"Of writing in white. White writing that cannot be seen on white paper unless when pulled trough pure water. Take alum, crushed to powder, and put it in a clean bowl. Pour fresh water over it, not to thin. Take a freshly cut feather, and write with the water on the paper whatever you want. Let it dry. When one wants to know what you've written, give him the letter so he can put it in wide hand basin with fresh water. Leave the letter in and after a quarter of an hour, writing i white will slowly appear."