Here's how to improve your chances for a long-lasting document when using a traditional or commercial iron gall ink.
Step 1: Iron Gall Ink Considerations
1. Stable inks require the correct proportion of galls to iron sulfate (copperas) -- the 1800's chemist Dr. Stark wrote that 3 parts aleppo blue galls to 2 parts copperas is the most stable. (I don't know what these proportions would be for pure-grade chemicals, since I use plant sources.)
2. Aleppo galls from the Middle East are the Rolls Royce of galls -- any other gallo-tannic source in the plant kingdom will be of lesser quality by comparison. And of these, blue galls (where the wasp inside has not yet emerged) are superior to the white galls (where the wasp has exited the gall, leaving a hole behind).
3. Avoid oxidation during storage -- including the air inside the bottle (collapsible bottles or tanks with floating lids, like the kind used in photography darkrooms are ideal for large quantities of ink -- I'm still trying to figure out a way to insert a floating lid inside an ink bottle for smaller quantities. The best you can do is transfer the lowering ink to smaller bottles as you use it up. I've also tried wadding up balls of Saran Wrap to take up the air space inside a bottle. This is messy, however, and I don't yet know if the acid in the ink would eventually eat it to oblivion.) Oxidation problems apply to lowering ink levels inside of ink converters as well, especially if the ink is not used daily.
4. New ink is more stable than aging ink, so use it daily. Montblanc puts expiration dates on their iron gall ink bottles.
5. Ink that has dropped sediment is no longer fit, if longevity is your goal. (IAMPETH has a .pdf article by Dr. Vitolo for freshening iron gall inks. I have not tried it. I'd be inclined to throw the ink out, myself, and make fresh ink, especially if permanence was my aim. But that is an option.)
6. According to David Carvalho, recipes with added dyes are suspected with hastening the browning of the ink, thus contributing to instability. Logwood is especially unstable. I personally prefer to keep my recipes simple (galls, copperas, distilled water and gum arabic) and avoid added dyes.
7. Though they take much longer, fermented, cold-process iron gall inks are of better quality than cooked-down or instant recipes.
Step 2: Nib Considerations
8. Dr. Stark advised avoiding metal nibs-- they prematurely age the ink in the bottle, tarnish the nib, and compromise the stability of the ink on the page too, since a chemical reaction has occurred between the ink and metal during the process of writing with it. Use a feather quill, reed pen, or glass pen if your quest is absolute permanence. If you really must use a metal dip pen, set a small amount of ink aside in an inkwell to dip from, so as to avoid shortening the life of an entire bottle of ink. (Make sure the inkwell is non-metal as well.)
9. If you still must use a metal nib even after reading #8 (and we all love convenience), choose a stainless steel or gold-plated nib (dip or fountain pen). Even then, you'll likely encounter premature browning of the ink on the page. And I've found that gold-plating does not last forever.
Step 3: Paper Considerations
10. Animal skins (vellum and parchment) last the longest.
11. For other types of paper, select acid-free cotton rag.
12. Gelatine-sized papers (such as those found in high quality watercolor papers) seal the paper and prevent the ink from penetrating too deeply into the fibers, thus protecting the paper from corrosive effects from the ink acid.
Step 4: Storage Considerations
13. Heat, humidity and light are enemies to written documents, so minimize these.
14. Avoid contact with acidic storage materials (such as cardboard boxes).
15. Use acid-free and PVC-free storage materials.