Good question. An ink is a liquid containing various pigments and/or dyes used for coloring a surface to produce an image, text, or design. Ink is a complex medium, comprising solvents, pigments, dyes, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescers, and other materials. The components of inks serve many purposes; the ink’s carrier, colorants, and other additives are used to control flow, thickness, and appearance of the ink when dry.
Ink Ingredients: Industry Research Report
http://www.reportlinker.com/p090591/Worl…
HOWEVER, GEL inks are particularly resistant to erasure and frequently bear recommendations from forgery experts. ANIMAL collagen is added.
I hope this helped.