On Friday May 30th, Ms. Fernanda Mokdessi Auada presented an account of the joint salvage effort undertaken by the Nucleus for Conservation of Public Files of São Paulo (APESP) and the Nucleus of Restoration-Conservation Edson Motta, Laboratory del National Service for Industrial Apprenticeship (NUCLEM-SENAI) following the 2010 flooding of São Luiz do Paraitinga, Brazil. Collective gasps went up from the audience as Auada showed photographs of the devastated city. Among the images was the city all but subsumed by the Paraitinga river, and shots of devastating structural damage to the city’s principal church (São Luiz de Tolosa) and its municipal library.
Thousands of documents, over 15 linear meters in total, were immersed in the flood waters for over 20 days. The papers related primarily to the population’s citizenship and legal identity, making it vital for conservators to save the information contained in the wet and moldy files. Despite the grave condition of the documents–and the challenge of having virtually no money or trained support staff–the overall salvage was a success, Ms. Auada said.
The documents arrived for salvage in three allotments. The first two allotments were treated manually, using traditional flood damage salvage procedures. First, the documents were separated and air dried flat on top of absorbent paper. The documents were then individually documented and inventoried during dry cleaning, these steps carried out in a dedicated cleaning area. Documents that could not be separated mechanically after drying were separated while immersed in an aqueous bath. Papers soiled with heavy accretions of dirt and mud were washed to recover legibility. The papers were then mended, flattened and rehoused in paper folders and corrugated polypropylene boxes. Incredibly, 95% of the documents in the first and second allotments were recovered.
The third allotment, from the Public Ministry, proved to be more problematic, calling for radical treatment. These documents arrived at the APESP three months following the flood, after having been stored wet and housed in garbage bags. Upon drying the materials, it was determined that the extensive mold damage would be impossible to treat using traditional methods. Representing a “worst-case” scenario, this allotment of 176 files was submitted to decontamination by gamma irradiation. The moldy documents were packed in corrugated cardboard boxes and sent to the Radiation Technology Centre for Nuclear and Energy Research Institute (CTR-IPEN) at the University of São Paulo. While still within the cardboard storage boxes, the documents were dosed for disinfection (not sterilization) at 11kGy. This was the first time this type of salvage procedure had been carried out in Brazil.
Following irradiation, the papers were separated and dry cleaned using brushes. The dry removal of the mold spores proved easier and faster than the first two non-irradiated allotments, with sheets separating easily. Perhaps most importantly, the biohazard was eliminated, eliminating the need to quarantine the documents during documentation and dry cleaning. Ms. Auada described the costs of the treatment as acceptable, even within the project’s meager budget. The irradiated documents will be monitored for long term effects of the radiation, with polymerization of the cellulose being of primary concern.
For the last few years, Ms. Villafana and her co-authors have been refining a new microscopy technique for conservation to create “virtual” non-destructive cross-sections. This is a very exciting development for our field, particularly for those of us working with materials–such as works of art on paper–that don’t typically allow for sampling. And for paintings conservators more accustomed to taking traditional cross-sections, this technique has promise for in-situ analysis of paint layers through varnish.
To summarize, the virtual cross-section image is created using pump–probe microscopy, a non-linear optical microscopy technique developed for the biomedical field, which allows non-invasive detection of biological pigments indicative of skin cancer. Because skin tissue is highly scattering, this technique was developed to be inherently confocal, meaning that the signal is generated only at the focal point, creating less scattering, and less spectral noise. The approach is naturally suited to the highly scattering pigments, binders, and supports making up materials of cultural heritage. However, the complexity of art objects render the technique more difficult to apply.
Pump–probe microscopy achieves high resolution in three dimensions with a maximum image area of up to 1mm square. The penetrating depth ultimately depends on the material composition of the object under study. The technique is typically operated at two wavelengths: 810nm and 720nm and modulated to create a series of images at different inter-pulse delays. These images can then be colored according to the molecular composition of the specific material and stacked to create a 3D rendering. With this presentation, Ms. Villafana shared case studies illustrating ongoing research into cultural heritage materials using pump–probe microscopy. The first project investigates applications of pump–probe on paper substrates bearing coatings of lapis lazuli pigment. With this technique, it is possible to produce an image illustrating the physical structure and condition of paper fibers underlying the paint layer. She observes that the pigment particles cluster around the fibers, as seen in the slide below. She is interested in further investigating the natural heterogeneity of lapis lazuli crystals, noting that samples from different parts of the world exhibit different delay behaviors. She plans to complement her pump–probe analysis of lapis lazuli pigments with SEM-EDS, Raman, and FTIR. Villafana also presented on preliminary research using pump–probe to investigate historical methods of pottery manufacture. After finding that pump–probe delays of hematite are dependent on firing temperature, Ms. Villafana started using mock-up clay bodies fired under different conditions (Oxidized at 1800F and 2300F/Reduced at 1800F and 2300F) to examine the difference in delay behaviors from the exterior to the interior of fired clay. She has found that higher temperatures and oxidation both result in shorter lifetimes. Further study will focus on phase change and particle size.
I quite curious to see how this technique develops in the near future. Will pump-probe (or something like it) be able to replace traditional cross section techniques within the next 5 to 10 years? What other techniques are being developed out there that might be able to achieve similar results?
See the following two links for more information: Villafana, et al., full-text PDF of recent research published in The Proceedings of the National Academy of Sciences of the United States of America Article about Pump-Probe Microscopy in Science News, from Science, AAAS