more info for the interested:
How do they work?
http://www.temperature-indicators.co.uk ... ucatio.htmThe key component at the core of the label is a number of active layers – and remember we’re talking very, very thin layers. One of these layers is a carefully formulated white wax which is located over a layer of black paper. When heat is absorbed by the paper it expands – like a sponge – whilst melting the wax. The wax seeps through the black paper, giving the visual colour change from white to black. A red paper can be used to give a white to red colour change. This whole process takes a maximum of about 4 seconds, with that time decreasing if a indicator temperature is exceeded very quickly. All the labels are backed with adhesive and a good seal to the surface is part of ensuring the indicator works well.