1. How Receipt Printers Make Text Without Ink or Ribbons
Standard printers use ink or ink-soaked ribbons pressed against paper to create text. However, the machines that print receipts contain neither of these. Instead, a component that generates heat simply touches the paper directly [1]. This method is said to have several advantages. Because the heat source contacts the paper directly, ink does not bleed or scatter, resulting in clear printing. It also allows for immediate printing upon powering on, and since no ink or cartridges are needed, the machine’s structure becomes simpler [1]. So, why does the paper turn black without ink? The answer lies not in the machine, but in the paper itself.
2. Thermal Paper's Colorless Dye and Acid Coating
The surface of thermal paper is coated with a dye (leuco dye) that turns black when it touches acid, and an acidic substance (developer). Normally, both exist as separate solid particles, so no reaction occurs [2]. When heat is applied, these two substances melt and come into contact, causing a chemical reaction that produces black color [3]. Paper manufacturer documents note that substances to speed up this color development are sometimes added [3]. This is similar to how red cabbage juice changes color when vinegar is added, as both involve acid triggering a color change. However, the difference is that in thermal paper, solid particles melt and meet due to heat, rather than liquids mixing.
3. The Thermal Head's Row of Tiny Heating Elements
The part that touches the paper is called a thermal head. It consists of a ceramic plate with a row of tiny heating resistors. When electricity flows through them, the resistors generate heat, which passes through a protective film to the paper, creating color [1]. Each heating element (heater) is said to be tens of micrometers in size (a small fraction of a millimeter). Explanations state that the temperature can instantly rise to several hundred degrees Celsius [4]. Text is formed by heating only the specific heaters needed, arranging small black dots. Some heads have 300 dots per inch (25.4 mm) [1].
4. Other Uses of Thermal Paper: Tickets, ATM Statements, Medical Images
Thermal recording is said to have spread in the 1980s for use in fax machines. Today, besides receipts, it is used for tickets, ATM statements, and medical image outputs like X-rays, CT scans, and MRIs [1]. Because there is no need to refill ink and the structure is simple, it is suitable for machines placed in fixed locations that output large amounts of short text or images. However, this wide range of uses is also tied to the nature of the paper.
5. What Makes Thermal Paper Fade: Heat, Light, Oils, and Chemicals
Paper manufacturer documents list factors that cause thermal paper to fade, including contact with moisture or oils, high temperature or humidity environments, and direct sunlight or fluorescent light. Contact with items containing solvents or plasticizers has the same effect [3]. According to an explanation by the Chemical Society of Japan, color can disappear over time or when chemicals like alcohol touch the paper, as the dye and developer can separate, among other causes [2]. Depending on the wallet material or storage environment, the receipt may come into contact with factors that cause fading. It makes sense to think that because the color-producing mechanism is a reaction driven by heat, it is easily affected by heat and chemicals.
6. Longer-Lasting Thermal Paper and How to Store It
As a countermeasure to fading, thermal papers with improved preservation properties have been developed [2]. Manufacturer documents state that if stored appropriately, discoloration and fading can be largely prevented [3]. How long a receipt lasts varies greatly depending on the type of paper and where it is kept. For records you want to keep for many years, making a copy or taking a photo are also options to consider.
Things to Try Yourself
If you want to investigate further, you can try a simple experiment at home. Take one receipt and cut it in half. Place one half in a sunny window and the other in a drawer. Check the darkness of the text every few days to see if there is a difference. Sometimes there is no difference, and sometimes changes do not appear immediately, so record the results even if nothing changes. Do not use fire, irons, or chemicals. You can also compare old receipts at home with recent ones to see if the text darkness or color differs. Differences may not appear depending on the paper type and storage method. To learn more, explanations of the color development mechanism can be found on the Chemical Society of Japan’s "Chemistry Club" site and on the thermal paper pages of various manufacturers.