Art Under The Microscope

Most modern inks and paints get their color from pigments. Pigments, as opposed to dyes which are water-soluble organic compounds, are particulates, usually composed of water insoluble inorganic compounds suspended in solution using polymetric material. When the ink or paint is applied to substrate, the small particles aggregate, and the solvent evaporates. Glittery inks may contain metal such as aluminum or copper to achieve the shimmery appearance. This project aims to investigate the composition of different paints and inks using various imaging techniques. This project analyzed two different types of pens, a gold sharpie and a gel pen, as well as three different types of acrylic paints with varying quality. The samples were imaged using the following techniques: (1) Secondary Electron (SE2), (2) InLens secondary electron, (3) Backscattering (BSD), (4) Energy dispersive x-ray spectroscopy (EDS) mapping, and (5) Brightfield mode on a light microscope. For effective SEM imaging, all samples were (6) sputter coated in gold. The unique microstructures and compositions for each pen and paint were and observed and compared!

2. Methods and Materials

A gold sharpie and a gel pen were applied normally onto a ~5x6cm sample of mixed media paper. Similarly, three acrylic paints of varying quality were diluted with water and a thin layer was applied to a ~5x6cm mixed media paper, typical to how acrylic paint is intended to be applied. The samples were then left to air dry for several days. Once airdried, all samples were analyzed under the Olympus BX51 Light Microscope located at the University of Rochester’s Institute of Optics with a 10x magnification objective under brightfield mode for imaging true color. Each sample was then mounted onto a sample stub and secured using carbon tape. The samples were then sputter coated with gold. This was achieved by placing the samples into bell jar of the sputter coater. The bell jar was then sealed and pumped down to down to ~200mTorr using a rotary pump before allowing the argon gas to fill the chamber to ~500mTorr. This process was repeated 2 more times before the system was pumped down to ~200mTorr. Then, 15mA of current was allowed to run through the cathode for 60 seconds to deposit 60 angstroms of gold. Then, carbon tape was used to ground the pen/paint samples to the sample stub. The sputter coating allowed for good imaging on the SEM with little charging or distortions at an accelerating voltage of 10kV if imaged on the same day. However, subsequent days showed significant charging and drifting, so an extra 60 angstroms of gold were sputtered onto the samples each additional day of imaging. The Zeiss Auriga Scanning Electron Microscope Tool at the University of Rochester's Institute of Optics was used to image all samples. All BSD images were taken at a working distance of around 8mm and an accelerating voltage of 10kV. SE2 and InLens images were taken in the same position with the same working distance and the same accelerating voltage for direct comparison. SE2 images were also taken at a working distance of 5mm for higher resolution images. All of the EDS analysis was performed at a working distance of 5mm and an accelerating voltage of 10kV in order to maximize the x-ray signal while minimizing charge and drift effects. Additionally, drift corrections were applied to each of the EDS mapping analysis with the exception of the gold sharpie, since it was the most conductive sample.

3. Results and Discussions

3.1 Gold Sharpie

Figure 1 shows bright field light microscopy images at 10x magnification at the edge of the applied gold sharpie ink with the paper in view, and in the middle of the sample, with no paper visible. Because the sample was imaged using bright field, the images depict the true color of the sample. Figure 2 shows the micrographs taken with an SE2 detector of the flaky structure of the gold sharpie on the course mixed media paper. A clear boundary between the paper and the sharpie ink is visible. This was hypothesize to be due to the vastly different composition between the paper and the ink. \ This hypothesis is supported by Figure 3, depicting BSD, InLens, and SE2 images all with the same location, working distance, and accelerating voltage. The BSD image shows a large contrast between the ink and the paper with the ink being significantly brighter than paper. This indicates that the ink contains atoms with a much higher Z than the likely organic composition the paper it is applied to. Additionally, the InLens image depicts topological contrast of the sample and thus shows much less contrast between the ink and the paper. The brighter contrast of the ink compared to the paper is due to the fact that the ink is applied on top of the paper and therefore is slightly closer to the InLens detector which sits directly above the sample. Figure 4 shows BSD, InLens, and SE2 images, all in the same location, working distance, and accelerating voltage, at a greater magnification where the flaky texture of the ink is more distinguishable. Again, the textured topology of the flakes are evident through high contrast in the InLens image, whereas the uniformly bright BSD image shows the homogeneity of the ink. The elemental composition of the gold sharpie was investigated via EDS. It should be noted that not all the elements identified by the software were included in the spectra or the elemental composition used for mapping. Due to EDS’s low spectral resolution, it is common for errors to be made in labeling elements. Thus, it is vital to use the knowledge of the sample to determine whether an element is likely to exist in the sample. For example, due to Vanadium’s rarity, it is not likely to exist in the gold sharpie ink despite being identified by the software as in the spectra. Thus, it was not included in either the reported spectra or the the elemental mapping. Figure 5 contains the spectra obtained by EDS analysis and it is clear that the gold sharpie contains copper, likely giving the sharpie it's gold appearance. Additionally, organic elements, such as carbon and oxygen, appeared on the spectra as well, likely mainly originating from the paper. In order to get a clear idea as to which elements were comprised in the ink and which were comprised in the paper, elemental mapping was performed. Figure 6 clearly shows that the copper x ray signal comes from the ink, while the majority of the carbon and oxygen signal come from the paper. Additionally, a significant amount of x ray signals attributed to calcium were mainly located on the paper rather than in the ink, while significant amounts of sodium appeared in the ink. This leads to the conclusion that the gold sharpie is mainly composed of a copper salt.

