Diagnostic study of ancient Durium phonographic discs

Sound recording and reproducing were first obtained by T. A. Edison in 1877 by inventing the phonograph cylinder. In 1929 a technology to produce phonograph discs based on the coating of a cardboard substrate with phenol-formaldehyde resin, impressed in order to form sound grooves, was introduced in the United States of America. The commercial name of such discs was Durium and they soon became a cheaper and more easily used alternative of the shellac and bakelite used for traditional record productions. The Institute for Sound and Audiovisual Heritage (ICBSA) of the Italian Minister of Cultural Heritage and Tourism holds a collection of about 1500 Durium discs produced in the United States, United Kingdom, and Italy dated from the '30s to the '50s. However, in most cases, such Durium discs have shown overtime strong bending deformations that make them unreadable with the classic turntables. Therefore a project started aimed at the chemical, physical and morphological study of the constituent materials of the discs for the planning of innovative procedures for their restoration with the final goal of their digital preservation. Here we report on the non-destructive analysis based on ultraviolet-visible and infrared spectroscopies for the characterization of the constituent materials of the Durium discs owned by ICBSA. Quantitative information on the morphology of the different layers and grooves has been obtained by microscopic imaging, laser scanning confocal microscopy and by an innovative Terahertz imaging approach. Results confirmed the presence of a phenolic resin, as the resorcinol-formaldehyde one, for the Durium layer. The analysis also showed the chemical homogeneity of materials employed in the production of discs, even if produced in different countries and at different times. The Durium layer was found to be about 110 µm thin, while cardboard is about 600 µm thick, suggesting the important role of the latter one in the bending deformations. In addition, inhomogeneities were found in the overall thickness of discs, most likely due to the carboard layer thickness variation. Overall these results provide useful information for the planning of restoration interventions suitable for such sound heritage made by coupling cardboard and plastic material. In particular, they suggest that an appropriate restoration intervention will have to focus on the recovering of the cardboard deformation which drives the overall deformations of the Durium discs.