Another trick you can try is writing white letters on a black background, using the directions in Girolamo Ruscelli's book from 1558:
Mix the yolk of a raw egg with a little water. Use the mixture to write your message on white paper. After the paper is completely dry, paint over the message with black paint or ink. When the paint is dry, rub over it with a rag: the letters will appear white.
This was a popular trick 400 years ago—many books in the Folger collection mention it!
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