At this point, the interaction of the combination solvent with the hydrophilic group of the pigment used or hydrophilizing agent for the pigment should be considered in order that the resultant ink has a zeta potential not less than 20 mV. [0078] Specific examples of the volatile solvents include polyhydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glyco