Hydrographic paper

As hydrographic paper is called a paper on which one with pure water can write clearly.

To make it, paper is dipped in a weak decoction of gall apples or in a solution of yellow blood liquor salt and, after drying, powdered calcined iron vitriol is rubbed well into the paper. When in contact with water then black, resp. blue lettering.

Due to the discoloration caused by water and moisture, the hydrographic paper must be stored dry and airtight.