Master Drawings with Self-Destructive Ink
One of the big conservation problems at Statens Museum for Kunst is that many of the finest foreign and Danish drawings are executed with a self-destructive type of ink: iron gall ink.
Drawings of Unique National Importance are disappearing
Iron gall ink is both self-destructive and detrimental to the paper. This type of ink has been used from about 1400 and right until 1900, and it presents a conservation problem that after a few hundred years - depending on the quality of the ink and the paper - the ink starts to discolour the paper, and then to eat into the paper fibres under the ink, making the ink line crackle and fall out.
The result is that the drawing itself disappears. In the end, only the hole where the ink was is left.
A Rembrandt in distress
The problem is apparent in the Rembrandt drawing Cain Slaying Abel, c. 1650. There are drawings on both sides of the paper, and in the middle of the motif you can see that the drawing on the back is about to "bleed" through the front.
The damage to the Rembrandt work has not reached the stage where the ink is ‘falling out’.
However, that is the case with Bertel Thorvaldsen’s small drawing of Apollo below, where part of the ink line by the quiver has disappeared. The same is seen on the woman’s head by Paolo Farinato, where the line around one eye and part of the mouth are now missing.
Searching for the best method
Many different methods have been applied in order to restore and conserve motifs drawn in iron gall ink, including
* Re-gluing to seal the ink line
* Pasting onto new paper
* Stabilising by means of paper patches on the back
None of these methods have struck at the root of the evil, the iron gall ink itself.
The ink must not be destroyed
Attempts to neutralise the acid produced by the ink have turned out to be problematic. A basic solution with a pH value of more than 8 for neutralising the ink acid will change the chemical structure of the ink irrevocably. Besides, the ink degradation of many works is so advanced that they do not tolerate any kind of water treatment.
Conservators have also tried to neutralise the acid from works by placing them between papers prepared with a basic buffer. But moisturising may in some cases cause iron ion migration from the ink onto the paper thus increasing the damage even more.
Re-mounting is not sufficient
Neither is the solution to re-mount the works, so that they are provided with new acid-free or possibly basic-buffered passepartouts. In fact, the degradation products in the iron gall ink are so potent that they will quickly destroy the cardboard. This is apparent on the cardboard behind Abildgaard’s drawing Recumbent Woman Symbolising the Jewish Religion. In UV light the incipient degradation of the cardboard is seen as a "copy" of the drawing. In this area the basic buffer is quickly used up and therefore no longer offers proper protection of the drawing.
The incipient degradation of the cardboard can be seen already after about 10 years. Considering that older kinds of paper, like for instance rag paper made from linen or cotton, have turned out to keep for 500 years, 10 years is a short time in relation to preservation. Besides, we have to take into account that the degradation takes place not only in the cardboard, but in the work itself!
Are antioxidants the answer?
The solution conservation scientists are working on at the moment is treatment with antioxidants, rather like what is happening in the food industry where they are used for prolonging the durability of foodstuffs.
But as we do not have any experience with the long-term effects of treatment with antioxidants, Statens Museum for Kunst has started a co-operation with the Royal Library and the research centre Centre de Recherches sur la Conservation des Documents Graphiques in Paris to find out whether the conservation methods now used have unwanted side-effects.
Although we have a special responsibility for preserving works of Unique National Importance right now, we must also examine whether the methods are sound before applying them on a grand scale.
MSc Conservation, Art on Paper
Read more about Niels Borring
On works of Unique National Importance
The restoration was undertaken thanks to the special appropriation from the Ministry of Culture for the preservation of works of Unique National Importance. Read more about works of Unique National Importance.