Can Green Overprint Turn Black?

I found a Lourenco Marques stamp in my collection. The Scott Catalog
says that the overprint should be Green. I looked under several bright
lights, and it sure looks like Black to me. Can it really change this
much over the years? Thanks.

You would think that if the green ink oxidized you would also see some evidence of this around the overprint on the stamp itself.

Not sure if green oxidizes (suphurize?) at all really anyway. But the rest of the stamp looks OK.

Is there a second printing of this in black?

The only overprints that I could find were in 1911 issue, and later in
1914-15. No mention of 25 overprint anywhere but 1911, when it was
stated as a green. I'm sure someone else will point out my error, but
the chase is on!

Green can oxidise to black,
I have only seen it in stamp printing ink, (portugal)
never in an overprint.
What you have is rather odd.

Have you looked at it under magnification?

The later overprint is sans serif (accd'g to Yvert), and your cancel looks to be 1913 (pre-1914 overprint).

I make it 1911 only
The opt looks fairly well done
the only possible diff is the B and the A in the type
when compared to Portugal opt.

Following up on rod222 suggestion, I looked at it under my best
magnifying glass and I do think that I see a very strong, dark green
in the first few letters. At 1200 dpi scan, you can see that the
green looks visible against the white background on the first few
letters, but VERY dark to black everywhere else. If someone has the
same stamp, could they please scan for comparison? Otherwise, what
I think we have is just a heavily overinked green that could have
oxidized over the years.



Thanks for everyone's excellent work, yet again.

That dark green shading ,almost black is normal on that stamp .

If that is the case..solved. Well done FT
I didn't have a copy of this issue in my collection.