This is ferric acetate. It is the amorphous, stinky, bubbling, fascinating, terrifying compound that is the magic behind the blackness of the rabbits.
I stumbled upon the process of ebonisation quite by accident. The internet told me that if you stuff some superfine wire wool into the bottom of a jar and top it up with spirit vinegar then a day or a week later (depending on who's writing) you'll have a solution that can stain pale wood black. It sounded so improbable that I had to try it. My first attempt was with substitutes - a wire scrubber and some sarsons vinegar - and a week later nothing much was happening. But by then I was suitably invested in the project to procure the proper ingredients. It started bubbling almost immediately! By the next day the vinegar was turning brown and soon I was greeted each morning by a sort of rusty scum that had to be dispersed to encourage the process along.
Ebonisation occurs when the iron reacts with the tannins in the wood. Since I was using a pale birchwood for the rabbits, I needed to fortify them with extra tannins. I brewed an insanely strong pot of tea to get the rabbits thoroughly prepped. Quite forgetting myself I drank a cup to get involved in the process and then required a short lie-down.
The rabbits were soaked with black tea, left to dry, and then brushed with the ferric acetate. This process was repeated several times until I was satisfied that they had truly passed over to Inlé. Finally, a few coatings of danish oil to seal and finish and an unbearably long 24 hour drying stage.
And voila! The warren is ready.