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.The copper and zinc may have other sources. How about using of brass mortar to crash the oak galls into ink?
This beautiful mortar comes from Austria, around 1451, and today belongs to the Metropolitan Museum of Art (here)
Probably accidently this mortar has a frog ornament that looks very close to a drawing in the Voynich manuscript f 101v.
The use of brass mortar in medicinal recipes is described in 14th century British manuscript
called The Physicians of Myddvai (or Meddygon Myddfai) here.
...Get linseed, pound in a brass mortar, make an emulsion therefrom with pure water, boiling it as you do porridge...The physicians also used brass pots, vessels and basins.
...Take a handful of mallows, of snails shells, of pennywort and linseed, pound them in a brass mortar...