"In the first U.S. stamp application of thermochromic ink, the Total Solar Eclipse stamps will reveal a second image. Using the body heat of your thumb or fingers and rubbing the eclipse image will reveal an underlying image of the Moon (Espenak also took the photograph of the Full Moon). The image reverts back to the eclipse once it cools.
Thermochromic inks are vulnerable to UV light and should be kept out of direct sunlight as much as possible to preserve this special effect. To help ensure longevity, the Postal Service will be offering a special envelope to hold and protect the stamp pane for a nominal fee."
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