I noticed that some old lenses contain black substances (residues, coatings or powders) on some elements, especially the internal surfaces located between the first and the second element. The substance is not visible (so no picture). I only know that it is there after wiping the lens and seeing what appears to be a black powder on the rag.
The lenses seem to be 1970-1980s. Maybe also the 1960s, not sure. Recently, I noticed it on a Tamron SP 28-135 / 4-4.5 BBAR ™ lens.
It seems to me that this product can be intentionally placed on the lens as an anti-reflective coating. Although I hope not, even if that is the case, it would still be advantageous to clean, because the lenses improve with a sharp open wide after cleaning. The stopped performances are negligibly affected, and I do not notice any particular change in the combustion.
Does this black substance exist intentionally? If not, what could be penetrated inside the lenses to cover the elements?