AIC's 47th Annual Meeting

Until the commercially successful introduction of chromogenic color prints by Kodak in 1935, colored photographic pictures mainly were produced by hand-coloring B. & W.-photographs, e.g. Daguerreotypes, albumen and gelatin-based prints (1). This was common practice with portraits on postcards (Figure 1). Until 1860, inorganic and natural organic pigments in a binding medium were applied. Later, inks, containing synthetic dyes were introduced. As many of these dyes are very light-sensitive, caution must be taken when exhibiting these artefacts. Little is known about the practice of hand-coloring. Therefore, a proper identification of these dyes would help to understand the coloration process and assess the risk of fading. Hydrogels, e.g. the “Nanorestore Gel”, or an agar gel, can be used to gently extract water-soluble dyes from the gelatin layer of a photograph, e.g. for cleaning purposes (2, 3). These gels could be useful for micro-sampling. Surface Enhanced Raman Spectroscopy (SERS) is a very powerful technique for dye identification, especially for synthetic dyes (4). The silver colloid, added to the sample taken from the object, can enhance the Raman signal up to a factor of 109, making it even possible to detect single molecules, e.g. crystal violet (5). According to research done by Doherty et al., methylcellulose films, doped with a silver colloid can be used to extract dyes from the artefact’s surface for analysis by SERS (6). In our research, this principle has been applied by doping the “Nanorestore Gel” and an agar gel, with colloidal silver (prepd. according to a description by Lee & Meisel (7). A small piece of the gel is immersed into the colloid and left there for a while to be impregnated by the colloid. During the short contact time (minutes) of a very small cube (area: c. 1 mm2) of the hydrogel, water-soluble dyes migrate into the gel and come into contact with the colloid. They then can be analyzed by SERS. This way, dyes on 4 hand-colored photographs from a private collection could be extracted and successfully identified by SERS. The spectra were measured with a Renishaw RAMAN microscope with a 785 nm Laser. In order to identify the dyes, their Raman spectra were compared with spectra taken from the RCE’s large reference collection of synthetic dyes. The red dye in the roses on the postcard in Figure 1 turned out to be eosin. *Corresponding author Many historic gelatin-based photographs show silver mirroring, the formation of colloidal silver at the surface, caused by redox cycling of silver in the gelatin layer (8). The silver mirroring on some of these photographs was shown to function as a SERS-active substrate, enhancing the Raman signal of the dye in contact with it.