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Why do thermal receipts fade back to white?

Thought this would be an entertaining question...

We all know the basic principles of a thermal receipt printer - it doesn't actually print anything onto the receipt paper, rather the paper is exposed to some kind of radiation (light? heat? something else? I'm not actually sure) which that causes parts of it to turn black.

These receipts fade out over time and eventually you can't read the text anymore, especially if you leave the receipt in your wallet for a long period of time.

As a result, many people, particularly small business owners, get into the habit of photocopying these receipts before filing them away for tax or expense recording purposes so that they're still legible years later.

But why, exactly, do they fade back to white? What's going on at a scientific level?

Is the plastic actually changing color back to white? Is it just rubbing off? Is it exposure to the sun that causes it to fade?

A corollary to the question would be: Would the label fade out even if you stored it safely in a cool, dark place? What can you do to preserve the label without photocopying it?

Thanks in advance for the discussions!
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heat caused the paper to turn black. With age the black fades and the background gets blacker until the contrast = zero.
Cold storage would retard, but not eliminate, the aging process.
The thermal paper that came with the Sinclair printer (went with the zx81) used to turn completely blue if you tried to photocopy it. I expect both paper and photocopiers have improved since then.
Distinguished Expert 2019

here some answers : heating it can make it readable again :  http://answers.yahoo.com/question/index?qid=20070507235729AAkhuBC
dhsindyRetired considering supplemental income.

Heat, light, and some chemicals can affect thermal paper which is a chemically treated paper (various chemicals are used).  To just store, put it away in a manila folder in a dark cool place.  For long term storage, scan the receipt to a digital file and/or print it with regular printer.
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Standard POS grade thermal papers use dye and coreactant technology to form an image. This produces an image that is sensitive to ultraviolet (UV) light. With extended exposure to office light or shorter exposure to intense UV light, the image may start to fade. The stability of the printed image will ultimately depend upon the degree to which the image was originally developed, the individual product design, the intensity of the UV light and the character of UV (percent of UV range in light source).
Because thermal paper is sensitive to ultraviolet light, it must be stored properly. Thermal documents should be stored in a dark place at a relative humidity between 45% and 65% and a room temperature below 77°F (25°C). If these conditions are met and the equipment used to create the thermal documents is set to image the paper fully, the images should remain legible for at least five to seven years.

In addition to UV light, exposure to the following materials may lead to image degradation and should therefore be avoided:

• Organic solvents (i.e. Alcohols, Ketones, Esters, and Ethers)
• Cleaning Fluids
• Plasticizers
• Petroleum Solvents
• Wet-type diazo copy paper
• Carbon papers
• Castcoated papers
• Papers containing tributal phosphate
• Dissimilar thermal systems
• Carbonless papers
• Ammonia
• Certain Oils
• Water (prolonged exposure)