best way to ebonise wood?  

This gives you different
ways of ebonising, depending on the finish/shade you want:
http://www.egcc.biz/workshop/e/ebonising.htm

I tried the vinegar & steel w...
I tried the vinegar & steel wool home brew method and all it did was turn my walnut chair an unattractive greenish color with tiny black flecks. Then I tried permanent water-based India ink and was thrilled with the results. It covered in one coat with just touch-ups needed in a few places. I used a small foam brush and I think only used maybe 2-3 ounces for the entire chair.
The finish was a soft, warm black with the wood grain still visible. It was still rubbing off a bit even after it was thoroughly dry, so you do have to put another finish on top. I used beeswax and now the ink is stable.
I've also used anilene wood dyes, though not for ebonizing. I like them but for ebonizing I'd use ink.

My experience
I've used both the india ink method and the steel wool / vinegar method and have had great success with each. The trick with the steel wool method is that it's best to experiment with different mixtures. Steel wool works fine, but I've found rusty nails work better because it speeds up the reaction. Also, the longer you let the steel wool or rusty nails sit in the vinegar, the darker the ebonizing effect will be. (I have let the mixture sit for only a short time, and the walnut turned a greyish-brown).
Overall, I prefer india ink, as it is a super fast, no hassle finish, where you know exactly what to expect. I wipe on the india ink, wipe off the excess and let dry for only 30 minutes. They I use satin wipe-on poly to seal. Looks great.

Ditto the india ink.
Unless dealing with a museum piece, I'd leave the chemical reactions to the alchemists.

India ink
is the most satisfying and easy to apply. Any art store will have it.
Like said above, you don't need much. But do test it first. Some can be
quite blue/lavender. I often need to adjust it to warm it up.
I would attempt to fill any nail/brad/pin-holes. Use a stainable filler
and apply with a single edge blade and lightly sand. Also good to test
first...(a dark stain will not necessarily hide imperfections)
I do like and use the iron/vinegar but i've got a lab/shop and have
developed formulas that give me exactly what i need for different
projects. And have been doing this for 20 yrs.
Also, if you type your thread title into a google search, you will get
pages of hits that will help you decide which way to go. Many
discussions.

This is how it is done! REAL ebonized wood I did.
I like to sculpt as a hobby.
This is Cherry hardwood ebonized and rubbed with clear wax.
To get proper ebonized wood, you need...
Apple Vinegar
(I've tried basic vinegar and it never worked well)
Iron. Real good iron. Some peopel claim nails and such but I used the shavings from my bench grinder. I have used to grind lawnmower blades and such.
Mix the two together and wait wait wait. it takes a good week to get a potent concoction.
But when applied to raw wood (And it can't be sealed in any way)it will start turning dark. Within a minute it is 50% grey. Within 10 minutes it is the darkest, blackest black.
It looks like coal.
Different woods respond differently.
I do not believe you would get a good dark color on a piece that has been previously finished.

thanks guys,food for...
thanks guys,food for thought.
I'll show the result when ready.

Paso
Thanks for the pics. That makes me want to try the home brew method again! I guess my mistake was using steel wool instead of iron. It got very rusty but didn't do much to the wood.