Max von Pettenkofer (1818–1901) a German chemist and hygienist invented the regeneration technique, a method to improve the saturation of blanched varnish layers on old master paintings. In 1870 he published a small book on this method: Über Ölfarbe und Conservirung der Gemälde-Gallerien durch das Regenerations-Verfahren. Esther van Duijn, a painting conservator and art historian who researches the history of conservation of the paintings in the Rijksmuseum, provides some interesting insights in its use.
In the Netherlands, the so-called Pettenkoffer method was almost immediately adopted by one of its best known restorers, Willem Anthonij Hopman (1828-1910). Hopman’s translation of Von Pettenkofer’s book was published in 1871. His enthusiasm for the method convinced museum directors and other specialists of the benefits of the technique. The method was straightforward: a soft cloth was attached to the bottom of a shallow box and sprinkled with alcohol. The box was placed over the painting, which needed to be small enough to fit underneath. The alcohol vapours inside the box would soften and regenerate the degraded varnish, restoring its uniformity, and hence making the varnish translucent again.
It is not surprising that the method became very popular at the end of the nineteenth century as at that time removing varnish layers was much more hazardous to the original paint layers; restorers simply did not have the knowledge and array of methods at their disposal that we have nowadays. And indeed, a method that regenerated the varnish without any manual intervention, must have seemed a small miracle. Of course the Pettenkoffer method did not remove the yellow tone of the varnish. But this yellowness was much appreciated in those days, especially on paintings by Rembrandt.
After the publication in 1871 of Hopman’s book on the technique, numerous paintings at the Rijksmuseum were regenerated by Hopman. The high-point must have been the regeneration of Rembrandt’s Night Watch in 1889. It was a big success. Local newspapers expressed their praise and mentioned how inappropriate the name Night Watch was, due to the much improved legibility of the painting.
The regeneration of the Night watch was documented in an extensive report, which explained that the box had to be specially made in order to fit through the hallways of the museum, as the treatment took place in situ. The actual regeneration lasted exact 22 minutes, but was preceded by two weeks of preparation during which the existing varnish was rubbed with a dry resin powder to even out the surface. The painting was touched by the restorer’s hands after all!
The most important drawback of the method was that its effect did not last; in the end the varnish would degrade again. For this reason, the Night Watch was regenerated eight times between 1889 and 1936, each time with less satisfying results until in 1936, there was no improvement at all. This resulted in the decision to remove most of the varnish layers after WWII by H.H. Mertens.
During the first half of the twentieth century the awareness grew that the method was not as safe as it had previously been assumed. The alcohol vapours could potentially also soften the paint layers underneath the varnish. Additionally, questions were raised about the safety of copaiba balm, a natural oleoresin that was often applied to the surface of a painting after the regeneration to slow down the inevitable degradation of the varnish layers. This, together with the fact that yellow varnish layers on paintings were less and less appreciated, made the regeneration method obsolete after WWII. However, it is important to know the extent to which this method has been used, as knowing what happened to paintings in the past, makes it easier to take care of them, both today and in the future. Further research in these ‘historical’ conservation methods is ongoing.
For anybody interested to hear more on this subject: the Rijksmuseum is organising an international symposium on the conservation history of paintings by Rembrandt on 8-9 November 2018. Early bird fee until 21 October 2018.
With many thanks to Esther.
Esther van Duijn’s research, not only focusses on individual restorers from the past (see previous blog Har Mertens hot-tempered character or sensitive restorer?”), but also on historical restoration methods, which often are no longer applied. Insights in the use of these methods, is crucial for understanding both the present condition of paintings, and the results of scientific analytical research on the paint and varnish layers.