Structural properties of 19th century phonograph discs by photogrammetry and terahertz imaging

• Photogrammetry has been used to measure 19th century phonograph disc deformations. • THz pulse time-of-flight has been used for measuring disc layer thickness. • Gluing cardboard to deformed discs worsens warping over time. • Durium disc deformation is associated with a thinner Durium layer. • Lacquer discs are produced by spinning liquid resin. This study investigates two types of phonograph discs from the 1930s to 1960s, conserved at the Institute for Sound and Audiovisual Heritage (ICBSA) in Rome, Italy: Durium cardboard discs and instantaneous discs (Decelith and lacquer). Durium discs, composed of synthetic resorcinol-formaldehyde resin on a cardboard substrate, often exhibit bending deformations due to aging and environmental factors, rendering them unreadable on standard turntables. Attempts to mitigate these deformations by gluing a secondary cardboard layer to their backs resulted in further deformations over time. Instantaneous discs, such as Decelith (polyvinyl chloride) and lacquer (metal base coated with cellulose esters), also face preservation challenges like plasticizer migration, cracks, and deformation. Two non-invasive techniques were employed to address these issues. THz time-domain spectroscopy measured layer thickness, and Structure from Motion-Multi-View Stereo (SfM-MVS) photogrammetry analyzed Durium disc deformations, even on featureless, reflective surfaces. Photogrammetry revealed that Durium discs exhibited bending deformations, with deviations from planarity of up to 2 cm, which worsened after the removal of the glued secondary cardboard. Applying a central weight effectively reduced deformation to within 2 mm. THz imaging associated Durium disc deformation with thinner resin layers (0.20–0.25 mm). The Decelith disc were thinner at the edges and thicker near the center (1–1.3 mm), while lacquer disc showed inhomogeneous dielectric layers (0.14–0.28 mm), thicker at the edges most likely due to centrifugal deposition of liquid resin. This study provides valuable insights into material composition, degradation, and historical production techniques, informing conservation strategies critical to preserving phonograph discs and ensuring access to their sound heritage.