Oxidation - darkening of the ink on certain stamps caused by contact with air or light. Some inks used in printing stamps, especially oranges, may in time turn brown or black.
Oxidation: Darkening of the ink stamps when they come in contact with air and sunlight.
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Packet: A pre-sorted selection of stamp ...
Oxidation: a chemical process where the color of the stamp is changed due to exposure to elements present in the air or by improper storage; is considered a "damaged stamp." ...
Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged.
Untoned. With no tarnish or oxidation, and with original cartwheel (frosty) or prooflike lustre. Copper coins are considered brilliant if they have full original red.
Brilliant Proof ...
Stamps that have changed color since they were printed are called color changelings. This can occur through exposure to sunlight or oxidation.
Stains and toning are usually the result of improper storage plus heat or humidity.
 See also: Stamp, Plate, Used, Applied, Official
  
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