Probably chemically altered or sun faded.
You are correct - these were issued in only one color; orange.
You can change this stamp back to its original color.
http://stampsmarter.com/learning/HowToRemoveOxidation.html
Don
Thanks for that link, Don!
Bill is sorely missed!
Actually the orange color did not change to a darker hue due to oxidation. It is sulphurization that caused it. Using water peroxide will actually oxidize the pigment to return it to its original color.
Thanks a lot for these clarifications.
The link is much appreciated and through philately learn chimical and this brings me back several years back to the university benches where I prepared some quick fixes.
Abderrahmane
re: US air mail
Probably chemically altered or sun faded.
You are correct - these were issued in only one color; orange.
re: US air mail
You can change this stamp back to its original color.
http://stampsmarter.com/learning/HowToRemoveOxidation.html
Don
re: US air mail
Thanks for that link, Don!
Bill is sorely missed!
re: US air mail
Actually the orange color did not change to a darker hue due to oxidation. It is sulphurization that caused it. Using water peroxide will actually oxidize the pigment to return it to its original color.