Interesting. Thank you for this addition. I am still surprised that, since the felt is quite porous, the black die did not go through at all...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):
Thank you Bruno for the additional picts.
Maybe we have two kinds of treatment of felt...A thick one as on your helmet and on the saxon I showed. And as Sandy found out, a more fluid one with oil, that does not show a hard crust.
Here is another example that may illustrate Sandys documentation:
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Such blackened felt Tschakos can also be seen on some period picts:
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