Controlled water evaporation from ink jet inks

An ink jet printing apparatus for ejecting an aqueous ink from a reservoir through a nozzle to form high precision images on a recording material, said apparatus comprising an ink reservoir for containing an aqueous ink, an airspace for receiving water evaporated from said aqueous ink to form a humid atmosphere within said airspace, and a ventilation port in said reservoir open to ambient air outside the reservoir to permit the release of water from the humid atmosphere within the reservoir airspace to the ambient air outside the reservoir whenever the humidity of the ambient air is lower than the humidity of the airspace within the reservoir, and a moisture barrier positioned between the ink in the ink reservoir and the ventilation port, the moisture barrier being formed of microencapsulated water beads adapted to provide a sustained release of the water to ambient air through said ventilation port, in order to maintain the water content of the ink at an equilibrium.