Fast Fills for Big Gaps Author: Rachel C

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Article: Fast Fills for Big Gaps Author: Rachel C. Sabino Source: Objects Specialty Group Postprints, Volume Twenty-Four, 2017 Pages: 454–471 Editors: Emily Hamilton and Kari Dodson, with Tony Sigel Program Chair ISSN (print version) 2169-379X ISSN (online version) 2169-1290 © 2019 by American Institute for Conservation of Historic and Artistic Works 727 15th Street NW, Suite 500, Washington, DC 20005 (202) 452-9545 www.culturalheritage.org

Objects Specialty Group Postprints is published annually by the Objects Specialty Group (OSG) of the American Institute for Conservation (AIC). It is a conference proceedings volume consisting of papers presented in the OSG sessions at AIC Annual Meetings.

Under a licensing agreement, individual authors retain copyright to their work and extend publications rights to the American Institute for Conservation.

Unless otherwise noted, images are provided courtesy of the author, who has obtained permission to publish them here.

This article is published in theObjects Specialty Group Postprints, Volume Twenty-Four, 2017. It has been edited for clarity and content. The article was peer-reviewed by content area specialists and was revised based on this anonymous review. Responsibility for the methods and materials described herein, however, rests solely with the author(s), whose article should not be considered an official statement of the OSG or the AIC.

OSG2017-Sabino_tip_layout.indd 1 12/3/19 5:46 AM FAST FILLS FOR BIG GAPS

RACHEL C. SABINO

The best method for creating a very deep fill in semi-translucent stones such as marble is to gradually build up successive layers of material(s) to achieve a satisfactory match in both color and density. However, if the treatment must be completed in a shorter time than this more controlled method requires, the materials best suited to produce a quick set over a large volume pose problems of opacity, cost, and/or reversibility. This tip instead makes use of high-density polyethylene foam covered with a thin skin of bulked and tinted epoxy resin to achieve a time-effective, economical, reversible, and aesthetically pleasing fill.

KEYWORDS: Loss compensation, Marble, Ethafoam, Epoxy resin, Onyx powder

1. THE RELIEF

This large fragment of a marble relief reflects in spirit and style the avid return around 1500 to classical models and their incorporation in contemporary sculpture (fig. 1). While nothing is known of its original context or function, it most likely formed part of a commemorative or triumphal monument in Rome. Both the cornucopia beneath the putto’s outstretched proper left leg and the terminus of the swag draped over his shoulder contain oak leaves—the emblem of the formidable Della Rovere family of Bologna, from the ranks of whom two Renaissance popes hailed: Sixtus IV and Julius II. The crossed keys under an umbrella-like canopy—the armorial device for the Holy See when the papal throne was temporarily vacant—provide another potential clue as to the original source of the fragment.

2. SCOPE

The object entered the collection of the Art Institute of Chicago in 1962 and received only one documented treatment in 1977, when unsightly incrustations were mechanically removed and it was cleaned with an aqueous surfactant. Consigned to storage, it languished for the better part of three decades until 2016, when it was designated as one of nearly 700 objects to be installed in the newly designed Deering Family Galleries of Medieval and Renaissance Art, Arms, and Armor opening the following spring. The relief was to be installed at considerable height in a gallery containing other stone works similarly reminiscent of the early antique. Since so many objects needed to be prepared for installation, the relief could not be given the luxury of a gradual, stratified approach to filling the largest losses.

3. CONDITION

The carving is largely intact and the stone is in overall sound condition but with some surface weathering and resultant loss of detail. The papal insignia appears to be a later modification recarved from a previous form—most likely additional vegetation at the end of the swag hanging from the proper right shoulder. Recarving is suggested by the relative whiteness and less-weathered appearance of this area; its position slightly lower than the plane of the rest of the relief; the roughly keyed surface of the marble just below it; and the style of carving that differs appreciably from the rest (fig. 2). The lower half of the putto’s missing proper right arm does not lie in the same plane as the insignia, and the nature of the break edge and the condition of the exposed marble along it gives the impression that this damage predates the recarving. The front of the proper left foot is also missing. A large square notch is present at the top proper right corner, perhaps made during extraction of the relief from its original context.

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Fig. 1. Black-and-white image of the relief before treatment. Italian, School of Andrea del Verrocchio, Putto with Papal Insignia, 1475/1500, marble, 149.9 × 73.7 cm, The Art Institute of Chicago, 1962.100 (Courtesy of the Art Institute of Chicago)

More crucially, there is a massive triangular fragment on the top proper right corner with associated chips and lacunae of varying dimensions along the break edge. In a previous campaign of stabilization, this fragment had been secured in place with four iron cramps. Over time, the expansion of the rusting metal caused the stone to cleave away from the cramps at the top and proper right-side edges, resulting in losses so significant as to expose the cramps themselves. The loss at the top edge measures approximately 30 cm long and 4 cm deep (fig. 3). The loss on the proper right side measures approximately 45 cm long and

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Fig. 2. Detail of the papal insignia, likely a recarving (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

4 cm deep. The primary void also extends to a significant portion of the putto’s outermost wing feathers (fig. 4). On the verso, lesser spalls are present in association with the cramps, as is a copious amount of disfiguring rust staining, but overall the morphology of the damage is not the same as that on the exposed sides (fig. 5). Perhaps this more benign condition is attributable to their relatively more sheltered position. A significant amount of rust staining has migrated through to the front of the relief, lending it a decidedly yellow appearance.

