US Scott 834/834A Oxidation Question In Relation To Expertization Certificate.

Recently had a block of stamps expertized (what I believed to be 834a) which came back as being 834, with an opinion that states the carmine red was oxidized to appear red brown. I found a similar situation referenced here:
http://virtualstampclub.com/lloydblog/?p=6861

which in pertinent part states, Addendum: The Philatelic Foundation found that the stamp "is not Scott 834a, rather it is a Scott 834, carmine and black, the carmine color oxidized." PF did not provide any information as to how they reached that conclusion.

I am curious if any specialists or experts in the community can elaborate and provide further insight into this occurrence.

My next post will show two images, one of a carmine red 834 that I possess and the other of the one I had expertized, side by side.

Thank you all in advance.

One way to try to see the original color of a stamp or cancel is to look at it under high magnification. You can often see individual grains of color that are in the original color. In this case, they might have spotted some bits of red among the darker bits. This would have pointed to oxidation of most of the red, as opposed to having been printed from red-brown ink. You can do the same when looking at cancellations that appear dark green. Often, there are black bits with an oily substrate that makes the cancellation appear greenish.

However, in this specific case, I cannot say what the PF did or did not do, only what I've done in the past.

C.

Thank you chipg, I appreciate the insight and will look at the stamp under 30-60x magnification to detect the bits of red that you mentioned may be among the darker bits.

Will update the thread.

The ink on stamps can change color due to dirt, moisture, chemical contamination. The Great Britain penny red can be turned to black if exposed to Sulphur fume, Or so I understand. I bow to Chipg superior knowledge on how to detect the difference. GB Penny blacks with Perfs are sulphuretted penny reds.