Among the color pulps used for coating paper, there are some that bear the reputation of giving difficulty on the coating machine, while others run perfectly under the same apparent conditions. Since Bingham first called attention to the plastic flow of solids in suspensions, a great deal of work has been done on the flow of dry pigments dispersed in oily vehicles. By applying similar methods of study to pigment pulps used in an aqueous medium, some light has been thrown on a bothersome coating problem which has hitherto defied solution.