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.
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.
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?
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.
On top, there is an orangey-brown deposit around the edge and crystalline flakes floating on the surface.
The filter paper has dried, leaving a fine reddish-brown powder.
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.
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).
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.
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.
- 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.
- 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?
- 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 :)
- 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”.