When looking at old documents, you may encounter several problems. Chief among them is the problem of faded ink. Faded ink can prevent you from understanding what you are reading, as well as straining your eyes if you continue trying to read the document. However, with careful work and patience, you can restore the faded ink on a document, making it readable and useful.
Determine what has caused the fading. In most cases, ink will fade from time alone. However, it may also have faded as the result of someone's trying to erase or eliminate information on the page. If someone has tried to erase the information, take the document to a professional; you may not be able to restore it on your own, and may only make the problem worse. If you cannot tell what the cause of the fading is, it is most likely time and you can proceed as usual.
Paint over a small section of the writing with sulphide of ammonium. Test out a small area, preferably a word you can already partially make out that does not seem to be important (such as an article like "an"). Using a light brush, such as one made from camel hair, cover the word with a small amount of the solution. It may become black immediately, or nothing may happen.
If nothing happens, proceed with hydrochloric acid. Using a new brush or your earlier brush after it has been thoroughly cleaned, paint the acid over another small area of the writing. You may have to wait several hours or even overnight for the acid to dry and for the writing to become visible.
After part of the writing has become visible using one of the two above methods, cover the rest of the writing with the solution that worked. Wait a day and let it dry before attempting to read the writing.
If neither process works, consider bleaching the page around the writing using either bleach or hydrogen peroxide and later painting it over with a diluted solution of galls, tannic acid or acidulated ferrocyanide. However, these chemicals can be dangerous, and you may consider taking the page to an expert restorer rather than risking your health and the writing, which you may damage beyond repair.