How do stickers that fade to transparency work?

Xoder — Wed, 24 Jan 2007 08:18:52 -0800

At my (security-sensitive) work we all get badges. Sometimes we leave them at home. Sometimes we need to bring a visitor in. Then we get temporary badges. These badges have an interesting fading sticker on them. How does it work? The sticker is actually in two parts, which I've seen assembled thusly: First, a red "EXPIRED" sticker with lots of bright red lines is placed on the plastic badge. Then a white sticker with the division name is affixed directly on top of that one.

As the day progresses the "EXPIRED" text becomes more and more visible. I've not yet had the gumption to take one home and see if the white sticker fades completely, but it appears that is what would happen. How does this work?

I asked one of the receptionists that passes them out, and he said that the red ink was bleeding into the white sticker. I find this implausible for two reasons: the sheets with the red stickers on them are stored folded and have no protective covering for any wet ink, and generally the backs of stickers are pretty impermeable to all but some nice solvents (at least in my experience) that would cause the adhesive to fail.

In addition, the red and white stickers become permanently bound together (I futzed with them at work a bit), but it's trivially easy to remove the red sticker from the card itself (good for the staff as they clearly have to do this every day).

Thanks in advance!

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How do stickers that fade to transparency work?