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Poe, Paper, Ink, and Heat
Name: Andrew
Status: student
Grade: 9-12
Location: CA
Country: N/A
Date: May 2007
Question:
In Edgar Allen Poe's "The Gold Bug", a character mentions
that: "chemical preparations exist, and have existed time out of
mind, by means of which it is possible to write upon either paper or
vellum, so that the characters shall become visible only when
subjected to the action of fire. Zaffre, digested in aqua regia, and
diluted with four times its weight of water, is sometimes employed;
a green tint results. The regulus of cobalt, dissolved in spirit of
nitre, gives a red. These colors disappear at longer or shorter
intervals after the material written upon cools, but again become
apparent upon the re-application of heat." I was wondering if this
is true, and if it is, why the heat causes the writing to disappear
and reappear.
Replies:
Andrew,
What a wonderful question!
Both "zaffre" and "regulus of cobalt" are oxides of cobalt. Treating
them with "aqua regia" (a mixture of hydrochloric acid and nitric acid)
and "spirit of nitre" (an old name for nitric acid) will cause these
oxides of cobalt to change oxidation states and presumably become
colorless - cobalt is known to quickly change oxidation states.
Heating the reduced form of cobalt will cause the reduced form to be
speedily oxidized once again (oxidation can happen over time, but
heating it would speed up the reaction with oxygen in the air) and
become colored once again.
This is rather similar to the well-known use of lemon juice to write
on paper. The writing is invisible, but when heated the different
acids in the juice become oxidized and appear.
Greg (Roberto Gregorius)
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