6 Mint Moller 85 chairs  

That is certainly a...
That is certainly a possibility but I have never seen Oak that dark and the end grain of the chair legs LOOK like Teak
and in lightly sanding one it did not change??
Thanks for all you suggestions I have sent a picture and email to the restoration company in Denmark and maybe they will be able to help??
Dave

Re Moller # 85 chairs
The answer from the Danish Homestore was:
What is happening is that you have sanded away the thin layer of teak which has aged from daylight and got to the ?fresh? wood underneath.
As the chairs are not that old this will still be a more green colour compared to the rest.
There is little or nothing you can do to this unless you can get hold of some oxalic acid as this will bleach the timber.
Thanks again to all for their suggestions
dave

Blue-black stains caused by the
reaction between iron and tannin-rich wood species, like oak, can be often effectively removed or lightened by repeated applications of oxalic acid. This usually won't change the natural color of the wood, though. Teak, like most woods, tends to darken as it oxidizes, so it doesn't follow that sanding through an aged surface would reveal darker fibers below.
Also, I don't recommend "wet-sanding" furniture with oil. Though often promoted as a technique to fill and smooth open-grained wood, I've found that the slurry created only muddies the final results and diminishes the reflectivity or "chatoyance" that enlivens the surface and allows the grain to visually "pop". If a satin-smooth surface is what you're after, using a grain-filler followed by film- forming finish would be a better method.
I don't like to burnish bare wood with steel wool, either. Tiny fibers can break off and remain stuck in open pores only to turn black over time, resulting in little dark freckles. If you insist, bronze wool is available for this and won't cause the reaction.
The cause of your experience with the chair, though, remains a mystery to me. I must be missing something.