Thermochromic ink (also called thermochromatic ink) is a type of

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...

that changes color when

temperatures Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

increase or decrease. Often used in the manufacture of many toys or product packaging, as well as thermometers. Thermochromic ink can also turn

transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...

when heat is applied; an example of this type of thermochromic ink is found on corners of an examination mark sheet. This proves that the sheet has not been edited or

photocopied A photocopier (also called copier or copy machine, and formerly Xerox machine, the generic trademark) is a machine that makes copies of documents and other visual images onto paper or plastic film quickly and cheaply. Most modern photocopiers u ...

, and also on certain pizza boxes to show the temperature of the product. Use on

packaging Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...

can be to detect temperature history during shipping and to indicate proper heating in an oven.

Examples

On June 20, 2017, the

United States Postal Service The United States Postal Service (USPS), also known as the Post Office, U.S. Mail, or Postal Service, is an independent agency of the executive branch of the United States federal government responsible for providing postal service in the U ...

released the first application of thermochromic ink to postage stamps in its Total Eclipse of the Sun Forever stamp to commemorate the

solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six month ...

of August 21, 2017. When pressed with a finger, body heat turns the black circle in the center of the stamp into an image of the

full moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...

. The stamp image is a photo of a total solar eclipse seen in

Jalu Jalu, Jallow, or Gialo ( ar, جالو) is a town in the Al Wahat District in northeastern Libya in the Jalo oasis. An oasis, a city, and it is the main center of the oasis region in eastern Libya. It is located at the confluence of longitude an ...

Libya Libya (; ar, ليبيا, Lībiyā), officially the State of Libya ( ar, دولة ليبيا, Dawlat Lībiyā), is a country in the Maghreb region in North Africa. It is bordered by the Mediterranean Sea to the north, Egypt to Egypt–Libya bo ...

, on March 29, 2006. The photo was taken by retired NASA astrophysicist

Fred Espenak Fred Espenak is a retired emeritus American astrophysicist. He worked at the Goddard Space Flight Center. He is best known for his work on eclipse predictions. He became interested in astronomy when he was 7–8 years old, and had his first tel ...

, aka "Mr. Eclipse".

References

Thermochromism Dyes {{materials-stub

Examples

See also

References