Mads threw the bait and I just couldn’t resist the urge to chew it up.
Since I saw quite a bit of alchemy around this stuff on the web, with very interesting experiments but without the scientific background, I thought I would cover that part here, not to kill the magic, but to help those who want to try it know what happens and how to get accurate results and avoid the frustration of uneducated experiments.
We’re dealing with an acid here, and it happens that acids bleach wood. Vinegar is acetic acid quite diluted (between 4% and 10% usually). We’re not talking recipes with caramel, nay nay, white clear pure effin’ vinegar. Forget about the fake balsamic which not only is a fake but doesn’t bring anything serious.
So we’re dissolving iron turnings, shavings, pieces (nails) or steel wool into acetic acid. Mind you, there are many forms of iron acetate actually, we’ll stick to the first two for clarity:
- put the iron and vinegar into an air-tight container, with very little air above, and the iron will completely dissolve with the solution remaining completely colorless: this is iron (II) acetate, and it’s clear:
(If you’re using stel wool, since steel is made of iron and carbon, you will get an insoluble layer of carbon at the bottom: just ignore it and add more steel wool or nails!)
Actually the iron (II) acetate happens as Fe2+ ions and CH3COO- ions in the solution:
- open the lid and rather quickly oxygen from the atmosphere will oxidize the stuff, because iron 2 ions are very prone to that when there’s oxygen around: Fe2+ ions become Fe3+ ions in the presence of water and oxygen, and they stay that way:
So by keeping the container open, which almost everyone might be doing, you get the classic rusty solution of iron 3 acetate, or better said, iron triacetate .
This principle can be applied to many other metals that acetic acid can dissolve, or if it doesn’t, their carbonate form can be used very effectively. Some metallic ions are healthier than others, though, and we’ll come back on that on future installments. For now we’ll just stick with the iron and this very method. So far, so good.
Now let’s make some tea, shall we?
Tea, like wood or bark, or actually any vegetal, gives off tannins when infused, giving the beautiful colors that we all know. Tannins are beefy molecules, composed of a glucose element surrounded on all sides by organic acids made by the cells of the vegetals: we’ll stick to the simplest one, which is gallic acid, but we’ll come across others too:
So tannins from the tea are made of a glucose molecule to which five molecules of gallic acid are stuck:
This is the basic block, since all the single -OH links you see (hydroxyl groups) can be attacked to form what are called esters, which are subject to an equilibrium with water: here in France all the kids at the college see during the chemistry classes the esterification equilibrium:
Thus, these tannins can be hydrolyzed by some agents, and can react with whatever metallic ions might be present.
This is exactly what happens when we sum these two steps: iron acetate on its own will darken a tannin-rich (ie with a saturated color) wood and turn it way darker, sometimes black (more on that later). Flooding tea over a piece of wood builds up a tannin-saturated layer on the top (between the open fibers) when the water diffuses into the wood or evaporates. When flooded with the iron acetate solution, this top layer of tannins turns into a very high concentration layer of complex iron esters, among which iron (tri)gallate, which is black-blue! No molecular formula here but a shot of one of my test tubes: left above a 500W halogen lamp, right against the white wall.
Now the $1M question: why on Earth does the wood sometimes turn brown instead of black? Well, remember what we said earlier about acid and wood?
Righty: acid bleaches wood.
Actually, the best acid to use for that very purpose is oxalic acid. It forms oxalates that are, well, far less colored than the gallates. So, what happens if your iron acetate solution was not completely exhausted of acetic acid, I ask you?
Yup, something more happens.
So, the best thing is to put waaay more iron than your vinegar would dissolve (you can do the math actually, but to abide by the K.I.S.S. principle, the best practice is to put a LOT of iron and let it sit for a week, checking if there remains solid metal and if not add more) to ensure you destroy all the acid in the bath and only get iron acetate (which you can crystallize by evaporating the solution, by the way: easier to store and helps make a known concentration solution every single time).
If you want to get a slightly acidic solution to change the effect of the ebonization, mix some of your iron acetate with a bit of vinegar in a separate container. Lab measuring devices are great for that, since knowing exactly what quantities you used will allow for reproductible results (you can buy cheap plastic measuring cylinders online, the precision is enough for that kind of thing). I haven’t done color tests on that matter yet, since I’m in Paris at the moment and my wood stash is at the parents’, but I’m pretty confident that Mads will do the thorough test himself ;)
Now a word about the real thing: what do you think happened to the swamp oaks (which, as a reminder, spent a LONG time in iron-ladden swamps)? And what do you think lacks today’s ebony versus past times ebony?
You got it: iron in the soil!
So, for those of you in the countries that are actually producing ebony, why don’t you try using grass fertilizer (iron sulfate, which is harmless to the environment) on a regular basis and over the full trees’ growing span, to help them develop that natural jet-black color that only the old trees produced back in the day when the ground where they grew had plenty of it?
By the way, all the Dalbergiae would also benefit from that. Betcha oaks would too! I’ll try this myself, but we won’t have visible results before years. But since ebony in its black form is almost nonexistent in today’s crops, it’s high time the producers get a kick in the butt to start thinking about it, educate themselves, and understand why the same plant has gone colorless without anything else than the SOIL CONTENT changing. Ye be warned Gaboon peeps!
-- Thomas - Pondering the inclusion of woodworking into physics and chemistry classes...