element is described the ink is substantially the same color as the imaging element background and remains substantially hidden until the imaging element background changes color when heated time temperature exposure indicating labels can be prepared using a printer with direct thermal type and ink jet type print heads application_number US 36519006 A description by way of example various embodiments of the invention are described in the material to follow with reference to the included drawings variations may be adopted fig1 illustrates a thermally sensitive label 10 with hidden opaque printing 20 before exposure to excessive heat the label 10 comprises a substrate with one or more thermally sensitive coatings e g permitting direct thermal printing 30 on the label 10 in a manner well known in the art the label 10 may also include printed material such as a logo 40 added by direct thermal printing inkjet printing lithographic process flexographic printing or the like conventional direct thermal printing paper can be used for the exemplary labels 10 in the example shown in fig1 the initial background color of the label 10 is white the hidden opaque printing 20 would match this background color so as to be essentially invisible before the label 10 is exposed to excessive heat in this example.