I just had to try the invisible inks mentioned in your review of Kristie Macrakis's book Prisoners, Lovers, and Spies ("Under Cover of Ink," Winter 2015, p. 46). At the end of the review there is a recipe for an alum-based invisible ink. I couldn't find alum at the grocery store. Then it dawned on me: alum is the stuff in a styptic pencil. I wonder if some of these inks may have been used by prisoners of war to get messages out.
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