I think there are many types of indicators. The mechanism is usually very simple e.g. steam- and temperature-sensitive copper thiosulfate (yellow) decomposes (relatively quickly) to black copper sulfide under sterilization conditions. However, autoclave tapes should be used only for a quick (visual) indication that your materials were exposed to the steam process, not that the process was successful.
When I had to use sterile materials, the tape tells me whether something was already autoclaved. Also, having the tape change color because of the heat in the autoclave also ensures that the machine was working.
The autoclave tape containing temperature-sensitive barium salts which changes colour by heating.
I think there are many types of indicators. The mechanism is usually very simple e.g. steam- and temperature-sensitive copper thiosulfate (yellow) decomposes (relatively quickly) to black copper sulfide under sterilization conditions. However, autoclave tapes should be used only for a quick (visual) indication that your materials were exposed to the steam process, not that the process was successful.
I agree with Andonis that autoclave tape is not a guarantee that your items were sterilized, merely exposed to high temp and steam.
Our autoclaves were updated recently, and programs changed such that my 10 L fermenters were coming back non-sterile - if I let them sit a day or two, the media would be cloudy! We then used Bacillus stearothermophilus inside the vessel (in an ampule) during the cooking process to test new cycles to ensure proper sterilization.
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