The Palermo Observatory glass plate negatives collection, thanks to the cooperation with the Course of Cultural Heritage conservation of the Palermo University, was conserved and made safely accessible to scholars.
The Astronomical Observatory of Palermo still operates in the original 18th-century premises where a wealth of heritage has stratified, including historical instruments, books, archival documents and photographs. Among the photographic collections a number of glass plate negatives have survived several moves and internal rearrangements of the spaces, especially of the library and archive. Some years ago, a fruitful cooperation with the leaders and students of the course on Cultural Heritage Conservation of the University of Palermo provided an opportunity to design and carry out a preservation project to clean, stabilize and digitize all the historical photographic negatives of the collection. The project was carried out by the students under the tutelage of an accredited conservator and allowed, with a small investment, to dramatically increase the life expectancy of such an invaluable piece of heritage and to make it safely accessible to scholars.
The Astronomical Observatory of Palermo is a shrine of scientific and documentary heritage, and especially interesting because the historical collections are all interconnected and refer to each other. In the observatory, historical instruments, their original purchase invoices, and the catalogues of 19th-century instrument makers from which they were selected, can still be found side by side. There are also annotated proofs and finalized editions of books, pictures of people and activities carried out within the observatory premises and abroad during expeditions, with related correspondence and archival records . The observatory was established in 1790 by Giuseppe Piazzi, professor of astronomy in Palermo since 1786 , within the uppermost quarters of the Royal Palace of Palermo, where it is still located. In 1923 the Observatory became a branch of the University of Palermo. It continued to acquire scientific instruments and books, but performed only didactic activities. Eventually, in 1988 it joined the Italian observatories network and, since 2002, with eleven other Italian observatories, it is part of the National Institute of Astrophysics (INAF) .
Since then, thanks to an agreement with the University of Palermo, current owner of the historical collections, the Observatory cares for this invaluable heritage.
In 2013 one of the professors working both at the Observatory and at the Department of Physics and Chemistry proposed to the course in book conservation of the University of Palermo, run by the same department, to establish a formal partnership with the Observatory to allow the students and future conservators to practice on original materials and, at the same time, to benefit the University’s collections in the care of the Observatory. So, after some preliminary contacts, four students and their tutor-conservator moved to the Observatory to start a first collection care program for the library and archive.
Because the curriculum of the course covers the theoretical and practical aspects of conservation of books, archival records and photographs, the observatory asked also for an assessment of the conditions of some glass plate negatives from the early 20° century, still in contemporary cardboard boxes (see figure 1), and to have them conserved if necessary.
The importance of conserving the plates is not only related to their status of cultural heritage items but also to the role that these plates can still play as source of historical information. The subjects of the plates were already partially explored and some images (see figure 2) have been used for research purposes on the history of the observatory and its instruments, therefore the negatives were also scanned for a safer and easier access to the content.
The plates also witness the presence and use of many instruments. In one of the pictures taken during a famous solar eclipse observation in Tunisia in 1905, Temistocle Zona, astronomer of the Palermo Observatory can be seen posing with one of the telescopes still in the observatory’s collection1 (figures 3 and 4).
In the images captured in the glass plate negatives there are also some curious details, for example, there is a photograph of a painted portrait of Giuseppe Piazzi, which had been taken outdoors somewhere in the Observatory grounds to be photographed, and supported for the purpose by a cast iron stool (see figure 5) which is nowadays used as a pot stand at the entrance of the Observatory (see figure 6). In the published report of the expedition to Sfax , where many of the glass plate negatives images were used to illustrate the booklet, another interesting curiosity related to the Observatory’s collection was discovered: The meteorological hut used in the astronomer’s camp (see figure 7) in Tunisia in 1905 found its way back to Palermo where it is still in use today (see figure 8).
More images of the instruments and some of the historical photographs can be found in the dedicated page of the Palermo Astronomical Observatory website2.
The primary agents of deterioration include adverse levels of relative humidity (RH) and temperature, two of the most serious threats to photographic plates and can determine their longevity. High temperature and RH can lead to microorganism growth as well as attract pests. Heat can also be a threat because it can accelerate any type of chemical deterioration that occurs, while fluctuations in RH can strain the adhesion of the gelatin to the glass plate, with dangerous expansion and contractions, causing the emulsion to crack or separate along the edges  (see figure 9).
High levels of humidity have also been responsible in this case for alterations in the photographic image, leading to fading or to the typical alteration of silver processes called “silver mirroring”. The particles dispersed in the binder that make up the image can undergo oxidation processes, ionizing and migrating to the surface, then reducing themselves back to metallic silver. The image in these areas takes on a “mirror” appearance, metallized and tending to a bluish color, especially in the parts which are darker and therefore richer in silver, and along the edges, as the process is facilitated by the presence of moisture  (see figure 10). Another common deterioration process affecting the plates was biological, like insect erosion, causing severe material losses and compromising the image.
Glass plates were stored in their original paper boxes. The browning of the paper inside the boxes was due to oxidation processes. Both cellulose and lignin within the paper can be oxidized. In cellulose, oxidizing the hydroxyl groups to aldehydes, ketones and carboxylic acids leads to discoloration . Lignin contains several chromophores with conjugated aromatic rings and carbonyl groups that absorb in the near UV spectrum (300–400 nm). When these chromophores absorb light, they can decompose into yellow-colored ketones and quinones, turning the paper yellow  (see figure 11).
The conservation process started recording information about the plates, like inventory numbers, data, materials and damage. Then dust and dirt were removed using a soft brush and then a cotton swab with hydroalcoholic solution 50:50 v/v (see figure 12).
The digitization of astronomical plates, using a special scanner, has been carried out in order to enable free and easy access to this unique astronomical data, and we considered it one of the best approaches to preserving them.
Storage and housing of photographic glass plates is also an important part of their preservation. Photographic glass plates were housed in four-flap enclosures made with Heritage Archival pHotokraft paper, emulsion side up, preventing them from being pulled in and out and further deteriorating the image through flaking and abrasions, with special precautions for the broken ones  (see figure 13).
Custom-made boxes were made using Archival Folding Boxboard with alkaline buffer (see figure 14).
Furthermore, the original boxes were lined with white acid-free Heritage Archival pHotokraft paper, in order to avoid the introduction of acid substances to items stored with it, that can potentially damage them.
We would like to express our gratitude to prof. Giovanni Peres for prompting the cooperation between the Conservation of Cultural Heritage course at the University of Palermo and the Astronomical Observatory, to Prof. Stefana Milioto, former director of the course, for accepting the suggestion, to Dr Giusi Micela, former director of the Palermo Astronomical Observatory for welcoming the conservator and the students to the Observatory, and to Dr Donata Randazzo and Ileana Chinnici for their warm welcome, their constant support and for generously sharing their vast and profound knowledge of the Observatory’s heritage.