USA #178 Dark Portrait Background

Looking at the Andrew Jackson on the right I determined it to be a SC#178 (through paper quality). In comparing it to my existing 178 (on the left) I noticed that the background colour in the oval, behind Mr. Jackson's portrait seems almost black, compared to the vibrant vermilion of its counterpart. Even ignoring the cancel, and the fact that the right hand stamp appears a little dirty, the ink in the oval itself seems dark.
Can anyone explain this to me, or am I seeing things?



I think there may have been another posting with a similar question recently, but I wouldn't know where to find it.
Thanks

The surface ink has been contaminated with sulfur pollutants from the air. In your example, it's more noticeable within the oval because there is more ink coverage within the oval.

See this older thread for a comparison of contaminated and uncontaminated stamps.

https://goscf.com/t/15068