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Liver of sulfur on bimetallic parts- sterling/silicon bronze
For a line of buckles to be produced that are made of cast and assembled (hard soldered) silicon bronze and sterling parts I tried a warm liver of sulfur bath. The LOS concentration was the normal used here on a daily basis for imparting a deep black to sterling parts. The result was a deep black buildup on the bronze while the sterling went a faint grey/pink. In other words the bronze elements seemed to retard the patina on the sterling. By the time the sterling finally got a deeper black the bronze had too much of a buildup sitting on top of the metal. Overall not a good look, even when it was relieved by a cut/polish.
Does anyone have any experience in colouring such bimetallic pieces? I am looking to blacken the sterling areas, which will then have their highlights polished for contrast. The bronze areas I would like to have a "soft" antiquing so as to not show tarnishing as readily. The result will not be sealed or lacquered.
product designer - Providence, Rhode Island, USA
August 31, 2011
"The Colouring, Bronzing & Patination of Metals"
by Hughes & Rowe
on
AbeBooks
or
eBay
or
Amazon
(affil links)
Try Hughes and Rowe book on patination and bronzing.
there you can find some info on that subject. Hope it helps and good luck!
- Cerovski vrh Croatia
September 5, 2011
That is a good book, have owned it for decades. However it does not address why the liver of sulfur deposits on the bronze while seeming to retard the reaction of the LOS on the sterling portion of the piece in the bath. The bronze seems to prevent the sterling from reacting to the LOS as it normally would.
Jeff Deegan- Providence, Rhode Island, USA
November 30, 2011
November 30, 2011
Hi, Jeff.
That's an interesting observation, which I would not challenge.
There is a well known technique for removing tarnish from silver by having it touch aluminum while immersed in a conductive solution (see letter 4785), which causes the sulfur to bind with the aluminum, removing it from the silver. I suppose you are seeing bronze (which is a more active metal than silver) playing the same electrochemical role as the aluminum.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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