Why do train tickets and movie tickets fade? How should we preserve them?

When collecting paper tickets such as train tickets, we often find that the words on the tickets gradually fade over time. The reason behind this phenomenon is related to thermal printing technology.

Thermal printing technology is a printing method that does not require ink cartridges or toner. Its principle lies in the special coating on the ticket. This coating contains a colorless dye with a lactone ring structure. When it comes into contact with a developer and is subjected to heat and pressure, the lactone ring opens to form a colored compound, thereby achieving the "printing" of information. This technology is widely used in train tickets, movie tickets, supermarket receipts and some photo printers because of its low cost and convenient operation.

A notable feature of thermal paper is that the heat and pressure generated by friction can leave marks on the ticket surface, which is why we often see a black mark left by swiping a finger on a ticket. However, thermally printed tickets are difficult to preserve for a long time because the color reaction is reversible and exposure to sunlight and air will cause the writing to gradually fade.

In order to preserve thermal paper tickets for a long time, they need to be isolated from air and light. Plastic sealing is not recommended because the heating during the plastic sealing process may cause the ticket to turn black as a whole. Some collectors choose to use small bags and only plasticize the four sides of the ticket. Although the effect is limited, the operation is more cumbersome. The composition and process of different thermal papers determine the length of their preservation time, and train tickets have a relatively long preservation time among thermal papers. For example, a train ticket from ten years ago that is kept away from light can still be recognized, while a movie ticket from half a year ago may have no trace of the words.

In summary, although thermal printing technology provides convenience for ticket printing, it also brings challenges in preservation. For collectors who want to preserve tickets for a long time, it is important to understand the characteristics of thermal paper and take appropriate preservation measures. Nevertheless, the fading of thermal paper tickets is inevitable over time, which may be the unique mark left by time on these tickets.

Author: Chai Knows

Reviewer: Sun Mingxuan, Professor of Shanghai University of Engineering Science

The article is produced by Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting.