Copper resinate

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Copper resinate is a green and transparent glaze that is mainly used in painting. This is made from a combination of copper salts and resins. It is also called transparent copper green.

properties

Copper resinate mainly consists of copper salts and resin acid . Depending on how it is made, it has a neutral to warm shade of green.

In order to test the light resistance, tests were carried out with different light sources. The color changes color when exposed to intense ultraviolet radiation and takes on a brownish tone. Daylight or a shorter exposure interval have little to no effect on the appearance.

In addition to the radiation, the resin used is another criterion. In comparison, Venetian turpentine has few acid groups and can therefore react more poorly with the copper in verdigris . As a result, copper resinate can only be formed poorly and the bond is less stable and can be more easily attacked by exposure to light. In contrast, with its many acid groups, pine resin offers a significantly more stable bond, resulting in a light-resistant copper resinate. When heated, it melts and turns brown. At high temperatures and over a long period of time, the material decomposes to black copper oxide and carbon.

The addition of organic solutions such as benzene or chloroform ensures the solubility of copper resinate. However, the paint is not soluble when enclosed in a dry film of oil.

Manufacturing

Since the definition of copper resinate is so broad, there is no uniform production method. One of the oldest methods of making copper resinate emerges from the records of the 17th century medical doctor Théodore Turquet de Mayerne . In the Mayerne manuscript he described the preparation as follows: Mix two ounces of Venetian turpentine and one and a half ounces of turpentine, then add two ounces of crushed verdigris and cook over hot ash. The color quality can be checked when applied to glass. Finally, the copper resinate must be filtered through a linen cloth. This process produces a thick and transparent mass that dries very slowly over several months. The end product can either be processed immediately as a glaze or powder.

Another production method is recorded in the records of a Florentine book by Birelli from 1601. This describes the method of making a green color that can be used on glass: combine a pound of white rosin, three ounces of mastic and half an ounce of wax and boil over a moderate fire on charcoal. The mixture must be constantly stirred. Then everything is poured into another container and one ounce of verdigris is gradually added. The constant stirring enables the ingredients to be mixed evenly. Finally, the vessel is placed back in the charcoal ash. Reheating the mixture ensures that the paint liquefies again and can therefore be applied more evenly. The same author describes a second possibility of creating a so-called emerald-like shade of green consisting of linseed oil, alunite and high-quality verdigris, to which pine resin is later added. In today's production, a copper salt solution reacts with a liquid sodium resinate solution.

Another approach is to melt resin with reactive copper salts.

history

Copper resinate was most used between the Renaissance and Baroque periods .

The first suspected evidence can be traced back to the Middle Ages. It was used in illuminated manuscripts. But it was mainly used in the Dutch and Italian oil paintings of the sixteenth century. After that, the use of the paint decreased significantly across Europe. It is assumed that this is due to the brown discoloration of the copper resinate.

However, none of these discoveries can be assumed as facts, since the copper resinate could also have been formed by a reaction with the light radiation or other pigments. Up to now, the scientific knowledge is insufficient to prove or disprove this evidence.

proof

The copper content in transparent shades of green is not sufficient for proof of copper resinate, as it does not show any individual pigment particles under the microscope. Instead, the resin acid is detected using thin layer chromatography or gas chromatography . In practice, gas chromatography is seldom used, as it requires a color sample of approximately one milligram, which in very few cases can be made available. Therefore gas chromatography with mass spectrometry coupling is the most suitable. In this process, free acid groups are removed from the resin and allowed to react to form methyl esters, in order to then determine it with its mass spectrum. This enables the stable dehydroabietatic part of the resin layer to be demonstrated.

literature

Web links

Individual evidence

  1. ↑ Ashok Roy: Artist's Pigments. A Handbook Of Their History And Characteristics. Volume 2, 2012, p. 150
  2. ↑ Ashok Roy: Artist's Pigments. A Handbook Of Their History And Characteristics. Volume 2, 2012, pp. 150-151
  3. ^ David A. Scott: Copper And Bronze Art. Corrosion, Colorants And Conservation. 2002 p. 296
  4. ↑ Théodore Turquet de Mayerne: Mayerne manuscript
  5. ↑ Ashok Roy: Artist's Pigments. A Handbook Of Their History And Characteristics. Volume 2, 2012, p. 149
  6. ↑ Ingo Klöckl: Chemistry of colorants: In painting. 2015, p. 13
  7. ^ David A. Scott: Copper And Bronze Art. Corrosion, Colorants And Conservation. 2002, p. 294
  8. ^ David A. Scott: Copper And Bronze Art. Corrosion, Colorants And Conservation. 2002, pp. 295-296
  9. ↑ Ashok Roy: Artist's Pigments. A Handbook Of Their History And Characteristics. Volume 2, 2012, p. 151