Example of citric acid writing

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In several instances, the examination of a document with short wavelength radiation (ultraviolet light) allows to reveal unexpected plots. In this case, for example, the document appears to be free from any suspect visible sign under normal light (photo #1);

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by observing the same piece of paper with ultraviolet lighting (photo #2)

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one can clearly notice the surfacing of the writing done with citric acid - a trick often resorted to in order to fool penitentiary guards' mail control.

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