Natural history museums worldwide house more than one billion objects in wet collections, dry mounts, and on microscope slides, and these collections underpin research on biodiversity patterns and evolutionary processes. Microscope slides are particularly important in entomology. When researchers dissect the genitalia of certain insect taxa to examine internal structures, they often remove the abdomen, treat it with chemicals such as potassium hydroxide to dissolve non-sclerotized tissues, and embed the resulting preparation permanently on a slide, a standard practice for groups like Lepidoptera. Different life stages of various insect groups are also stored this way. For the preparation to remain useful, the specimen and the permanent slide must be perfectly labeled so they stay correctly linked; if the label becomes illegible, the dissection may become scientifically orphaned.

The problem is that labels are typically made of fragile materials. Paper and ink are susceptible to environmental degradation from humidity, light exposure, and temperature fluctuations. Collections of micro preparations are often stored in cabinets that can become dusty and damaged over time, and even when the preparations themselves remain intact, the labels can become unreadable over the years. Curators have used pencils, pens, inks, and inscription tools of many kinds to document data on labels, and today printers are common. But printer choice matters: inkjet printing is not recommended because the ink can fade in light and is vulnerable to water and solvents, while laser-printed labels have long raised concerns about durability, particularly in fluid preservation contexts, where both the letters and whole labels tend to flake off and become unreadable.

Paraloid B-72, an acrylic resin composed of ethyl methacrylate and methyl acrylate copolymers, is renowned in the preservation field for its stability, ease of application, and protective qualities. Conservators widely use it on archaeological and fossil artifacts of wood, copper, and other materials, where it forms a protective layer that shields objects from environmental damage. Its effectiveness stems from the low chemical reactivity of its copolymer components, which minimizes interaction with substrate materials and reduces the risk of deterioration. The resin also resists ultraviolet radiation and extremes of temperature and humidity, and it is expected to last more than 100 years while remaining easily soluble in organic solvents, a property that matters because any future conservator could reverse the treatment if needed.

The resin is not universally suitable, however. Previous work outlined its use for preserving marking inks on glass, metal, wood, and ceramics but advised against applying it to paper, textiles, some plastics, and unstable surfaces. Related polymers have their own trade-offs: Paraloid B-67 shows similar aging characteristics but lower stability during thermal degradation, while water-based polymers such as Aquazol and Acrysol WS-24, though stable, are water-soluble and therefore unsuitable for collection labels. Butvar B-76 performs well in appearance but has exhibited chemical change due to weathering on archaeological material. Whether Paraloid B-72 could safely coat paper labels on microscope slides was therefore an open question the researchers set out to test directly.

To do so, the team tested various paper types and writing materials used for microscope slide labels. The papers included acid-free archival paper and card at 80 and 160 grams per square meter, red paper and yellow paper at 160 grams, and standard recycled paper at 80 grams. The writing materials were inkjet print, laser print, archival ink, pencil, ballpoint pen ink, and fountain pen ink. Paraloid B-72 was dissolved in ethyl acetate at concentrations of 15, 20, and 30 percent, with periodic stirring over the course of a day to ensure a homogeneous mixture. Attempts to dissolve the resin in 75 percent ethanol failed, producing a gelatinous solution unsuitable for coating. Coatings were applied by immersing the slides in a beaker glass for one second, allowing the excess to drip off, and drying them on small pieces of wood, with one- and two-layer series tested and roughly three hours of drying time between layers.

The results contained a surprise. When labels were initially immersed in pure ethyl acetate to test their stability, the laser-printed letters smudged in half of the trials, while the inkjet-printed labels remained unchanged. Notably, this smudging did not occur in the solutions containing Paraloid B-72, suggesting the resin itself alters how the solvent interacts with the printed surface. Across the coating trials, inkjet prints proved highly stable when exposed to the Paraloid B-72 solutions, as did archival ink, pencil, and fountain pen ink. Ballpoint pen ink, by contrast, proved unsuitable because it smudged during the immersion process, a finding that curators will want to note before treating older labels inscribed with everyday pens.

The paper type and resin concentration both mattered considerably. All paper types darkened slightly and acquired a sheen after coating, though legibility remained consistent across samples, and a paper weight of at least 160 grams prevented bleed-through and maintained label integrity, whereas the lighter 80-gram recycled paper allowed the coating to bleed through. Among the concentrations, 15 percent provided minimal coverage and risked insufficient stabilization with a single layer. The 20 percent concentration delivered optimal results, with effective coverage and manageable tackiness, although minor bubble formation was noted. The 30 percent solution was impractical: excessive bubble formation impaired both label legibility and ease of use, particularly after a second layer. In practical terms, two layers of the 15 percent solution provided the best stabilization with a reduced risk of smudging, albeit with a longer processing time, while a single-layer application of the 20 percent solution was a faster alternative with slightly more handling risk.

The researchers recommend using Paraloid B-72 solutions when coating large quantities of slides that are not classified as type material, citing the method’s efficiency and practicability when processing many samples at once. For type material of particular scientific importance, however, they advise considering alternative methods, since older slides and the inks and papers used decades ago might react differently to the treatment compared with more recent material. Before applying the solution to any label, the team stresses that it must be verified whether the inscription can be dissolved by the ethyl acetate and Paraloid B-72 mixture, and monitoring throughout application is recommended to prevent labels from flaking off during drying. Careful application is crucial to prevent adhesion issues as the coating dries, indicating the technique still needs refinement.

Even with those caveats, the study offers natural history collections a promising, sustainable tool. Improving label preservation matters across a broad spectrum of collection-based fields beyond entomology, including geography, botany, zoology, paleontology, parasitology, microbiology, and medicine, all of which depend on the data labels carry. By extending the durability of these fragile sources of information, the coating approach minimizes the need for elaborate label restoration later, aligning with the growing emphasis on sustainable preservation practices in museums. As collections face mounting pressures from aging infrastructure and environmental change, simple preventive treatments like this one may prove essential for keeping a billion objects, and the stories written on their labels, readable for the next century.

Subject of Research: Coating of microscope slide labels: a possible solution for their long-term preservation

Article Title: Coating of microscope slide labels: a possible solution for their long-term preservation

Article References: Müller, S., Rajaei, H., Wendt, I., Werner, M. J., & Wanke, D. (2025). Coating of microscope slide labels: a possible solution for their long-term preservation. Natural History Collections and Museomics, 2, 1-9. https://doi.org/10.3897/nhcm.2.153194

Image Credits: AI Generated

DOI: 10.3897/nhcm.2.153194

Keywords: Coating, microscope, slide, labels, possible, solution, long-term, preservation, scientific research