Review: Visualizing Indented Writing on Thermal Paper by the Controlled Application of Heat

Emily C. Lennert



Patterned Evidence


indented writing, thermal paper, receipt paper, latent evidence

Article Reviewed

Johnson, M.; Bond, J. W. Visualizing indented writing on thermal paper by the controlled application of heat. Journal of Forensic Sciences. 2017, 62(5), 1366-1371.


The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.


Indented writing, or writing that is imprinted on paper beneath the sheet that has been written on, may have forensic applications as a form of latent evidence. Indented writing becomes increasingly faint on subsequent sheets of paper, with the sheet immediately below providing the most clearly defined indentation. One form of indented writing detection is by using an electrostatic document apparatus (ESDA). ESDA is proposed to work by an electrical charge variation, which originates from the indentations in the paper. Thermal paper, i.e. receipt paper, is a common paper that may be used for indented writing analysis. Thermal paper contains a thermal reactive layer, which has a color developer and color former that appear white and crystalline at room temperature. This layer liquefies and undergoes a chemical reaction that turns the color developer and color former into a black dye when heat is applied. In this study, the controlled application of heat is used to visualize indented writing on thermal paper. The indented handwriting of volunteers is examined by both the controlled application of heat and ESDA for comparison.

Samples were produced either by controlled indentation or by volunteers. After indentation, thermal paper was heated on a hotplate to 49-51 °C, held in place by a glass plate, to visualize the indented writing. A modified version of the Bandey scale, developed originally for fingerprint evaluation, was used to grade the visibility of indented writing, as seen in Table 1 within the study.

First, indentations were produced in a controlled manner with varying pressure using a letter stamp. A steel “I” letter stamp was used and pulled with constant pressure, from a known mass applied to the stamp, to create a continuous line. Beginning with the mass of the stamp alone, approximately 21.4 g, and increasing in increments of 10 g until ten consecutive samples produced visibility scores of 4. Then, the mass was increased in increments of 50 g up to approximately 387 g, which was selected as a likely mass greater than that applied during writing. Applied pressure was calculated based on mass and surface area of the stamp. This sampling was conducted with one sheet above the thermal paper, then repeated with two and three sheets. Figure 2 within the study displays visibility grading as a function of pressure for one, two, and three sheets. With one sheet, indentations produced higher visibility grades at lower pressures compared to two and three sheets. Visibility grading rapidly increased from 0.5 to 4 at an applied mass of approximately 86.5 g, correlating to a pressure of 3861.8 kPa. For one sheet, all pressures after 3861.8 g produced visibility grades greater than or equal to 3.5. Two sheets produced lower visibility grades compared to one sheet, with higher pressures being required to produce grades greater than or equal to 3.5. Three sheets required the greatest pressure to produce visible grades, with scores of less than or equal to 2.

Then, indented writing samples were collected from twenty volunteers writing with a ballpoint pen. Volunteers wrote a pangram, i.e. a sentence containing all letters of the alphabet, in both uppercase and lowercase on a sheet of paper above thermal paper. This was repeated with two and three sheets above the thermal paper. With one sheet, all samples produced visibility grades of 4. With two sheets, sample grades ranged from 1 to 4. Finally, with three sheets, six volunteers produced no visible indented writing, while eight volunteers produced samples with grades of 3 or greater. Pressure, and therefore indentations, varied between volunteers and also varied within an individual’s writing sample. No difference was observed in clarity between upper and lowercase samples when samples were graded a 3 or greater. However, at lower grades, uppercase developed with greater clarity than lowercase. This may be due to uppercase writing being less practiced, and consequently written with greater pressure. No significant difference was noted between male and female sample grades.

The authors offer a possible explanation as to why controlled heat application leads to the development of indented writing. Compression thins the paper, and condenses the color developer and color former in the area of the indentation, as seen in Figure 6 within the study. This compression may lead to the development of the ink at lower temperatures, i.e. the controlled application of heat.

Finally, indented writing samples were developed by ESDA and controlled heat and compared. Again, twenty volunteers wrote a pangram in uppercase and lowercase on paper above thermal paper. This was repeated with two and three sheets above the thermal paper. Samples were cut in half and developed by both techniques for comparison. The frequencies of visibility grades are shown in Figure 7 within the study. With one sheet, all samples for both ESDA and heating produced grades of 4. For two sheets, both techniques produced a range of visibility scores between 1 and 4. Finally, with three sheets, ESDA produced samples with a range of 0 to 3, while heating produced samples with a range of 0 to 4. Statistically, heating produced more samples with grades of 3 or greater for three sheets. For samples producing visibility grades of 3 or greater, no statistical difference was noted between upper and lowercase samples.

Scientific Highlights

  • Controlled application of heat allowed for the visualization of indented writing on thermal paper at varying pressures and with up to 3 sheets of paper above the thermal paper.
  • The development of the dye at the heat used may be due to the compression of the color developer and color former.
  • With three sheets of paper above the thermal paper, controlled heating produced more samples with visibility scores of 3 or higher than ESDA.


The controlled application of heat offers an alternative technique to the commonly used ESDA, and utilizes common laboratory equipment, making it a low cost technique.

Potential Conclusions

Controlled application of heat may be a viable alternative for the development of indented handwriting compared to ESDA.