A substrate card, in the form of a Metrocard.RTM., farecard, keycard, passport, boarding pass or the like, presented for controlled gateway passage at an automatic reader, is photographed and the captured card image is digitized. The card then is heated to a point where trace explosive present on the card is subjected to an energetic reaction imparting a char smudge signature. The card is cooled and a second image is captured and digitized. The digitized images are subjected to a subtractor stage resulting in an isolated char smudge signature image possibly in the form of at least a partial fingerprint residue recognizable by a computer. Upon char image detection, which may include a fingerprint matching process, the computer directs the controlled gateway to restrain the card presenter and secure the card as evidence. The card may be pre-coated to avoid ignition and to enhance trace material adhesion.