Iron gall inks normally contain iron, sulfur and carbon, and frequently potassium. Small amounts of copper and zinc are little unusual. Sources for these elements may be as minor contaminants in the iron source, or possibly due to the use of a brass inkwell; the actual source is unknown.While the contamination theory as boring as it is may be the likely truth, the brass inkwell suggestion makes much more interesting story.
Quick research in the inventory of some major museums shows that brass inkwells 15th century and older in their possession come from Iran (Mesopotamia in general). Few of them have the Zodiac engravings like these two found in the Metropolitan Museum of Art. (here and here)
Christi's sold a Flemish brass inkwell from that period. In 15th century Europe the centers for brass production were Belgium and Germany - they supplied mostly the church - brass was somewhat a precious metal at the time. Brass production in Europe generally declined after the fall of the Roman Empire.
Brass inkwell in 15th century would be a luxury item in Europe. Most of the fancy European inkwells of the time I found in the museum were bronze, silver-plated, leather or ceramic.
So if we ignore for a moment the likely true (but boring) explanation about the copper and zinc in the ink - we can dream about VMS European author with brass inkwell who is wealthy enough to afford such luxury, may be associated with the church, may be located in Belgium or Germany, may be in possession of imported inkwell from Mesopotamia, may be interested in brass as part of his/her alchemy work.
So, the little frivolous remark that the ink researcher made while analyzing the Voynich manuscript materials makes a nice story... as always it doesn't move us any closer to the solution.