A picture of the article Audio letters on direct cut discs, written by Katrin Abromeit and Johann Hinterstoisser. You can see several direct cut discs - Tonica disc, Melograph disc, Phonoson disc, Pathépost disc - and you learn from which material they were made of.

Letters cut directly

Katrin Abromeit, Johann Hinterstoisser (2024): “Audio letters on direct cut discs. Initial results from the SONIME collection, restoration and research project – a technical focus”
In: The Lindström project: Contributions to the history of the record industry Vol. 13, 2024, pp. 44-52.

Article about the technical findings on direct cut discs within the SONIME project:

“As pure gelatine films are very thin and sensitive, they were sometimes laminated onto cardboard. Both materials are hygroscopic, but to different degrees. While the gelatine layer can absorb water over the entire surface, this is only possible at the edges of the cardboard core. As a result, bent edges can often be observed. In addition, cracks in the gelatine are common due to drying shrinkage or buckling of the cardboard core.” (p. 51)