One of the oldest previously-classified US documents, written over one hundred years ago, explains how to make secret ink: “Take one ounce of linseed oil, 20 ounces of liquid ammonia, 100 ounces of distilled water. This mixture must be shaken up before using with a quill pen. Write in free space between words written in pencil. To make this writing appear, dip the whole letter in cold water, and read secret writing while wet. Upon drying the writing disappears.”