coloring agent is improved by selecting as the colloid an anionic copolymer of acrylamide and acrylic acid having a weight average molecular weight of about 200 000 to about 400 000 and containing about 8 to about 15 weight of carboxyl groups the ink is free of surfactant associated with the copolymer such an ink is capable of forming dots of coagulated colloid which are resistant to abrasion without producing undesirable background on the electrocoagulation printed images application_number US 43001899 A description according to a preferred embodiment use is made of a copolymer of acrylamide and acrylic acid having a weight average molecular weight of about 250 000 to about 350 000 preferably about 300 000 a particularly preferred copolymer is one containing about 10 weight of carboxyl groups the carboxyl content of the copolymer can be modified by neutralization elimination of the surfactant s generally associated with the above colloid can be achieved by subjecting a dilute dispersion of the colloid to ultra filtration the colloid is preferably used in an amount of about 6 5 to about 12 by weight and more preferably in an amount of about 7 by weight based on the total weight of the ink water is preferably used as the medium for dispersing the colloid to provide the desired colloidal dispersion the ink also contains a soluble electrolyte preferred electrolytes for causing the desired breakdown of the passive oxide film on the positive electrode.