In the system of the present invention the ink used is a low concentration aqueous solution of a compound that forms a visible percipitate with a metal ion. The compound and the cooperating ion are selected to produce the desired precipitate color and for greater convenience is selected to react at room temperature. The metal ion is imparted to the printing surface either by inclusion during the manufacture of the paper stock or by treating the paper surface at some subsequent time including during the printing cycle prior to the printing operation.