In the presence of water iron reacts with the tannins in the oak to produce a potent black dye. (a good reason to not use steel wool on oak) The only way I know to get rid of it is by sanding it away, unfortunately it can penetrate quite deeply so sufficient sanding to get rid of it may not be possible without compromising the piece. You can sand away the majority of it and then use a dark stain to mask it or consider painting it.

Oh jeez, that all sounds really really bad! I swear i've used steel wool on oak before although that may have just been between poly coats, now that I think about it. Sounds like I'll get out the sander and cross my fingers! Thanks so much!

Look into "fumed oak"as a possible finish.

Oxalic acid will remove tannin-iron stains. But you'll need to strip it first.

http://www.woodcentral.com/bparticles/woodbleach.shtml

Wow, oxalic acid looks amazing. Watched a few videos on it. We sanded the frame yesterday and a fair amount of the black stain away. Hoping a second pass will lighten up the grain more, but if not the oxalic looks like it might do the trick! Interesting to learn that its the main ingredient in Barkeepers Friend - no wonder the stuff is so great!

Emsy, did you take pictures? I'm curious to see it :)

I wish I did! I'll try to get some post-sanding pictures this weekend.

Oxalic acid for the win! The picture of the table leg was formerly black. It was sanded 2x and the acid lifted the dark grain and gave it a very nice clarity. Then was oiled up and looking beautiful.

I do have a follow up question:

The chair in front was sanded & oiled (3-4 applications) -- but looks dull dull dull. The other 3 chairs were treated with oxalic acid and oiled only 2 times, and are beautiful! How would you recommend I get the dull one to look like the nice ones? Keep oiling? Or put that one through acid too?

Thanks again!

What kind of "oil" did you use? There are many products labeled some type of oil that are partially or not at all oil. http://www.popularwoodworking.com/techniques/finishing/oil-finishes-their-history-and-use

An oil-varnish blend will add some gloss to the look and can be applied over an oil finish (assuming it's a drying oil like tung or linseed oil)

We used Watco Teak oil

I've been refinishing for 45 years and in my travels anytime did I ever put any type of oil on old old would it turns black instantly every time never oil would old would ever never ever, What you have to do is wash it with soap and water let it dry very well and then apply a thin coat of seal coat wax free shellac on it then you can put whatever type of an oil you want but first you must condition with wax free seal coat shellac made by zinsser

That also includes mineral spirits it also hasn't oil in it stay away from that to 1st you must shellac

Seems like after you told them the oil you used, they just abandoned you 😟 I'm sorry! The next advice was 4 yrs later. I hope your gorgeous table and chairs turned out the way you wanted!! ✌🏻🦋🌈

Watco Teak oil is a very dilute traditional (rosin-based) varnish with what look like some UV inhibitors added. It is not pressed from teak trees or seeds.