Monday Morning Brief | Putting thermochromic ink to the test – Linn’s Stamp News
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Monday Morning Brief | Putting thermochromic ink to the test
Linn’s Stamp News To protect the properties of the thermochromic ink, the Postal Service sold what it calls a “protective sleeve,” a black envelope sized to hold a 16-stamp pane of Total Solar Eclipse stamps. Printed on the back of the envelope is this rather cryptic … |
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