it and got the same colour as the old stain produces.

So, lo and behold these crystals are the substance that does the job. I hope it is clear how to get them: follow my procedure and let them grow on the surface of the concentrate (on the second image in your post I see them — “the crystalline flakes”, as you said — floating on the surface). I got a ton of them from one piece of steel wool (see images). Since the crystals do not dissolve quickly, it is easy to rinse them with water and then make the final solution without any acetic acid in it. It is definitely nicer to work with because it smells like water.

In addition to the questions you initially asked, I’d like to know what is this crystalline substance and why it grows on the surface (the crystals actually sink if stirred and float only when they have a dry top). Hope this helps.

(To moderators: I've read the guidelines and I understand that I'm in violation, strictly speaking, but I judged that the contributed information outweighs the offence. If not, sorry)

The crystals

This is a 1 litre jar. All of these crystals grew from the one piece of steel wool

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Since I had some rusty scrap steel (mostly nails) that were not going away anytime soon, I put them into a mason jar with 5% white wine vinegar; but there may have been some contaminants like dust or oil on the nails.

The solution I made slowly but steadily went from clear to orange-brown in color with a white powder forming on top of what remained of the nails. There was never a time when my solution turned green or purple even though I was checking the solution about every half hour in the first several hours of starting and a couple times each day afterwards. My solutions were about a week old before I tried filtering one at which point both solutions were staying dark orange/brown, with the what looked like a 'crust' starting to form on top.

I put my solution through a coffee filter which caught the crust, but after a couple weeks, the solution did not change color; it instead formed a new dark brown crust on the top. This new layer of crust seems to be getting thicker as the water level decreases, but it's hard to tell how thick because some of the crust stays on the side of the jar where the water level used to be. The filtered solution was inside another mason jar during this part of the setup.

From what I read about Iron (II) acetate, the hydrate can be made by the reaction of ferrous oxide or ferrous hydroxide with acetic acid.

Iron (II) hydroxide can be made by mixing hydrogen peroxide with acid and iron which would make a green solution that can then react with excess acetic acid to form Iron (II) acetate. I went with the easier method of using iron that oxidized in the presence of oxygen and water over time. I don't think that the nails I used were galvanized, but it is a possibility that some of the zinc came of and allowed rust to form on parts of the nails.

I would have thought that there might be a slight green color if I mostly had iron (II) acetate, so maybe much of the Iron (III) acetate was suspended in the water and managed to make it through the filter somehow. Maybe the iron (II) acetate changes into iron (III) acetate over time or under the right conditions?

After awhile (a little over a month), I got back to my a mason jar of filtered solution and found a brown powder that looked sort of like coffee grounds. I broke it apart into a powder and added some tap water. My powder seemed the get absorbed or at least suspended in the water. A small amount of the solution was poured onto a piece of wood that I had on hand (I think that the wood was spruce). The solution had some suspended particles, but seemed to mostly be liquid. The wood visibly started changing color in a minute; after a few minutes, the spruce was almost the same color as walnut. I think either both types had formed alongside each other, or maybe I only had iron III acetate which was merely suspended in water, but helped the iron acetate ebonize the wood which was awesome (sorry, I have no pictures of this).

In conclusion, my altered version of this investigation failed to produce both the green solution and the purple one. I wish I knew where the purple color came from; I am completely stumped on that point. In any case, the iron acetate worked, regardless of whether iron II or iron III acetate was created. Also, it does not seem to matter what the source of iron is; my rusty nails worked wonderfully and they were free.

This was an easy solution to make; I am going to make my own homemade stain using old nails and vinegar from now on.

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  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – A.K. Aug 12 '18 at 23:13
  • $\begingroup$ Honestly, the question is so long that I no longer know what the question is and whether this answers the question. If the observations are not reproducible, then the chances of having a definite answer to "what chemistry is going on" are automatically reduced, anyway. I'll undelete it in the meantime... $\endgroup$ – orthocresol Aug 26 '18 at 6:43
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SEARCHES : iron buff, wood vinegar and steel wool stain, giggle juice (found manly by people looking to add paints i have found) and ebony wood.

WHAT'S THE GOAL : iron acetate II (reddish in color after filtering and may have to sit a few days to obtain color)

HISTORY : Iron buff is one of 3 main chemical stains. It goes back to the 1890's where they used it to turn maple gun grips gold by heat treating it after iron buff was applied for the flintlock guns and was used commonly up to the 1960's.

BREWING : While there are many ideas on how, what is clear is that a 1 qt jar of glass or plastic is best, with a plastic top, or one that wont rust. It's not a big deal if you have to use a mason jar lid. Lid can't be airtight do to the hydrogen gases created by the chemical reaction of the acid in the vinegar and the iron in the wool. This also makes temperatures of 100+° from my understanding and experience. Oxygen also helps with oxidizing. One qt of vinegar (distilled white is best) to 1 steel wool (the finer the better, or faster). Also you can rinse the steel wool to clean off the protective oiltiand fluffed or cut up, which all will speed up brewing as well. The brewing time varies, but average is 24 hours to 1 week (I believe that a controlled environment at room temperature, witch I have seen on sites as a suggested temp for reliable results). Also, those sites don't talk about heating up the vinegar or adding peroxide. Filter with coffee filter or cheese cloth to stop brewing by taking out any steel wool left behind. I seen it come out apple cider color after filtering and reddish if cider is what you get let sit sealed up over night or until reddish (purplish is OK but close to being burnt. Yes, over time the solution will go bad since there's no way to fully stop the reaction. Yes you could filter more but I have found there is a thing as to much filtering).

NOTE: You can burn the solution by brewing to long. The color will be black. I just now read that you can renew the solution by adding peroxide to. However, I dont know how much.

OPTIONAL ADDITIVE : Hydrogen peroxide can be used to speed up brewing. (Anything else, or why, I don't know and never looked into it). You can also heat up the vinegar to speed up brewing.

ADDITIONAL INFORMATION: I have found that over all, no matter what you really do to make it, the end color is the same. I have even found a project site were they used black tea and iron buff ended up with the same color in the end, with pine. What I have found is if its too light when applied to wood, wait a day or two, plus when clear coats applied it darkness it. It doesn't fade in sun so it's good indoors or out. Now there are many ways to age wood using things at home. I hear stove cleaner is great for graying wood, so the different questions you have could work just as well, but it's all up to how you make it and the results your looking for.

BACKGROUND: I helped make a solution for my job a few years ago. We make about 30 gallons at a time. I have since moved departments so I don't play with it anymore but lately I started experimenting again and always enjoy learning more about it

Hope this helps if your still working with it.

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  • $\begingroup$ The question is not about ebonizing wood, it's about understanding the chemistry as thoroughly as possible. $\endgroup$ – Emmet Feb 26 '17 at 1:37

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