Attempts to characterise modern oil paintings through simple surface measurements: a case study of works by Karel Appel

Alexia Soldano, Klaas Jan van den Berg, Louise Wijnberg *

Introduction

Several recent studies have shown the use of surface conductivity and pH measurements of acrylic paints in predicting

the extent of swelling during aqueous cleaning and minimizing risks during aqueous cleaning (Ormsby et al. 2013).

Conductivity and pH has been less explored in the case of modern oil paints. Although attempts to adjust the conductivity

of cleaning solutions to oil surfaces were not successful (Soldano 2014), it was concluded that the ionic activity measured

on the surface could still be valuable information about the paint film and help when making conservation

considerations.

A case study of works by Karel Appel

Three oil paintings by Karel Appel were selected for this investigation: Grande Fleur de la Nuit and L’Homme, Stedelijk Museum, Amsterdam and Femme et Oiseau, Musee

National d’Art Moderne, Centre Pompidou, Paris. The paintings date from 1953 and 1954 and were executed in when Appel was living in Paris.

Paintings by Appel provide interesting surface characteristics: impastoed medium-rich areas alternating with dry and cracking ones over a wide range of pigments. Similar

paint compositions and pigment use have been identified, yet the paint layers respond differently to aqueous swabbing – some surfaces exhibit sensitivity while others do

not. The works conserved in Amsterdam have proven to be complex cleaning treatments in the past (Burnstock et al. 2006).

SEM exploration of water extractable ions from surface

Droplets of water used to measure surface conductivity

and pH were dried and observed using SEM-EDX. The

obtained images are representative of the relative

concentration of ions contained in a drop of water after

lying on an area of the painting for 1 minute. For example,

white paint, presenting a low conductivity produces a

much smaller quantity of residues than the blue paint on

Grande Fleur de la Nuit.

AcknowledgementsMeta Chavannes, Sandra Weerdenburg, Stedelijk Museum

Veronique Sorano-Stedman, Sylvie Lepigeon, Centre Pompidou

Bronwyn Ormsby, Tate

Luc Megens

Annegret Volk

ReferencesBurnstock, A., van den Berg, KJ., de Groot, S., Wijnberg, L., 2008, An Investigation of Water-Sensitive Oil Paints in 2 Century Paintings. In: Modern Paints Uncovered, ed. Learner T. Los Angeles: Getty Conservation Institute, pp 177-188. Dillon, C.E., Lagalante,A., Wolbers, R., 2012, Acrylic emulsion paint films: the effect of solution pH, conductivity, and ionic strength on film swelling and surfactant removal. Studies in Conservation, 59 Vol.1: pp.52-62. Mills, L., Burnstock, A., 2008, Water sensitivity of modern artists’ oil paints. In: ICOM Committee for Conservation 15th Triennial Meeting New Delhi Preprints, ed. J. Bridgland, pp 651-659. Paris: ICOM Committee for Conservation.Ormsby, B, Barker, R, Hellen, R, Smithen, P., 2013, Cleaning acrylic emulsion paintings: Case study treatments, evaluation and reflections. In: What’s changing: Theories and practices in the restoration of contemporary art, ed. Rava, M.C., Mundici, A. Milano: Skira.Silvester, G., Burnstock, A., Megens, L., Learner, T., Chiari, G., van den Berg, KJ., 2014, A cause of water-sensitivity in modern oil paint films: the formation of magnesium sulphate. Studies in Conservation, 59, vol.1, pp.38-51 Soldano, A., van den Berg, K.J., 2014, Investigation into the Surface Conductivity of Water-Sensitive Modern Oil Paints. In: Issues in Contemporary Oil Paint, ed. van den Berg et al. Switzerland: Springer International Publishing.

White paint, Grande Fleur de la nuit, (sensitivity: none, conductivity : 40 μS/cm, pH: 7.0)

Blue paint uncleaned and cleaned, Grande Fleur de la nuit, (sensitivity: none, conductivity before: 1540 μS/cm, pH before: 6.2; conductivity after: 750μS/cm, pH after: 6.0)

Blue paint (ultramarine), L’Homme, (sensitivity: high, conductivity: 1090 μS/cm, pH: 5.8)

Detail of L’Homme, impasto and very dry areas of blue paint

Detail of Femme et oiseau, efflorescence on dark blue impasto

Results and Conclusion

Sensitivity assessed by number of rolls of swabbing with water before pigment/ colour lifting. Divided into following categories: 0-5 rolls = very sensitive;

6-15 = sensitive; 16-25 moderate; 25+ = none

Conductivity in μS/cm

Surface conductivity and pH measurements of the paintings were carried out using both deionized water droplets andagarose disks, depending on the fragility and sensitivity of the surfaces. The conductivity measurements on passages ofultramarine paint were the highest conductivity values by far (1000-1260 μS/cm) with comparable values on zones ofefflorescence. Selective cleaning of the surface proved to greatly affect the measurements taken subsequently,suggesting that the ions are mostly present superficially and do not originate from the paint itself. Interestingly, someareas of colour presented an increase in conductivity after swabbing. This may also be related to the level and type ofsoiling present on the surface of the painting.

Despite being very similar paintings, composed of the same paint, by the same artist and at the same time, surfacemeasurements differ greatly. High results might suggest a sensitive paint film, with formation of water-soluble salts(Silvester et al. 2014) or the formation of efflorescence. Inorganic analysis of the paints have revealed the presence ofzinc, sulphur and chlorine on most surfaces with additional presence of aluminium on the blue paints, coherent withultramarine pigments and explaining the highest conductivity values. On the contrary, low results might be the sign of aprotective medium of layer on the paint film and /or lack of superficial dirt. Modern oil paint films are complex and theexact interpretation of the measurements, conductivity or pH, may be difficult. However, when surface phenomena arenot necessarily visible by eye or through basic conservation equipment, surface measurements can be viewed as an extratool prior to intervention. The measurements are simple, cheap and free of risk and can be carried out when access toscientific analysis is not available.

Detail of Femme et oiseau, surface dirt trapped in paint asperities

L’Homme Grande fleur de la nuit Femme et oiseau

Sensitivity Conductivity pH Sensitivity Conductivity pH Sensitivity Conductivity pH

Glossy black Moderate 110 6.8 None 470 6.6 Moderate 380 6.6

White None 96 6.9 None 40 7.0 None 420 7.2

OchreVery sensitive

310 5.9 None 210 6.0 Sensitive 690 6.2

Ultramarineblue

Verysensitive

1260 5.8 None 1040 6.0 Very sensitive 1260 5.9

Burnt Umber Moderate 280 6.2 None 310 6.3 Moderate 300 7.2

Detail of L’Homme, migration of oxidized oil medium

Detail of L’Homme

* Alexia Soldano, Private painting conservator, Paris, Francealex_soldano@hotmail.com (author for correspondence)

Klaas Jan van den Berg, Rijksdienst voor het Cultureel Erfgoed, Amsterdam, The Netherlands

Louise Wijnberg, Stedelijk Museum, Amsterdam, The Netherlands