This is a 15th century recipe from Secretus Securitas for an invisible ink. The instructions allude to using this ink between the lines of another. When the ink is brushed lightly with the ‘revealer’, you’ll see the ink begin to take shape so you can read what was concealed. This is perfect for important details that need to be written but must be hidden from sight.
This item comes with about one ounce of the secret ink and about one ounce of the ‘revealer’.