| abstract |
Compositions, devices and processes related to etching of a very thin layer or fine particles of a metal are disclosed for monitoring a variety of parameters, such as time, temperature, time-temperature, thawing, freezing, microwave, humidity, ionizing radiation, sterilization and chemicals These devices have capabilities of producing a long and sharp induction peπod of an irreversible visual change The devices are composed of an indicator compπsing a very thin layer of a metal (e g, polyester film having an extremely thin, e g, about one hundred Angstroms layer of aluminum) and an activator, e g, a reactant, such as water, water vapor, an acid, a base, oxidizing agent or their precursors, which is capable of reacting with the said indicator The indicator retains its opacity and metallic luster, e g, silvery white, mirror like finish of aluminum layer for a long time The activator destroys the indicator layer including the naturally formed oxide layer |