activation temperature for revealing the hidden message 20 can be adjusted by changing the thermal sensitivity of the label 10 the substantially opaque white message 20 may be applied over a protective layer as desired alternatively or in addition a substantially transparent protective layer may be applied to the label 10 on top of the printing 20 the label 10 is not limited to white thermal paper and the hidden message 20 is not limited to white opaque ink other colors of paper and inks may be used similarly the label 10 is not limited to black thermal imaging other imaging colors being possible in another application of a thermal label or thermal paper 10 the hidden message 20 could be used as a security feature when the paper 10 is thermally printed an area can be intentionally thermal printed to expose the hidden print 20 authenticating the media the foregoing description above presents a number of specific embodiments or examples of a broader invention the invention is also carried out in a wide variety of other alternative ways which have not been described here many other embodiments or variations of the invention may also be carried out within the scope of the following claims as shown in fig3 a printer 50 for printing of time temperature indicator labels 10 can be constructed using a direct thermal printing print head 60 to print first indicia 30 on the labels 10 and an ink jet print head 70 to print second indicia 20 on the labels 10 using well known print head technologies the printer 50 includes a platen 80 opposing the thermal print head 60 and the labels 10 are presented for printing.