To Write Letters of Secret, That They Cannot Be Read Without the Directions Following
Take fine Allum, beat it small, and put a reasonable quantity of it into water, then write with the said water.
The work cannot be read, but by steeping your paper in fair running water.
You may likewise write with Vinegar, or the juyce of Lemon or Onion; if you would read the same, you must hold it before the fire.