Hello, This finish is known as forge black. I learned about it originally from a blacksmith quite a few years ago.
After a quick on-line search I found this modern "How To". I hope that this might be of some help in possibly determining why these have their black appearance.
How To Achieve Forge Black Iron Rustproofing and Metal Finish
This metal finish is known among aficionados of forged iron metal work as FORGE BLACK,
and it derives from a technique dating back over a thousand years. This method is an
attractive finish and a very early method for the rust proofing of iron.
The metal is first meticulously cleaned with 00 emery cloth or steel wool,
then washed with denatured alcohol, shellac grade. This is to remove all grease and dirt.
Do this with clean cloth or paper gloves on so that further grease from handling is not deposited.
As soon as this is dry, coat with linseed oil by wiping, brushing or spraying.
Immediately heat over the forge with air flow applied. This adds free carbon to the burning oil
and an enamel like finish will eventually result, once the oil is all burned off.
Do not heat above black. When the piece stops smoking, remove, cool, and wipe with a cloth to
remove free soot. This leaves a good black finish and has been used for ages as an initial treatment
for iron, with defeat of rust as its main ideal.
Robert
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