3.2 Gel Pen

Bright field images on the light microscope at 10x magnification were taken at the edge of the applied gel pen ink with the paper in view, and in the middle of the sample and depicted in Figure 7. This demonstrates the true color of the gel pen as well as the ink's nonuniform coverage on the paper. Figure 8 shows SE2 micrographs of the gel pen and two different spheres are clearly visible contained in the ink. The inconsistent coverage of these spheres compared to the gold sharpie aligns with the observation made in the light microscope images that the gel pen isn't uniformly covering the paper. Further investigation of the spheres' composition was performed on a close up of an aggregate using various detectors in the SEM. Figure 9 depicts BSD, InLens, and SE2 images all with the same location, working distance, and accelerating voltage. The contrast visible in the BSD image shows that the two different spheres are composed of different materials. The more even contrast in the InLens image show how the spheres lay on top of each other compactly. EDS analysis was performed but did not provide any conclusive information about the composition of the of the spheres. This is likely due to a few factors. First, the spheres are relatively small, with the large spheres between 200-300nm. This makes it hard for the EDS spatial mapping to resolve the of the composition of the spheres due to the large interaction volume of the beam, especially within the compact aggregates. Additionally, because the accelerating voltage is set at 10kV, not all atoms are able to be detected in the spectra, so it's possible that some elements that exist within sample are not detected by the EDS.

3.3 Acrylic Paints

Figure 10 shows bright field images on the light microscope at 10x magnification taken at the edge of each of the applied acrylic paints with the paper in view, as well as in the middle each of the samples. Each paint is slightly transparent and the paper is visible through the paint, but the coverage appears uniform. Additionally, the true color for each paint is visible in these images. Figure 11 shows SE2 micrographs of each of the various quality acrylic paints. None of the paints had as neat of a microstructure as either of the ink, however differences between the different paints is apparent. The two lowest quality paints are the "messier" than the high quality paint, with the smaller and more irregular shaped particulates. The highest quality paint had fewer small particulates, giving a "cleaner" look. EDS analysis was performed an all the paint samples. The spectra showed Oxygen, Carbon, and Calcium were all shown to be present in all three of the EDS spectra, however they were not included in this this figure since most of the signal was determined to comes from the paper, consistent with the conclusions drawn from the elemental mapping of the gold sharpie. An EDS mapping analysis of the lowest quality paint is shown in Figure 12. The main components found in the paint were silicon, aluminum, Titanium, and Magnesium. Figure 13 shows EDS mapping of the next lowest quality paint, containing silicon, aluminum and magnesium. Figure 14 shows the highest quality of paint and showed the fewest elements, with only magnesium and silicon being distinguishable. It is interesting to note that with increasing quality of paint, fewer elements were identified. These conclusions however should be met with skepticism however, for reasons outlined in the previous EDS discussions.

4. Conclusion

If you're like me, hopefully the next time you pull out your gold sharpie to mark off the end of the day on your calendar, or your gel pens to write out a thank you letter to a loved one, or your set of acrylic paints to express your creativity, you can appreciate the interesting and beautiful microscopic structure hidden within your artistic expression. :)

Thank you to Sean and Gregg for their support in this project, especially Sean, for helping me struggle through the EDS software.