In the magazine "Kunststoffe", volume 5 from 1915, there was something about these helmets. I don't own it, but found a snippet of it via Google. On page 107 it was written:
“Durch Matrizen werden Helm-Formen aus Filz hergestellt, die mit einer leicht und rasch trocknenden, deckenden, schwarzen Grundierpaste in Oel überzogen werden. In der Regel sind mehrere Grundierungen notwendig, da der Untergrund sehr porös ist.“ (Translated: „Felt helmets are made with matrices, which are coated with a light and quick-drying, opaque black primer in oil. As a rule, several primers are necessary because the substrate is very porous.“)
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Source: „Kunststoffe“, Volume 5 (1915), page 107 (
www.google.books.de):