Now That's What I'm Talking About

As Janice predicted oak gall in water is sepia coloured.  However those ancients knew a thing or two

Steel wool in vinegar makes a pale grey stain, but you can see what happens where they join forces....

And this is how they worked it out (I swear this is a genuine bit of historical reconstruction) - first they swished a bit of the oak gall stain onto some notes pertaining to their work, and commented to self that it is indeed sepia and although it does darken a bit as it dries, it isn't dark enough to use as ink. 

Later in the day, having set a ramekin of steel wool and vinegar on the windowsill and being unable to resist poking it occasionally while sitting at desk, it is eventually spilled on the windowsill.  Oops, better wipe that up.  Sits ramekin on the earlier swishings (now well dried and forgotten about), and dribbles off the bottom of the ramekin soak into the work notes.  (The vinegar by the way is still completely clear, nothing to suggest that, even alone, it would make grey marks on paper.)

Returns ramekin to windowsill.  Resolves to poke it more carefully.  Turns to see:

Ta Daaa!

Shame about the work notes.  

(NB: The original ancient page of the evidence is still buried somewhere in a pile of unexplored chronicles put together on a windswept and mainly barren island off a northern shore of a larger island in the eastern North Atlantic; somewhere you could freeze and starve and be raided by Vikings and feel closer to God as a result.)  

Don't quote me.