Colour changes due to the fading of Prussian blue in Danish Golden Age paintings

Visual observation of the surface in oil paintings by the so-called Danish Golden Age artists from the first half of the nineteenth century indicates that a significant degree of deterioration and fading has occurred in a number of cases in some of the blue coloured areas. The occurrence was initially suggested by the different appearance of the paint along the edges of the paintings where they had been protected from the light.
Technical examination of paintings from the Statens Museum for Kunst collection, by artists such as Christen Købke, Nicolai Abildgaard and C.W. Eckersberg, revealed the presence of Prussian blue, mixed with lead white and calcium carbonate as a component of the paint in the affected areas.

The Prussian blue pigment, a chance discovery of the early 18th century, is a hexacyanoferrate (II) compound. Its complex chemistry and various methods of manufacture have been discussed in the literature. One of the most significant additions to artists’ colours in the 18th century, Prussian blue is a marker for the emerging role of chemists in the evolution of painting materials. It has been demonstrated by experimental studies and is known from historical sources that, due to its high tinting strength, the pigment was often used – mostly in moderate proportions - mixed with other compounds and pigments. And further, that the durability of the colour changes dramatically when the pigment for instance was mixed with certain whites. Among its virtues, the relatively modest price would seem to account for its frequent occurrence in Danish paintings from the mid-18th century onwards, though reservations about the permanence of the pigment are found in several written sources already a few decades after its first appearance on the market. It is therefore doubtful whether this awareness or scepticism was to any extent shared by 19th-century users of the pigment, or whether Danish suppliers of the period were in general concerned with the exact nature of their product.
The colour changes have in several instances influenced the traditional perception of specific, well-known paintings. In the case of Christen Købke’s familiar View of Lake Sortedam from 1838 the changes to its original appearance imparted an altered tonal quality to the composition, with the effect that an erroneous reading of the composition placed the painting as part of a group of works from the second half of the 1830’s in which the artist introduced a new romantic atmosphere to his images. The recent technical examination has caused an art historical reappraisal of this and other works by Købke and his contemporaries. In a broader sense, the current interdisciplinary research, combining scientific analysis, archival, and art historical studies has led to the realization that paintings of the Danish Golden Age were probably in many cases created with brighter, more contrasting and striking colours than the rather subdued and harmonious tonality with which these paintings are often associated by present-day viewers.