Abstract: A method of coding an optically-readable medium comprises: applying to the medium at least one coating of a heat-sensitive material to produce a specified combination of at least two optical changes as a function of time when heated to a predetermined temperature above ambient; heating the medium to the predetermined temperature; and sensing the optical changes undergone by the medium to determine whether the sensed changes match the specified combination. The method and apparatus are described particularly to an optically-readable medium wherein the heat-sensitive coating is in a plurality of windows formed in the medium, each window signifying a value element.