Stamp Photos

I have three of this stamp, but one of them changes colour when you warm up the ink with your finger. Does anyone have any idea why one of the stamps does this, yet the other two are just plain?

Added by Sarah on 17 February 2012.

Hi Sarah, we found a website that suggests that some stamps were issued with thermochromic ink:
http://retrievia.wordpress.com/2008/01/06/the-stamp-of-the-chemistry-nobel-prize/

Added by Stamps Editor on 17 February 2012.

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