Certain kinds of thermal fax machines use thermal paper to print out the faxes they receive, using what's called the direct thermal method. This kind of paper became so synonymous with fax machines that people came to refer to it simply as "fax paper." However, these days, thermal paper is also a popular choice for printing machines that print receipts and barcode labels for retail products.
What makes the paper special is the coating of chemicals on it. If you've ever felt thermal paper, you probably immediately noticed that it feels different from plain paper. It also often has an odd chemical smell. The chemical coatings give it these characteristic textures and smells. They also lend it a slippery surface that makes it difficult to write on.
The chemical coating allows the paper to react to heat. The particular chemicals used vary depending on the machine and the manufacturer, but can consist of dyes, antioxidants, inert pigments, and waxes, among other things [source: Petrovic]. As the machine applies heat in the right places and those areas of the paper rise to the appropriate threshold temperature, the chemicals react and actually cause the paper to darken in those spots.
So, to take advantage of the chemical's ability to darken paper, the fax machine incorporates a printhead that applies heat in the right places. This printhead uses a heat resistor to convert electricity into usable heat. The principle behind this conversion is Joule's first law, which explains that a heating effect comes from the flow of currents and resistance. The heating element receives the encoded information, which tells the printhead where to apply heat and where not to apply it. In the end, the page comes out with readable information on it.
Thermal paper usually consists of at least three layers:
- Substrate layer: This is simply the paper layer.
- Base layer: This includes a binding element, which can consist of such things as starches, gelatin and certain alkali salts. Another component of this layer is what's called a porosity improver, which helps heat travel through the paper.
- Active layer: This layer contains a colorless chemical formula that reacts to heat.
In addition to these three layers, the paper might also include a protective layer to prevent heat from fading or even ruining the document after printing. The paper can feed into the machine in rolls, and an automatic paper cutter separates pages.
This kind of fax machine is said to use the "direct thermal" printing method to distinguish it from the machines that use the "transfer method," which we'll talk about next.