Page 422 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

RECIPE 4 BLUE VERDITER
There are practical difficulties in manufacturing blue verditer. Mactaggart and Mactaggart
(i98o) found that 15 g of cupric nitrate in 50 ml of water mixed with 5 g of chalk produced
pale green rodlike biréfringent crystals, unsuitable for use as a pigment. With 15 g of cupric
nitrate in 800 ml of water and 5 g of chalk, kept between 6 °C and 20 °C for 24 hours, a green
product was formed. This was synthetic malachite consisting of spherical particles with diame­
ters between 2 and 15 μπι. These particles were highly biréfringent with a dark cross between
crossed polars.
Finally, a blue verditer was produced on the third attempt using 200 ml of water and 15 g of
cupric nitrate kept at below 12 °C and stirred at i-minute intervals. It took 5-9 hours for blue
crystals to form. Repeating the process using the first product formed as starting material caused
the blue verditer particles to become better formed, and diameters up to 25 μιη were produced.
The low temperatures are probably needed to enable enough carbon dioxide to stay in solution
to form a carbonic acid solution strong enough to influence the formation of the product within
the azurite stability field rather than in the malachite field of stability predicted by the relevant
Pourbaix diagrams.

RECIPE 5 SYNTHETIC CONNELLITE
Pollard, Thomas, and Williams (1990 a) prepared connellite using 100 ml of water and, while
stirring, adding 0.725 g of cupric chloride dihydrate, 1.6055 g of sodium chloride, and 0.7 g of
sodium sulfate decahydrate. Carbon dioxide - free 0.05 mol sodium hydroxide solution was added
until the pH rose to 7.2. The fast addition of hydroxide yields a blue phase of claringbullite,
i
Cu 8 (OH) 1 4 Cl 2 -H 2 0. This precipitate will recrystallize to connellite within 48 hours f the mix­
ture is kept in a sealed container to exclude C 0 2 , and it is regulated with a thermostat to 25 °C.

RECIPE 6 SYNTHETIC POSNJAKITE AND MALACHITE
The syntheses employed by Naumova, Pisavera, and Nechiporenko (1990) are described here,
although not all of the reaction conditions were specified by the authors. Posnjakite was made
by adding powdered sodium bicarbonate to copper sulfate solution at an initial pH of 6.4-6.6.
Precipitation of posnjakite occurred in the broad range of pH from 5.7 to 9. The precipitate crys­
tallized on the second day. From a solution with an initial pH 6.7, posnjakite was mosdy pre­
cipitated as crystal twins and triplets and in the form of concretions. Posnjakite obtained from
a near-neutral solution appeared as lamellar crystals of rhombic or pseudohexagonal form.
For the coprecipitation of posnjakite and malachite, a sulfate solution was prepared from
copper sulfate containing 50 mg/1 of copper. A sodium bicarbonate solution was poured into
the sulfate solution and mixed with a magnetic stirrer. The sodium bicarbonate was used to

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