Although this can happen to any stamp, it is especially noticeable on US stamps that are printed in either yellow or orange ink. Over time, the stamps may begin to turn brown. I recently had a customer who sent me a copy of Scott #468 (orange 6Ę Washington head issue) that was a very dark brown. He was confused because the color is supposed to be orange according to the Scott catalog and he could not identify it.
In the simplest of terms, these stamps are rusting. It is very similar to rust that appears on automobiles.
Orange and yellow inks contain materials that can oxidize (read: turn to rust) under certain conditions. This can vary from a very light discoloration on a small area of a stamp to a stamp that is almost black in color all over.
In some cases, it may be possible to restore the stamp to its original color.
If the stamp is used, you may be in luck. A bath in a light solution of hydrogen peroxide may help remove the traces of oxidation and restore the original ink. Copies that are heavily oxidized may require a longer bath time. After the hydrogen peroxide bath, I recommend soaking the stamp in plain water to help remove any leftover hydrogen peroxide on the stamp.
If the stamp is mint (read: has gum), it is much more difficult. Soaking it in a hydrogen peroxide solution is not possible because it will remove the gum and decrease the value of your stamp.
If your copy only exhibits light traces of oxidation, it may be possible to take a cloth that is very lightly dampened with a hydrogen peroxide solution and gently rub it across the face of the stamp. You donít want to rub too hard and risk tearing the stamp. You donít want to use too much hydrogen peroxide solution because it will soak into the paper and eventually soak through to the gum area as well Ė either disturbing the gum in that area or leaving an area that has no gum.
If your mint copy is heavily oxidized, you may be out of luck. You could try the light application of hydrogen peroxide. However, youíre going to have to do this multiple times, allowing the stamp to sufficiently dry between each application. Otherwise, the stamp gets too damp and it will disturb the gum. Depending on how heavy the oxidation is though, even multiple light applications may not be enough to break through to the underlying ink color.
You can obtain hydrogen peroxide at most drug stores or grocery stores. These over the counter solutions are very dilute and suitable for use on stamps for short periods of time without damage.
You have to weigh the value of the stamp with the cost of trying to remove the oxidation. If you have a very poor used copy of Scott #723 that is heavily oxidized, this stamp is very common and inexpensive. Rather than going to the trouble to remove the oxidation, it may be easier to take $1 (or less) from your stamp budget and buy another used copy instead that is not oxidized.
If you have a mint NH copy of #C1 that you paid over $100 for that copy and itís starting to show some signs of light oxidation in one corner of the stamp, then using a lightly damp cloth to restore the ink color may be worth it, assuming you do not damage the stamp or disturb the gum in the process.
I hope you found this tip useful when dealing with your brown stamps that are not supposed to be brown!