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Keywords: paintings, varnish removal,

gel impregnated tissues, technique

development

AbstrAct In recent years a technique for cleaning paint�

ings, using tissues impregnated with cellu�

lose ether gelled organic solvents, has been

developed at the Stichting Restauratie Atelier

Limburg (SRAL). This pack system, whereby

the advantages of a gelled solvent can be uti�uti�

lised with minimal concerns regarding possi�with minimal concerns regarding possi�

bly harmful residues, was initially devised by

Van Och for removing the varnish from 17th

century canvas paintings in The Royal Palace

Huis ten Bosch, Den Haag, The Netherlands.

In many subsequent treatment situations the

technique has demonstrated various general

and practical advantages over using swabs

with free solvent. These benefits, and the

technique’s adaptability depending on the

individual requirements of the painting, will

be demonstrated in this paper by case studies

and research projects carried out at SRAL.

résuméCes dernières années, une technique de

nettoyage des tableaux à l’aide de chiffons

imbibés de solvants organiques gélifiés à

base d’éther de cellulose a été mise au point

par le Stichting Restauratie Atelier Limburg

(SRAL). Ce système intégré, qui tire profit de

l’utilisation d’un solvant gélifié tout en pré�

sentant peu de risque quant à la présence de

résidus dangereux, avait été conçu par Van

Och pour retirer le vernis des peintures sur

toile du xviie siècle dans le palais royal Huis Ten

Bosch à La Haye, aux Pays�Bas. Lors de nom�

breux traitements ultérieurs, cette technique

a montré différents avantages pratiques plus

généraux, par comparaison avec l’utilisation

de cotons�tiges imbibés de solvant libre. Ces

avantages, ainsi que la flexibilité de la tech�

nique en fonction des critères particuliers du

tableau, sont démontrés dans cet article à

Gwendoline R. FiFe*Stichting Restauratie Atelier Limburg (SRAL)Maastricht, The Netherlandsgwenfife26@hotmail.comJos Van ochStichting Restauratie Atelier Limburg (SRAL)Maastricht, The NetherlandsBascha staBikStichting Restauratie Atelier Limburg (SRAL)Maastricht, The Netherlandsnada MiedeMaStichting Restauratie Atelier Limburg (SRAL)Maastricht, The Netherlandskate seyMouRStichting Restauratie Atelier Limburg (SRAL)Maastricht, The NetherlandsRené hoppenBRouweRsStichting Restauratie Atelier Limburg (SRAL)Maastricht, The Netherlands*Author for correspondence

A pAcKAge deAl: the development of tissue gel composite cleAning At srAl

introduction

For cleaning oil paintings (specifically, the removal of applied non-original coatings) conservators typically employ organic solvents with swabs, a practice which has been valuably influenced by research into the swelling and leaching effects of organic solvents on (oil) paint films. For the informed paintings conservator contributions from Stout (1936), Graham (1953), Stolow (1957), Ruhemann (1968), Eissler and Princen (1968), Hedley (1980), Maschelein-Kleiner (1981), Feller et al. (1985), Michalski (1990), Erhardt and Tsang (1990), Tumosa et al. (1999), Sutherland (2001), Phenix (2002a), and Zumbühl (2010) among others, form the backbone for their solvent use, enhancing a continued development in their conceptualised modelling of oil paint film behaviour and enabling a fine-tuning in their cleaning approach. Reassuringly, some of the most recent research has indicated that ageing of a paint film reduces the magnitude of swelling (Phenix 2002b) and that ‘an individual cleaning treatment (using swabs and free solvent [sic]) may present a minimal risk to the paint layers from leaching’ (Sutherland 2001, 137).

However, given the inherent and yet unknown dangers of solvents on an individual paint film, the fact that swelling in these original layers may be difficult to observe, and that ‘leaching is a phenomenon which occurs, although at a low level, on each repeated exposure to solvent’ (Sutherland 2001, 137), cleaning remains one of the most fraught procedures for a paintings conservator. Whilst utilising the research findings to minimise the risk of using a swelling solvent or mixture, the logical conclusion is to reduce solvent contact with the original materials as far as possible. This idea corresponds with the concept of gelling solvents for cleaning applications. Increasing the viscosity of a solvent reduces its movement through cracks via flow and capillary action, thus preventing the opening of new diffusion fronts. The assumption that solvent penetration into original paint layers via diffusion will also be retarded when solvents are applied in gelled form has recently found some validation in a Getty study (Dusan et al. 2004). Accordingly, harnessing the advantages of using a gelled solvent whilst minimising the disadvantages of gel residues is an underpinning concept for SRAL’s tissue gel technique whereby tissues impregnated with cellulose ether gelled organic solvents are applied to the varnished paint surface to swell, dissolve, and absorb the non-original coatings (Fife et

