Methods and control systems are provided for operating a thermal printer adapted to print images by heating and transferring donor material from donor patch sets from onto a receiver medium, the printer being operable to print images in a manner that exhausts a full donor patch set or a fractional donor patch set during printing. The control system comprises a controller for identifying one of the donor patch sets on the web as a fractional donor patch set having sufficient donor material remaining to print an image. The controller is further adapted to determine that the fractional donor patch set is not to be used for printing when the identified fractional donor patch set has been used to print an image wherein at least a minimum area of at least one donor patch has been heated to a level above a threshold level.