I had intended to strain the crushed oak galls a few days after putting the jar on the windowsill to steep but, as so often happens, the days slipped by and it was a full week before I separated the murky brown liquid from the galls (1).
I strained it first through a sieve (2) and then through a coffee filter paper (3) to be sure of removing all the woody sludge, and was left with a cloudy, dark brown liquid. I then added two teaspoons of gum Arabic solution (4) to create a more viscous ink and to help it adhere to whatever paper it's applied to.
In order to darken the mixture I dissolved 1 teaspoon of ferrous sulphate grains (iron) (5) in a tiny amount of hot water and stirred it well into the mixture - which immediately turned to an impressive black (6).
The ability of iron to darken, or 'sadden' botanical colours used in all sorts of natural dyeing is very useful, but be sure to keep pots and utensils for iron use only as it stains everything. Gloves would have been a good idea (7)!
Applying the oak gall ink to watercolour paper initially gave a rather disappointing grey (8), but exposure to the oxygen in the air quickly turned it into a deep, solid black (9). I'll certainly be using these 'knopper' oak galls again!