A really fascinating blog post, which I missed back in September, about the materials which were used to write on and illustrate papyri, using the example of the New Kingdom papyrus, the Book of the Dead of the Goldworker Amun, Sobekmose.
The ink is made by burning organic materials such as wood or oil, and then pulverizing the material before mixing it with water. To keep the particles from clumping together, the black is mixed with a binder, probably a plant gum from the Acacia tree family. As a valuable source of timber in Egypt, its branches may have also been used as the source for the charcoal. As well as keeping the carbon particles suspended in the water solution, the gum binder helps to keep the ink adhered to the papyrus surface. This ink is very stable, does not fade, and does not deteriorate the papyrus below as some metallic inks can do.