Gilben

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A heavily yellowed varnish (2) is on the original paint layer (1) and a layer of tar particles (smoke) has deposited on it (3).

When yellowing is defined as the change of a colored paint layer and / or a varnish layer on an oil painting. This happens in the course of its aging process. The color effect of the painting becomes "warmer".

The same applies to the varnish layer on top of the paint layer. Both processes together ensure that the appearance of a painting, in the worst case, can change until the shape and color are unrecognizable.

Paint layer

Covered by the frame palatinate, the so-called primary yellowing has taken place on the lower edge of the picture, due to the light closure.

With the paint layer, a distinction is made between primary and secondary yellowing (age yellowing). Under primary Gilbert refers to a "dark reaction", painted in fresh and in the dark (box, safe) preserved paintings occurs. It depends on the type and composition of the drying oil, as well as its proportion in the paint layer, the layer thickness and the relative humidity. The yellowing is a question of time and a side effect of the drying process (oxidation). As can be seen from a letter to the court painter Justus Sustermann in Florence, Peter Paul Rubens was already familiar with the “dark reaction” of wrapped paintings. He asked the colleague to expose the painting sent to him to sunlight upon arrival in order to remove the yellowing. Primary yellowing turns into secondary yellowing over time, which also takes place in light. It is referred to as age yellow in painting. It is irreversible and is only perceived on closer inspection, for example in the case of originally blue colored areas that now appear greenish. Strictly speaking, the term yellow only applies to white colored areas; the differently colored layers of color lead to an irreversible discoloration from green to dark brown.       

intensity

The top layer of paint is so yellowed / browned that the shape underneath can no longer be seen.

The intensity of yellowing depends on the type of painting medium ( linseed oil , walnut oil , poppy seed oil ) and its composition, the thickness of the paint layer and the relative humidity. Linseed oil is more yellow than walnut oil, poppy seed oil is the least yellow. Under the same conditions, canvas paintings yellow more than wood panel paintings, because the textile image carrier also exchanges oxygen and moisture from the back.

The condition of many dark brown to black-brown colored areas (e.g. backgrounds) in which shapes are present but can hardly be recognized is due, among other things, to the higher proportion of binder that dark pigments need in order to be spreadable, or to a high-binder glaze technique. Examples can be found in the whole of European painting more and more since the 17th century, especially in the painting of the Caravaggists and paintings made with the glaze technique. It can also be observed on many of the previously blue Madonna robes well into the 16th century, which today appear green to dark brown due to additive color mixing (yellowed binder + blue pigment).

varnish

The layer of varnish on this medieval panel painting (detail) has yellowed so strongly that the gold ornaments only emerge clearly after the partial varnish has been removed.

Similar to the drying oils in the paint layer, the varnishes made from natural resins or drying oils that lie on the paint layer also yellow. They are also unsaturated compounds which, depending on the ingredients, can turn yellow to brown when oxygen is absorbed and then act as a “yellow to brown filter” over the entire layer of paint. This not only results in strong color shifts (blue turns green to brown, red turns orange to reddish brown, etc.), but also an adjustment of the brightness values. The painting loses the color and brightness that was originally conceived by the artist.

Degree of discoloration

The degree of discoloration depends on the resin used, its solvent and the age of the varnish layer. The historical varnishes made of amber, copal or rosin browned heavily, the dammar and mastic varnishes used today yellow, while the synthetic resin varnishes, according to literature, do not change, but slight yellowing, graying and absolute insolubility have been found in some.

The "yellow filter" made of a yellowed resin or drying oil lying on the layer of paint can be removed by removing the varnish so that the colors can be seen to a certain extent in their original color values.

Impurities

The surface of a painting can "yellow" or even "brown" beyond recognition due to contamination in the air, for example in rooms where there is a lot of smoke (tar particles). On superficial examination, this pollution can be confused with the yellow age.

literature

Knut Nicolaus: Handbook of painting restoration . Cologne 1998, ISBN 3-89508-921-4 .

Individual proof

  1. ^ E. Guhl: Artist letters . Berlin 1880, p. 190 .
  2. A. Eibner: Development and technology of panel painting . Munich 1928.
  3. ^ Theodor von Frimmel: Handbook of painting studies . Leipzig 1896.
  4. ^ Knut Nicolaus: Handbook of painting . DuMont Buchverlag, Cologne 2003, ISBN 3-8321-7288-2 .