I suppose it relates to the Bower-Barffing process:

The patination of iron by bower barffing
The bower-barff process was a patented method of developing a protective, aesthetically pleasing vevety black surface on iron. The process had two essential steps. The first was the intentional production of a light layer of flash rust by exposing the iron to steam. This was followed by conversion of the rust to magnetite, which is black, by hadrgen reduction using ordinary producer gas.
https://books.google.de/books?id=XgshBQAAQBAJ&pg=PA155&lpg=P...

In metallurgy, the Bower–Barff process in metallurgy is a method of coating iron or steel with magnetic iron oxide, such as Fe2O4, in order to minimize atmospheric corrosion.
The articles to be treated are put into a closed retort and a current of superheated steam passed through for twenty minutes followed by a current of producer gas (carbon monoxide), to reduce any higher oxides that may have been formed.
http://en.wikipedia.org/wiki/Bower–Barff_process

Domain Iron, steel and other metal industries
en
Term barffing
Reliability 2 (Minimum reliability)
Term Ref. CIATF. Dictionnaire idéologique de fonderie (Indexed Foundry Dictionary). Éditions techniques des industries de la fonderie, Paris. 1979
Date 08/11/2012

Term Bower-Barff process
Reliability 2 (Minimum reliability)
Term Ref. CIATF. Dictionnaire idéologique de fonderie (Indexed Foundry Dictionary). Éditions techniques des industries de la fonderie, Paris. 1979

fr
Definition traitement à la vapeur à haute température de produits ferreux afin d'obtenir la formation d'une couche d'oxyde brun noir qui résiste bien à l'usure
Definition Ref. CIATF Dic.idéologique fonderie 1979

Term barffing
Reliability 2 (Minimum reliability)
Term Ref. CIATF Dic.idéologique fonderie 1979

Term procédé Bower-Barff
Reliability 2 (Minimum reliability)
Term Ref. CIATF Dic.idéologique fonderie 1979
http://iate.europa.eu/SearchByQuery.do?method=searchDetail&l...

Subject Keywords French: Bâtiment; Métal; Métal architectural; Fer architectural; Fer forgé; Conservation; Protection; Revêtement; Histoire; Electrodepositi on;
Subject Keywords English 2: Building; Metal; Architectural metal; Architectural iron; Conservation; Protection; Coating; Electroplating; Cast iron; History – ICCROM
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