Instructions, 'Advice about a concealed way of writing' by Henry Oldenburgh [Oldenburg] Oldenburgh provides instructions on how to conceal messages. Notes the ingredients and method required to create a white ink, with which to write the 'secret' lines; a black ink, with which to cover up those lines, and; a substance with which to blot out the black ink and make the former lines visible. Subject: Chemistry 17th century