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Gum arabic is a water soluble golden-colored sap collected from Acacia trees native to North Africa. It may be purchased from art supply stores in the form of a liquid, a powder or as dried clumps or fragments. Gum arabic keeps the black pigment suspended in the liquid; otherwise, it would settle to the bottom of the container over time. It also helps to thicken the ink, allowing it to flow more easily from the pen or brush onto the paper.

More importantly, the gum holds the ink at the surface of the paper for a few extra seconds before sinking into the fibers. This influences the appearance and durability of marks made with the ink. The ink line is clearer and sharper than it would be without a binding agent, in part because the ink sinks less deeply into the paper fibers. However, too much gum arabic will cause the dried ink to become inflexible, and it can crack and flake off the surface.


link..^^
http://www.knaw.nl/ecpa/ink/make_ink.html