The bubbles are quiet possibly hydrogen as thats a by product of the chemical reaction.
Here is a post from the aaw (
http://www.aawforum.org/vbforum/archive/index.php?t-4403.html) about it:
"You’re joking…Steel wool and Vinegar?
If ever there was certain evidence that woodturning is an ancient art, the use of this witches’ brew for staining oak black must be it.
It works – there’s no denying that, but there is a quicker, simpler and cleaner method of achieving exactly the same result, so why not give it a try? But first…a chemistry lesson.
Everyone knows that vinegar is a very dilute solution of acetic acid (which is itself a very weak acid) and that steel is mostly iron. When steel wool is placed in vinegar a chemical reaction takes place:
Iron + acetic acid → iron acetate + hydrogen gas
It is the iron acetate that is wanted. This is what turns the oak black but, because the vinegar actually contains very little acetic acid, the reaction is slow and we get very little of the iron acetate. Instead, other (unwanted) reactions take place. In particular, most of the iron simply rusts, making the whole mixture into a filthy black mess which contains hardly any of the iron acetate we set out to obtain. Incidentally, the hydrogen gas is also produced in tiny amounts and probably goes unnoticed although you might see a few tiny bubbles on the steel wool in the first half-hour or so.
Bear with me, please, while I take the lesson a shade further. When metals (e.g. iron) react with acids (e.g. vinegar) the atoms change slightly and become a different particle called an ion. Iron acetate consists of two ions, as its name implies. These are iron ions (from the iron atoms in the steel wool) and acetate ions from the acetic acid in the vinegar.
End of chemistry, the rest is common sense (or ‘logic’ as we scientists like to call it). We know that vinegar alone does not stain oak black. Therefore it is not the acetate ions which do the staining. Therefore it must be the iron ions. Therefore anything which contains iron ions would be able to stain oak black. So why mess about with steel wool and vinegar for heaven’s sake?
There is a simple, clean, cheap and virtually harmless iron compound available. It is correctly called Iron(II) sulphate but is commonly known as ‘ferrous sulphate’. It is usually found in the form of pale green crystals which look very much like green sugar. Ferrous sulphate is the active ingredient in some moss killers and in iron tablets (for anaemia). It's available in the UK at places that sell garden chemicals - I guess the USA has similar stores. It isn't very expensive here.
Dissolving a teaspoonful or two of ferrous sulphate in a cupful of warm water will give a pale greenish yellow solution which will turn oak black if you paint it on, or make you very very constipated indeed if you drank it, or kill the moss on your lawn if you spray it on. I said it was virtually harmless and it is – but you wouldn’t want to drink it any more than you’d drink the steel wool/vinegar mixture. Normal care should be taken and is all that is required.
Sand your work to its final finish before applying the solution. Be warned that the effect is not instantaneous. You paint it on and nothing happens…the black colour gradually develops over the next couple of hours. It’s probably best left overnight. The water will raise the grain so sand down gently – the beauty is not much more than skin deep. You can repeat the application but the left-over solution will gradually ‘go off’ after a few days. Best use what’s left to kill moss on lawns/paths, and make it up fresh as required.
By the way, as if you care, I’m pretty sure the blackening is caused by another chemical reaction between the iron ions and the tannic acid in the timber (not just oak). I got a strong black colour when I added a few crystals to some cold tea. I’m guessing that ferrous tannate is an insoluble black substance. Just don’t drink the tea afterwards!
Bob"
Hope that helps
Paul