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travers des études de cas et des projets de

recherche menés au SRAL.

resumen En los últimos años se ha desarrollado en el

Stichting Restauratie Atelier Limburg (SRAL)

una técnica para limpiar cuadros empleando

papeles impregnados con disolventes or�

gánicos en geles de éteres de celulosa. Este

sistema, que permite aprovechar las ventajas

de un disolvente en gel con preocupaciones

mínimas por los posibles residuos nocivos,

fue concebido inicialmente por Van Och para

eliminar el barniz de los cuadros sobre lienzo

del siglo XVII del Palacio Real Huits ten Bosch,

en La Haya, Países Bajos. En muchos trata�

mientos posteriores la técnica ha demostrado

tener varias ventajas generales y prácticas

frente al uso de hisopos con disolventes. En

este artículo se demuestran estas ventajas y la

adaptabilidad de la técnica en función de las

necesidades individuales de cada cuadro, con

estudio de caso y proyectos de investigación

realizados en el SRAL.

al. 2010). With adaptations and permutations depending on the individual requirements of the painting, this system has been successfully employed in a variety of projects as will be outlined below.

generAl description of technique

The solvent (or mixture of choice) is selected in the normal manner based on varnish solubility and safety margin tests with swabs and free solvent. The chosen solvent is then gelled by adding 2 to 4 percent by weight of a cellulose ether, typically Klucel G (hydroxypropyl cellulose). The gel is used to impregnate a specific tissue, small sections of which are then tested on the painting to indicate the optimum length of contact time for dissolution of the non-original coatings, or desired ‘cleaning level’. In general, the gel impregnated tissue is applied to the surface for up to 60 seconds, during which time it may, or may not, be covered with a thin plastic foil to further reduce solvent evaporation from the system. During this working period the tissue remains semi-transparent and wet, allowing some monitoring of the cleaning process as the discoloured varnish residues (and sometimes also the old retouchings and overpaints) can be seen dissolving into the tissue-gel composite (Figure 1). After this working period, a second dry absorbent tissue is placed directly above the first tissue and gently pressed using a sponge roller or metal spoon to absorb the non-original materials and gel (Figure 2). The pack of tissues and absorbed materials is still damp when it is then lifted from the cleaned painting surface. Depending on the individual painting, additional cleaning of the paint surface or clearing of any gel residues may be carried out (Figure 3).

development of technique

Back in 1997, a team was assembled by Anne van Grevenstein for the research and conservation treatment of the Oranjezaal in the Royal Palace, Huis ten Bosch, Den Haag, The Netherlands. Many conservators were amassed for the enormous project of conserving and restoring the contemporaneous and large 17th-century canvas paintings decorating the walls of this impressive room. Whilst required, the cleaning of the paintings (varnish removal) posed many complicating issues:

• The team comprised many conservators with very different backgrounds, training and approach.

• Being an entire ensemble, the Oranjezaal treatment required a unified result.

• The total surface area of the paintings covered approximately 304 m2

and there was a strict deadline for completion of the project.• The paintings were unlined and extremely porous. Any application

of swabbed free solvent to the varnished paint film was immediately observable on the reverse of the canvas.

The paintings had not been frequently cleaned in the past. Their known conservation history dated to the 19th century after which the paintings

Figure 1Application of tissue 1 (gel impregnated). Discoloured varnish residues start to dissolve into gelled tissue 1

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had been cleaned only twice. Essentially a safe step-by-step technique for cleaning was required, whereby the paintings could be evaluated between phases guaranteeing controllability by the head conservators and thus ensuring unity in the final result. Hence, a system had to be conceived whereby:

• The specific solvent mixture used and its contact time with the varnished surface would be regulated, diminishing the effect of ‘different hands’, and ensuring an even cleaning.

• Varnish removal could be as safely efficient as possible. • Penetration of solvent through the painting structure would be

prevented.

Increasing the viscosity of the solvent was an obvious initial consideration. The solvent tests had indicated solubility of the varnishes whereby the cellulose ether, Klucel G, could be used in low concentrations as a thickening agent. Ageing tests have indicated this material is relatively stable (Feller 1990), and although any residues left on the paint surface may undergo some yellowing, it critically maintains its solubility behaviour, and is inert with regards to any chemical effect it may have on the original paint surface. Testing with a gelled solvent (2 percent weight Klucel G/volume of solvent) showed no penetration of solvent through to the reverse. The requirement for the application of a regular quantity of gel resulted in the conception of a gel impregnated tissue; the contact, and thus working time of which could be precisely controlled. After small tests, the use of a secondary dry tissue step was found to maximise effective absorption and removal of the dissolved residues.

