Switzerland - Colour Error?

I'm now tackling Switzerland and am on the early years. I have the same stamp shown here:
http://www.celestamps.com/product_i...7d8411e659c3

However, the stamp does not appear to be orange at all. I have used a microscope and various magnifiers and I am almost certain that it is the 20R and not the 5R in a dark brown colour. Is it possible this is a colour error or that the orange colour has somehow darkened? I can provide a picture if that will help unless someone knows something about this without one. Thanks!

Early orange pigments will slowly turn very dark (blackish) when exposed to polluted air. It's interacting with the sulfur in the air. The process is "reversible" by chemical treatment -- but there are differences of opinion regarding whether this constitutes altering the stamp and also whether one should be required to disclose this chemical treatment of the stamp.

Here is an example. Left stamp is a color changeling because of exposure to sulfur in the air. The stamp at right is the original color.

Wow is that ever dramatic! But very good to know. Thank you for the examples and this would be a mystery solved. Now, to spend my day determining the paper types of each. Thanks to you guys, I'm slowly learning a lot about the early year varieties and differences. Much appreciated!

Are you sure it's sulpher and not oxygen? Perhaps both? Many shades of orange and red contain iron oxide which might not be 100% pure oxide. Residual iron in the pigments will oxidize, causing this color change.

Rileysan poses a good point. I am not familiar with the exact pigment composition used to make the 15c orange special delivery.

However, in general, the problem is usually sulfur, not oxygen, that is causing the color changeling. The term oxidation used in philatelic circles is sort of a misnomer, if you will.

On the problem stamps, one of the main causes is the use of compounds such as lead chromate. In a nutshell, upon prolonged exposure to sulfur pollutants in the air, lead chromate changes to lead sulfide (black). Chemical treatment to "restore" color involves changing the lead sulfide to lead sulfate (whitish). This converts the overlaying thin "black" layer into a relatively invisible layer.

If increasing iron oxides were causing the problem, then the chemical treatment would not work because they oxidize the stamp even more. Therefore, the problem must be dominated by issues with lead chromate type pigments.

Something like that... I'm certainly open to correction...

Thanks for the education, khj! I had no idea. But the thought occurs to me ... could this be the cause of USPS employees going "postal"?

Hear me out!

Lead Chromate (esp if it contains high levels of hexavalent chromium) could leach into the gum. Years of licking stamps would eventually lead to dementia (from the lead) and even cause tumors (from the chromium) that can spread to the brain.

I need to report this to the CDC!

So then what you are saying is that I should just lick this stamp several thousand times. If I begin to experience dementia, it's a dye reaction problem. If not, it's a million dollar colour error?

Gotchya!

As long as you remember to lick the back of the stamp and not the front, you should be OK.

Unless of course, you lick on average 100,000 older yellow or orange stamps every year... Fortunately, there are not that many yellow/orange US postage stamps, so I should still be OK doing all the licking for my mailing lists.

Yes, you are right Rileysan; lead/chromate-based pigments are bad news, and as far as I know, are no longer used in modern stamp production -- but don't hold me to that. That is why you don't see these types of color changelings on modern stamps.

I don't know what is used to produce the yellow and orange ink these days. I'm old school. If it isn't carcinogenic, then it's not worth using or eating...