Hmmm, that messes with my brain. I have it in my head (I know not where from!) that registrars of births, marriages, and deaths are obliged to write (not type) a record that must last 100 years. That is why they use ferrogallic (registrar's) ink. I'm sure that they couldn't achieve that standard if they accidentally leave the light on in the office.
Yes, I know -- they keep their documents in deep dark dungeons to archive them.
Also, we were lucky enough to see an exhibition of the Lindesfarne Gospels in Durham in recent years. I suspect that there must have been some IG ink used by those monks 1400 years ago and it's still clearly legible -- well, if you're a Latin reader, it is.
Perhaps either a registrar or a 1400 year old monk might pop onto the forum to clarify the photo-stability of IG inks.
The next best thing is Amber's documentation of the (relative, subjectively interpreted) results of leaving some writing samples in south-facing windows in Las Vegas for .... quite a while, a year at least. Results published here. IIRC, Salix didn't fare so well; Diamine Registrar's retained "Excellent" legibility. My vague recollection of my own experience putting Chesterfield Archival Vault in a window for a few months or more was that it faded, but didn't disappear.
We have a forum for this sort of thing. My current test doesn't include any samples of IG inks, but others have tested them before, and perhaps the pictures are still available.
Edited by Arkanabar, 13 May 2016 - 16:12.