I am working my way through Jason Logan's book "Make Ink" and currently boiled up some acorn caps for a rich dark grey ink for paper. I foraged a big paper bag full of large, smooth acorn caps from old oak trees near my Mom's house in Louisville, Kentucky last winter. I let them dry out first, then gave them a good wash before boiling them for 4-5 hours on a low simmer, the time split over 2 days.
After the boiling, I strained out the caps, added ferrous sulfate (iron salts that oxidizes the tannins within the dye/ink) to turn it from brown to grey/black. I added a little at a time and took many samples of the ink on little swatches.
See the entire process squished down to a 1.5 minute REEL!