Removal of a Past Varnish Treatment from a 19th-Century Belgian Wall Painting by Means of a Solvent-Loaded Double Network Hydrogel

Abstract

Polymeric materials have been used by painting conservator-restorers as consolidants and/or varnishes for wall paintings. The application of these materials is carried out when confronting loose paint layers or as a protective coating. However, these materials deteriorate and cause physiochemical alterations to the treated surface. In the past, the monumental neo-gothic wall painting 'The Last Judgment' in the chapel of Sint-Jan Berchmanscollege in Antwerp, Belgium was treated with a synthetic polymeric material. This varnish deteriorated significantly and turned brown, obscuring the paint layers. Given also that the varnish was applied to some parts of the wall painting and did not cover the entire surface, it was necessary to remove it in order to restore the original appearance of the wall painting. Previous attempts carried out by conservator-restorers made use of traditional cleaning methods, which led to damage of the fragile paint layers. Therefore, gel cleaning was proposed as a less invasive and more controllable method for gently softening and removing the varnish. The work started by identifying the paint stratigraphy and the deteriorated varnish via optical microscopy (OM), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. A polyvinyl alcohol-borax/agarose (PVA-B/AG) hydrogel loaded with a number of solvents/solvent mixtures was employed in a series of tests to select the most suitable hydrogel composite. By means of the hydrogel composite loaded with 10% propylene carbonate, it was possible to safely remove the brown varnish layer. The results were verified by visual examinations (under visible light 'VIS' and ultraviolet light 'UV') as well as OM and FTIR spectroscopy.

Keywords: FTIR spectroscopy; SEM-EDX; XRD; deteriorated synthetic varnish; gel cleaning; optical microscopy; solvent-loaded hydrogels; varnish removal; wall painting.

Figure 1

(a) A black and white photograph of the chapel in 1897 in the Sint-Jan Berchmanscollege, when the wall painting was not yet executed. (b) The chapel in the 1930s, after Ernst Wante created the wall painting. The chapel was not yet divided into two floors at that time [33]. (c) A photograph of the wall painting in 2017 showing its state before conservation. The wall painting is now found on the top floor of the chapel.

Figure 2

(a) An optical microscopy image showing the intonaco, priming, paint layer (black arrow), and varnish layer (red arrow). (b) The same view under UV light. (c) A magnified field of the red rectangular present in ‘b’. The image illustrates the fluorescent layer belonging to the priming coat in addition to the paint and the varnish layers. (d) A backscattered SEM image (of the area in the blue rectangle indicated in ‘a’) revealing the intonaco layer and the superimposed four layers of the priming coat. (e) Elemental maps acquired by SEM–EDX of the same field are presented in ‘d’.

Figure 3

(a) and (b) XRD diffractogram of the intonaco and priming coat, respectively. (c) µ-FTIR spectra of the painting and priming coat.

Figure 4

Photographs showing the glossy varnish that was not applied to the entire wall painting, as indicated by the yellow arrows. (a) Left side and (b) right side of the wall painting.

Figure 5

(a) General view of the selected area for the first stage tests. (b) Selected spots before the treatment indicated by white circles. (c) The five hydrogel composites applied to the surface during three different contact times: 10, 20, and 30 min. The numbers 1 to 5 represent the hydrogels loaded with 10% EA, 10% PC, 5%/5% EA/PC, 10% MEK, and 5%/5% MEK/1-PeOH, respectively. (d) Treatment of the surface with a dry cotton ball after peeling off the hydrogel pellets. The arrow refers to the swelling of the varnish resulting in a whitish haze. (e) View of the selected area after gel cleaning treatments. (f) View of the same area under raking light. (g) The same view under UV light showing the disappearance of the varnish fluorescence and the severe damage that occurred at the treated spots with the 5%/5% MEK/1-PeOH hydrogel composite (right-most column of test spots).

Figure 6

ATR–FTIR spectra of hydrogel, unvarnished and varnished paint layers, and spots treated with the five tested hydrogels in the first-stage tests.

Figure 7

(a) General view of the selected area for the second-stage tests. (b) Selected areas before gel treatments. (c) The selected three hydrogel composites during application. (d) Removal of the swelled varnish after peeling off the hydrogel using a dry cotton ball. (e) View of the surface after the treatment. (f) The same view under raking light. (g) The same view under UV light.

Figure 8

(a) General view of the selected area for the third-stage tests. (b) Selected areas before treatment. (c) The surface after treatment with the three hydrogel composites. Spots treated with the 10% EA and 5%/5% EA/PC hydrogel composites were overcleaned. (d) The same view under UV light showing the same observation regarding the overcleaning effect.

Figure 9

ATR–FTIR spectra of hydrogel, unvarnished and varnished areas, and the spots treated with three hydrogel composites (5%/5% EA/PC, 10% PC, and 10% EA) in the third-stage tests.

Figure 10

(a) Photomicrograph showing the boundary between a treated and an untreated area of a green area of the wall painting. The untreated area shows the darkened varnish layer, while the treated area illustrates the total removal of the varnish layer, revealing the original color of the surface. (b) 3D photomicrograph of the same view revealing the presence of a thick, darkened varnish layer. (c) µ-FTIR spectra of the varnish, priming coat before and after the treatment, treated and nontreated paint layers, and hydrogel.

Figure 11

(a) The wall painting during the initial steps of conservation. (b) Final results of the wall painting after conservation and removal of the dark varnish layer.