Patent ID: 8567317

Claim:
A method of hiding and revealing a warning message to indicate whether a label has been exposed to a selectable activation temperature, the method comprising: at a first time, printing with ink of a first color the warning message on a first portion of the label having a thermally sensitive coating of an initial color which substantially matches the first color of the warning message to render the first color of the warning message substantially invisible against the initial color of the thermally sensitive coating and thereby to hide the warning message; at a second time which is after the first time, applying heat to a second portion of the label which is different from the first portion of the label to provide a non-warning message of a second color on the second portion of the label, wherein the second color of the non-warning message on the second portion of the label is different from the first color of the warning message on the first portion of the label; and at a third time which is after the second time, exposing the first portion of the label to the selectable activation temperature to change the thermally sensitive coating from the initial color which substantially matches the first color of the warning message to a different color to render the first color of the warning message visible against the different color of the thermally sensitive coating and thereby to reveal the warning message to indicate that the label has been exposed to the selectable activation temperature.