Short answers, pulled from the story.
Ink is a gel, sol, or solution containing at least one colorant such as a dye or pigment. It can also include solvents, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescents, and other materials that affect its flow, thickness, and dry appearance.
Ink was used in Ancient Egypt for writing and drawing on papyrus from at least the 26th century BC. Egyptian red and black inks included iron and ocher as pigments, along with phosphate, sulfate, chloride, and carboxylate ions, with lead used as a drier.
India ink was invented in China but got its name because its materials were often traded from India. Its manufacture was well established by the Cao Wei dynasty, which ran from 220 to 265 AD.
Pigment inks use solid, opaque particles and are more color-fast but more expensive, less consistent, and narrower in color range. Dye-based inks are stronger and produce more color per unit of mass, but because the dye dissolves into the liquid it tends to soak into paper and bleed at the edges.
Iron gall ink is corrosive and damages paper over time through acid-catalyzed hydrolysis and iron(II)-catalysed oxidation of cellulose. It makes items brittle and the writing fades to brown, threatening original scores by Johann Sebastian Bach, about 25 percent of which are in advanced stages of decay.
Indelible election ink marks a voter so they cannot vote twice, and some types use silver nitrate first applied in the 1962 Indian general election after development at the National Physical Laboratory of India. The method is not infallible, with reports of the ink washing off voters' fingers in Afghanistan.
The German poet Justinus Kerner invented klecksography, the art of making images from inkblots, and published Klecksographien in 1857. Hermann Rorschach kept a klecksography hobby in childhood, which eventually led to the development of his Rorschach test.