No detrimental effects on the paint surface were observed using this technique, and neither solvent nor dissolved varnish were visible on the reverse of the paintings (Figures 4 & 5). Hence, degradation resulting from acidic resinous varnish residues further penetrating the original canvas supports was prohibited – a factor considered especially pertinent for unlined paintings in general. Varnish removal in this manner provided a very revealing intermediate cleaning step whereby the effects of previous partial cleanings carried out during the paintings’ history could be clearly observed. This was of importance for the research and documentation aspect of the project, as well as informing (also with the aid of UV light) the head conservators where further cleaning was specifically required. A unified cleaning was therefore safely achieved with a minimum exposure of the paintings to solvent.

selected cAse studies

Grote Galerij portraits, castle amerongen, amerongen, the netherlands

SRAL commenced an ongoing conservation project of Castle Amerongen in 2000. This included 18 family portraits from the Grote Galerij in the

Figure 2Tissue 2 (the dry absorbing tissue) is placed over tissue 1. A thin plastic foil (held flat in a wooden frame) is laid above. Gently rubbing over the plastic with a spoon or roller pushes the gel and dissolved varnish residues into tissue 2

Figure 3After removal of tissues 1 & 2 the painting surface is dabbed with a tissue to remove some visibly remaining gel/varnish residues

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castle which date from the 17th to 19th centuries. These oil on canvas paintings did not have identical conservation histories, but many had remained unlined and had extremely cupped paint films. Essentially the remit of the Amerongen project excluded restoration treatments, however, for the majority of paintings in the collection the removal of the varnish(es) proved a conservation consideration. Effective consolidation, the reduction of stresses on the paint surface (which exacerbate cupping) induced by the brittle varnish, and as far as possible the removal of acidic resinous varnish materials from the painting surface and structure, were all conservation requirements. During testing, various problems using swabs and free solvent on such topographies were generally observed:

• Mechanical damage could be incurred on a macro scale whereby vulnerable cupped paint areas at the sharp raised edge could be broken off.

• It was difficult to achieve an even removal of varnish. Swabs removed varnish from the peaks of the paint film, whilst leaving more residues in the ‘cups’. In removing these latter varnish residues both more solvent exposure was required and these ‘tops’ of the paint film could be further abraded.

• The varnish was not effectively removed from the cracks, nor from under the raised edges. The paint film remained cupped after cleaning whereby subsequent consolidation would be less effective.

Tests with the tissue gel composite cleaning frequently proved advantageous on all counts. Macro mechanical action on the vulnerable and cupped paint films was reduced and to the extent that no paint losses were incurred. This was partly because there was no direct rubbing on the paint surface or ‘catching’ of cotton fibres as occurred with the swab. But another important factor was reduced pressure, since the surface area of the ‘tops’ of the paint film was effectively increased by the applied tissue layers, and the contact area of the spoon or roller was greater than that of a ‘typical’ swab. More effective removal of varnish from the ‘cups’ was achieved, resulting in a more even removal of varnish from the entire paint surface. This was confirmed under comparative examinations of test areas before and after both swab/solvent and tissue gel cleaning at 15x magnification and samples at 250x magnification.

Similar examinations additionally indicated that the technique, unlike swabs with free solvent, appeared to swell and dissolve some of the varnish in the cracks and under the raised paint edges, whereby this material could be gently pressed out during the absorbent tissue step. The swelling of these residues in the cracks may explain the frequent observation that application of free solvent to a recently tissue gel cleaned location no longer results in solvent penetration through to the reverse of the painting, even when this rapidly occurred prior to cleaning.

With the cupping in these paintings reduced and more of the non-original materials removed from the structure, more effective consolidation could be subsequently carried out. Thus during the treatment of the family

Figure 4Detail from reverse of an unlined canvas painting during cleaning tests with both solvent/swab (on left side) and tissue gel technique (on right side). Spot visible on left where swab�applied free solvent has penetrated to reverse. On right no visible penetration of solvent with tissue gel technique

Figure 5Same area in UV of reverse of an unlined canvas painting during cleaning tests with both solvent/swab (on left side) and tissue gel technique (on right side). Spot visible where solvent penetration has resulted in varnish residues being drawn through the painting structure to the reverse. On right no penetration of varnish residues with tissue gel technique

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portraits with these topographies and structural problems the tissue gel composite technique was used to remove the varnish layers.

