Material Used For Golden Glimmer On This Brown 5c Stamp

Hello,

what was used in printing production that is or has developed to such a golden glimmer like in the shown stamp?

I know this is not really gold :) and not rare as I have seen several like this. I just want to know the reason and name of the material / ink / print finishing and why it is not present at any other stamp colors or other stamp issues?

Kind regards,
stamperix

Looks to me like it is an atmospheric thing - this stamp is probably going to be a changeling.

Peter

It would be great to get a technical explanation of this. It looks like a precipitate on top of the ink.

I have seen older orange issues do the same thing as they oxidize.

I am fairly sure that older inorganic inks can have metallic elements in the pigments. Goggling will offer some info but perhaps we have a chemist among us that can assist?
Don

Yes, it's from the heavy oxidation; there is a metal component to the ink here. It's been badly stored, perhaps near a coal-fired furnace, coal-oil gas light or lamp, that contained a fair amount of sulfur/ sulfur compounds.

Excellent photo showing this.

Only if you have bad eyesight.

Thank you for all your answers, also hy-brasil's explanation. While I have seen this also on one or two other brown 5c, I never have seen it for any other stamp, also other countries. So it must have been some ink try for this brown color stamp in its first issue, which before and later wasn't done again. Maybe the ink was too expensive this way or not reliable enough. There is so much to learn and research at stamps, you never know where to begin and take your time...

I have fell for this curiosity myself, with Belgium.
I was convinced (at the time) this was a variety. I was politely advised otherwise.

If you click on the image, you can see the Value tablet has a silvery sheen.