Place the charcoal bricks in a barbecue grill outside. Heat the charcoal until it breaks down and becomes ash, approximately three to four hours.
Place the ashes in a large stone pot and crush as finely as possible with a blunt object, much the same way you would use a mortar and pestle, but on a larger scale.
Add the collected rain water to the stone pot slowly until it the mixture is the desired color. The more water you add, the lighter the ink will be. Add the gum arabic and stir. The gum arabic will act as a binding agent that will allow you to carry the ink on your writing tool.
Let the mixture coagulate overnight in a well-ventilated area without stirring. Test the color and add more water if necessary.
Transfer the India ink to a bottle or jar with a dropper. India ink will keep for approximately three weeks if stored in a cool, dark place. If the ink is used outside it will evaporate more quickly.