Another example. A cover I got this week. It is not an area I collect (French 19th century covers) but I have a couple for various reasons. I bid on this just because it was very clean and the stamps well struck. The letter was written 30 August 1859 to Monsieur Carre Kerisouet (minus the accent marks) in Plimet from Augustin Latimier du Clesieux in St. Brieuc. Both towns (cities?) in Northwest France in a region previously known as the Cote-du-Nord (north coast).
But the reason I am posting this here is that this cover and letter on the other side of this FLS were written with Iron Gall Ink and you can see staining of the letter penetrating the cover a bit. But only slight deterioration in 160 or so years. Probably good for another 2 or 3 hundred more years! with no conservation needed other than to store it in a dark, cool dry place.