A few makers, including C.F.Zimmermann, did instead of blackening the cast iron core, or painting the cast/ steel-sheet core, use different methods such as bluing the core with bluish finish or making it have a brownish finish called "Brünierung". This was done by burning different oils on the surfaces of the cores a few times to get this a bit exquisite finish on them.
Not exactly handbook text by me, as I can not express myself in the same way in English as I am capable of in Finnish or Swedish.
But I hope you get my point
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