Page 135 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Synthesis and use One way to synthesize sodium-copper carbonate is to precipi­
of chalconatronite tate the crystals from a concentrated solution of sodium carbon­
ate containing bicarbonate and copper ions. This explains the
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chalconatronite that Horie and Vint had identified on the Roman armor: the excavated metal-
work from this site had been treated with a sodium sesquicarbonate solution, and a reaction
must have occurred between the solution and the copper ions in the corrosion material. Sub­
stantial layers of chalconatronite have been found on a number of other bronze objects known
to have been conserved with sodium sesquicarbonate. The mineral is apparently deposited dur­
ing the long treatment regime designed to extract chloride ions from corroded bronze objects.
When viewed under a polarized light microscope, chalconatronite crystals appear as angu­
lar, colorless, crystalline fragments; in a melt-mount with a refractive index of 1.662, the crys­
tals appear in clear relief because all three refractive indices of chalconatronite are well below
that of the mounting medium. Under crossed polars, the crystals show a gray-white birefrin­
gence, with some particles showing a second-order straw yellow and red-purple tinge; most
particles have a clear extinction. Chalconatronite synthesized n the laboratory has optical
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properties that are similar to those of the naturally occurring mineral, with the exception that
extinction is usually undulóse and sometimes feathery, in the manner of anhydrite.
There is evidence that a green pigment synthesized in China during the early centuries C.E.
and referred to as synthetic malachite (Needham 1974) is, in fact, synthetic chalconatronite.
Needham mentions a text by the early Chinese writer Sun Ssu-Mo who gives instructions for
making this green pigment:

[T]o get a fine green pigment from copper one must calcine the rust [to make a copper
oxide] and then boil it with white alum in a sufficient amount of water. After it has cooled
it will be green, and one must add some natron solution [a naturally occurring mixture
of sodium carbonate, sulfate, and chloride] which will precipitate the green colour called
hsiao lu se. This is used in painting for the colour of plant and bamboo leaves. (Needham
1974:244-45)

Needham equates this color to a green verditer (synthetic malachite), which is the same as
Chinese thung lu. The reaction was replicated by the author in the laboratory, using cuprite as
the calcined copper rust and potassium aluminum sulfate for the alum solution. After several
hours of gentle boiling, the cuprite had partially dissolved, forming a blue-green solution with
a deposit of unreacted cuprite. The alum in this recipe reacts to form aluminum sulfate, alumi­
num chloride, or other salts that stay in solution; aluminum hydroxide is not precipitated. The
exact nature of the precipitated reaction products that form when natron is added to the cooled
solution probably depended on the composition of the natron used in the ancient recipe. For

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