How I Make My Oak Gall Ink
If you can get hold of Oak Galls (I picked mine from trees, but you can order them online I think) then it is a really easy process, and you might as-well try out !
Firstly - Prepping The Materials
In the second image you can see the difference between an un-ripe Gall and a ripe one. For the purpose of creating an ink, the ripe ones are best as they contain more tannings and so will create a better ink. However, even in the riper one, there is still internal spots of the lighter substance, which one can not see when picking them.
As pictured in the 3rd image, you grind down the Galls, and put to the side for later.
Images 4 & 5 - I couldn't get hold of iron sulphate for mixing with the tannings, so I instead marinaded steel wool in vinegar to create an iron oxide (rust) solution to mix with the tanning solution later.
Secondly - Making The Ink
Simmer the Oak Gall shavings to release the tannings, until you get a liquid like in the second image (or you can simmer for longer for a stronger colour pay off). Then strain the liquid from the solids into a jar, and add the iron oxide to it. There are no rules to what ration to use, so it is largely based on your own judgement and what colour you aim to get, generally the less iron oxide you add, the greener the ink will be.