Ferdinand Bol paintings, the peace palace, den haag, the netherlands

Between May 2006 and May 2007 an ensemble of four very large oil on canvas paintings by Ferdinand Bol (1616–1680) were technically examined, conserved and restored by SRAL whilst on public display in the Bonnefanten Museum in Maastricht. Previous partial cleanings, local but extreme cupping in the paint surfaces, and the project requiring a team of different conservators were again considerations. Furthermore, the paintings were coated in numerous thick varnish layers (Figure 6) and testing with free solvent and swabs indicated that, even with the most effective solvents for the varnish, considerable quantities were required to remove the majority of varnish from the surface. In almost identical test areas, the quantities of solvent required for applying both the swab/free solvent and tissue gel composite techniques were compared. Even on a brute scale (where the amount of solvent retained in the gelled tissues after their removal from the surface was not deducted) less solvent was required for varnish removal than with the tissue gel technique. In this case the system proved more efficient since the thick varnish layers were both dissolved and consequently removed everywhere the solvent was in contact with the surface (as opposed to swab/free solvent cleaning).

Despite setting up an effective extraction system in the museum studio, it was also considered critical that solvent exposure be limited and contained as much as possible. To this end an enclosed pack system was developed whereby multiple gelled tissues could be prepared in advance under excellent extraction conditions. This reduced the exposure of both public and conservator to solvent, and the time required for the tissue preparation.

portraits of landgraaf karl van hessen-kassel and his wife, castle amerongen, amerongen, the netherlands

These two oil on canvas paintings had been previously lined using an unusual adhesive which analysis indicated was composed of a drying oil with both lead and iron oxides. Over the intervening years, this adhesive had flowed from the reverse of the canvas through the cracks in the ground and paint layers and over onto the surrounding varnished surface. The intense red colour of the adhesive was particularly visible in the lighter passages and greatly disturbed the images in general as it emphasised the crack pattern. The possibility to remove the lining adhesive from the surfaces simultaneously with the varnish layers was explored and, in terms of their solubility behaviour, confirmed during solvent tests. However, using swabs and free solvent resulted in the solvent’s immediate penetration to the canvas reverse, and most critically in this case, smearing of the lining adhesive across the painting surface. Tests with the tissue gel composite

Figure 6Detail from a painting by Ferdinand Bol during varnish removal tests

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technique showed that the system could effectively remove varnish, old overpaints and lining adhesive residues from the surface without solvent penetration to the reverse and in a vertical manner whereby re-deposition across the painting surface was prevented (Figure 7).

investigAtions And further worK

As highlighted above the tissue gel composite technique encompasses some generally observed advantages that have proved highly beneficial in specific cleaning situations. Hence over the past 13 years the system has frequently surfaced as the conservator’s choice of cleaning method at SRAL. Temporarily employed conservators, interns and students at SRAL are also typically enthusiastic about the technique, continuing to employ and communicate the method after leaving to work elsewhere in the world.

However, some outstanding questions remain. By applying the solvent in a gel, the physical properties of the solvent will be influenced, including the evaporation rate for instance. It remains unknown how solvent diffusion at the paint/varnish interface will be affected when a solvent is gelled, a factor which may have significant consequences regarding the solvent’s degree of penetration into and thus potential swelling within the original paint film. In addition, given the current choice of gelators solvent selection is limited, and as with all gel systems there remain issues regarding clearance and residues (if not subsequently cleared with free solvent some cellulose ether residues have been observed remaining on the surface after tissue cleaning on samples at 250x magnification).

Alongside continued investigations and developments of the technique at SRAL, it is hoped that studies may be carried out within the framework of the upcoming multidisciplinary Netherlands Organisation for Scientific Research (NWO) Science for Arts Programme.

conclusion

The use of organic solvents is, and will likely remain, one of the most popular and reliable methods for removing non-original applied layers from oil paintings. It is not the intention of this paper to advocate the cleaning method outlined above for every situation. Rather it is to highlight that the development of techniques by conservators to manipulate solvent properties and their manner of application, in accordance with the latest research findings and the individual requirements of the painting, is an important step in refining organic solvent use for cleaning paintings.

AcKnowledgements

For their enthusiasm and observations the authors would like to thank their colleagues and all the conservators who have ever experimented with this technique at SRAL.

Figure 7During cleaning with gel tissue technique whereby the old red lining adhesive was removed without smearing across the paint surface

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mAteriAls list

Suppliers of Tissues

Tissue for gel impregnation: Hanotex (untreated), Joure, Nederland, article #2516, schoonmaakdoek RR, 33 x 40 cm

For absorbing tissue: Kimberly-Clark professional Kimtech 7506, dry absorbant towels