[Pigments and Binders]
[Pigments and Binders] |
Verdigris and Copper Resinate |
As first used as a pigment by European painters from the 13th to 19th century, Verdigris can be found in many early Italian, and German paintings. This intense green pigment was usually used for landscapes and drapery. Verdigris was most commonly used in the 15th and 17th centuries because of its transparent abilities as a glaze on paintings. Artists of this time would glaze verdigris over lead white or a mixture of lead-tin yellow combined with a layer of copper resinate to form a deep saturation of green color.
Verdigris or verte de Grece or 'green of Greece', was named for its
origins in Greece. It was originaly made by hanging copper plates over
hot vinegar in a sealed pot and would stay there until a green crust would
form on the copper. Another method of of obtaining verdigris pigment, which
occured in the Middle Ages, was to attach copper strips to a wooden block
with acetic acid, then the sealed block was buried in dung. A few weeks
later the pot is dug up and the verdigris is scraped off. One method used
in the early nineteenth century had to do with reacting copper sulfate
solution with solutions of lead, barium, or calcium acetate because all
of these are insoluable these solutions form percipitates and the copper
acetate remains in solution.
Chemical
Composition
Copper acetates of different chemical
coposition make up the pigment verdigris. This pigment ranges in color
from green, to green-blue, blue-green, and finaly just blue. Reactions
with copper acetate vary among substances such as the following: copper
acetates disolve in mineral acid, alkalies convert them into blue copper
hydroxide, oils, resins and proteins react to form green transparent copper
oleates, resinates, and proteinates. Of the many different types of Verdigris
each type can be clasified into either basic or acidic.
Neutral Verdigris is neutral copper acetate which occurs when basic
acetates are dissolved in acetic acid, or when basic verdigris is ground
up with strong acidic acid. Deomposition of neutral verdigris occcurs when
a solution is boiled. This verdigris is disolved in acidic acid. The shape
of neutral verdigris is hexagonal and rhombic with distinct bounderies.
Basic verdigris forms from the combination of air, water vapor, acetic
acid vapor, and copper or copper alloy mix. It forms a solid of blue, or
blue-green. It is often made up of fine needles. The chemical formula for
basic verdigris can include all of the following:
[Cu(CH3COO2]2. Cu(OH)2.
5H2O (blue)
Cu(CH3COO)2. Cu(OH)2. 5H2O
(blue)
Cu(CH3COO)2. [Cu(OH)2]2 (blue)
Cu(CH3COO)2. [Cu(OH)2]3. 2H2O
(green)
In the presence of HCl verdigris is soluable and forms a green solution.
From interaction with NaOH it is soluable and percipitates. Sulfer compounds
in the air darken all forms of verdigris. In the first three months of
use the verdigris formulations can change from blue-green to green. All
verdigris reacts with resin to form copper resinates. This copper resinate
is rather transparent and often used as an overpaint to increase depth
of saturation of an opaque green.
Aknowlegments:
Brill, Thomas B. Light: Interaction with Art and Antiquities Roy, Ashok
Artists' Pigments: A Handbook of Their History and Characteristics Vol.
2
Jason Payne, 1997.