This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Writing on old manuscripts, parchments, and old letters that has faded into nearly or complete invisibility can be restored by rubbing over it a solution of ammonium sulphide, hydrogen sulphide or of "liver of sulphur." On parchment the restored color is fairly permanent but on paper it does not last long. The letters however could be easily retraced, after such treatment, by the use of India ink and thus made permanent. This treatment will not restore faded aniline ink. It only works with ink containing a metal-like iron that forms a black sulphide.