[MR] Seeking inorganic chemistry (or metallurgy) help

[Brian Pfeifer]

Greetings Atlantians,

Emperor Qin of 220 BC China included an army of 7,000 life-sized terracotta 
soldiers in his tomb. The warriors were outfitted with bronze weapons, which 
are described as “finely coated with a 10-micron layer of rustproof chromic 
salt oxide” 
(http://www.travelchinaguide.com/cityguides/xian/terracotta/weapon.htm).

The question is, which chromic salt? Which chromic salts will easily bond 
with the bronze (copper 71-74%, tin 22-32%, lead .1-2%, I know it doesn’t 
add up, but that’s what the book says), and which chromic salts are readily 
available in China?

Chinese alchemists described several methods of applying permanent gold and 
silver, or similar, surface treatments to base metals, and I’m hoping one of 
these methods would work for the chromic salts. They can be roughly broken 
down into the following: heat fusing, amalgamation fusing, tinning, 
projection, and precipitation.

Heat fusing was most commonly used to apply silver to a copper substrate. 
When you place the two next to each other, and heat them to 800 degrees C, 
they bond, and since the two metals have similar mechanical properties, you 
can even shape them by hammering.

Amalgamation fusing was used for both silver and gold. The precious metal is 
dissolved in boiling mercury, and the resulting paste is applied to the 
substrate. The whole thing is then heated until the mercury boils off. I 
believe this is the same as the European fire gilding process. Many bronze 
mirrors were produced by this method.

Tinning is used to apply a tin (silver like) surface by dipping a cleaned 
bronze item into molten tin.

Projection exposes bronze or brass to arsenic vapor, or possible arsenic 
sulfide (water?) to produce a gold hue. I’m not entirely certain I have this 
one correct, but it’s close.

Precipitations sounds like the most promising process, but I lack the 
chemistry background to know if the reactions would work out with chromium 
salts. When certain types of alum, “are added to processed copper (powder), 
being made into a crude paste with vinegar and smeared onto the surface of 
iron, the iron is all turned to the color of copper.” (Joseph Needham, 
Science and Civilization in China, quoting Chhi Po who wrote around 510 AD) 
The author mentions other similar processes utilizing various alums and 
vitriols, which may point to the one we want. He also mentions gold, and 
silver being precipitated from their salts as well by the same method. Could 
this be used to precipitate a chromium coating on the bonze swords?

One complication to this whole process, is that nowhere in any of  the books 
I’m reading is chromium mentioned (except as one of the chemical components 
of jade). I would have expected to see a reference to it in one of the many 
Chinese documents. They wrote extensively on medical and alchemical 
processes and recipies. It is of course possible that the refernce exists, 
but the authors I’m reading lacked access to them. I do not know.

I’m researching this for a school project. I’m working on my bachelors in 
History of Technology at Goddard College. Any help you can give me would be 
greatly appreciated.

Thank you.

-----------------------------------------------------
Brian J. Pfeifer		Baron Sylvanus Perrin
Network Security Engineer	Order of the Sea Stag
brian_pfeifer at hotmail.com	http://sabletower.homestead.com

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