In a thermal dot-matrix printer which is adapted to both write and erase on a substrate which clarifies and opacified under different thermal regimes, the dot-printing and dot-erasing signals are selectively fed to the thermal printing elements of a single printhead, and all such signals are composed of a sequence of pulses. As described writing requires more energy than erasure, and a dot-writing signal therefore has more pulses than an erasure signal. The pulse patterns disclosed for both dot-writing and dot-erasure signals include:- (a) a regular sequence of rectangular pulses (50% duty cycle) (b) a broad pulse followed by (a) (c) a sequence of progressively narrower pulses (duty cycle > 50% --> duty cycle < 50%) The invention may be applied to erasing and rewriting a visible indication representing the remaining value of a magnetically coded consumable-credit card with thermal recording layer (28, Fig. 2). <IMAGE>
