Chemistry and colorimetry: preliminary investigation on chromogenic motion picture film

Color fading due to the degradation of the dyes is the main concern regarding the conservation and restoration of photographic and cinematographic materials. The intervention on the physical support is most of the time unsatisfactory and insufficient for restoration, which makes the digital conversion and digital restoration the only means to recover the film content. However, missing the non-degraded version of the support, the restoration is still dependent on the subjectivity of the operators that perform the work, even being experts. In this context, colorimetric studies are necessary in order to formulate hypotheses on the real evolution of the degradation process of the dyes as a function of time, with the purpose of obtaining information about the original appearance in respect to the analog materials. Different materials mean indeed different gamuts that lead to different colorimetric coordinates. The existence of a wide number of different films makes the conservation and restoration process very challenging. Indeed, in the photographic and cinematographic history, not only several supports (cellulose nitrate, cellulose triacetate and polyester) have been used, but also many techniques of coloration or color development (from early cinema to chromogenic motion films) have been employed, as well as many emulsions, dyes and couplers. In this scenario, knowing the material that composes the physical part of the film object could help the experts in all their work, from conservation to restoration.Unfortunately, the leading production companies have always been reluctant to divulge the compositions of the used compound due to the competitive logic that has always governed the film industry. Moreover, with the decline of the film market, the technical datasheets of films are harder and harder to find. This scenario is even worsened by the fact that a very limited number of studies have been reported on the identification of coloring substances of film materials. In this work, we want first to make the reader aware of the difficulties and problems we have encountered on this topic, highlighting the importance of scientific research on this cultural object that would otherwise be lost. Finally, we want to show the preliminary results that make Raman spectroscopy, and especially SERS (Surface-Enhanced Raman Spectroscopy), a promising method to individuate the dyes of motion picture films, revealing a new potential application of this technique in the field of conservation science.