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Fig. 3. The loss on the top edge of the fragment (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

4. TREATMENT OBJECTIVE

The tight time frame of this treatment, coupled with the logistical obstacles of performing radiography on such a large piece of stone, precluded exhaustive testing or definitive confirmation of the solidity of the cramps and the stability of the join. However, the familiarity conferred by repeated handling during initial examinations afforded some reassurance that the cramps, aided by a new mounting system, would continue to hold the pieces together safely. The scope of treatment was therefore confined to making aesthetic improvements with loss compensation in the two largest gaps; along the outside of the putto’s wing; along the break edge of the primary fragment on the recto; and within the square notch at the top proper right corner. In addition, wherever possible, the overall appearance of the stone would be harmonized by mitigating staining and discoloration.

5. THE DECISION TREE

There were any number of methods by which to approach this loss-compensation treatment, each method presenting advantages and disadvantages.

5.1 Plaster Plaster, whose economy and ease of use made it attractive, was initially considered for filling the largest losses. Most often, plaster is too dense for filling marble (even for weathered, ancient ones that tend to be more forgiving than the highly polished decorative ones of subsequent centuries), resulting in shadows on either side of the fill. But occasionally, with skillful retouching, plaster can be a viable option for marbles

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Fig. 4. The loss on the proper right side of the fragment (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

that are themselves quite dense and are not positioned in such a way to receive a good deal of transmitted light. This particular object was a borderline case. It would receive no transmitted light whatsoever, but despite being relatively dense, the tonality of the marble was comparatively warm relative to plaster. This tonal discrepancy could probably have been rectified by adding pigments to the plaster before hydrating it or by a campaign of calculated retouching.

Ultimately, the topography of the break edges and the presence of the metal cramps contraindicated its use. Even with the careful design and execution of dams or molds, it would have been all too easy to lose copious amounts of fill material through pinholes, slips, or faults, likely in a very messy way, nullifying the benefits of using plaster in the first place. In addition, the cramps sit slightly proud of the surface with a 3-mm gap around them (fig. 6). To look convincing, the new fills would also have to be set back this same 3-mm distance. It would be a formidable task to construct a dam or mold that would not create more problems than it solved: allowing the plaster to flow around and behind the dam, between the

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Fig. 5. The cramps and the resultant staining on the verso of the relief fragment (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

Fig. 6. The top cramp viewed from above showing its position slightly proud of the surface and the 3-mm gap in the stone around it (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

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cramps, and through to the verso of the object; entrapping the dam material within the plaster, preventing its extraction after casting; and requiring copious amounts of tooling and shaping after casting to replicate the texture on the top surface.

To eliminate the problem of liquid plaster flow and the concurrent necessity for faultless dams, it might have been possible to apply the plaster in a more controllable form at a more advanced state of cure. Working with curing plaster means working quickly, as the window of such plasticity is quite narrow. It also means doing multiple applications if the volume is in excess of what can be handled at one time, as was certainly the case in this instance. Not least, the chore of dealing with plaster dust and ghosting is something that conservators strive to avoid.

But the final determining factor weighing against the use of plaster was the inadvisability of placing a hygroscopic material (to say nothing of the actual water present in the admixture) in such close proximity to the iron, even when taking proper precautions by thoroughly sealing the surfaces with acrylic resin.

5.2 Thermosetting Resins A direct pour using a tinted epoxy was also an option. However, as was the case with plaster, any voids or pinholes in the dam or mold could produce results along a spectrum from inconvenient to calamitous. The directive to maintain 3 mm around the clamp would be even more difficult to achieve in resin and more difficult to reverse in the event that the pour went wrong. While it would most likely have been easier to separate the dam materials from the resin after curing, the risk of the epoxy migrating around the clamp or along the break edge of the fragment could not be justified. There is also the very real issue of cost: preparing a sufficient volume of a conservation-grade epoxy, accounting for the inevitable accompanying waste, would have been prohibitively expensive. Although the better-grade epoxies available to the marine and sculpture industries are more cost-effective, their aging properties are unacceptable.

Polyester resins are generally less expensive and better suited to casting very large volumes. However, they are challenging to use: they have extremely short working times and require resin-to-hardener ratios that are difficult to measure in the comparatively smaller volume necessary for these fills. They also produce a significant amount of heat, a potential danger not only to the object but also to the user.

5.3 Acrylic Resin Recent experimentation (Wolfe and O’Connor 2005) has led to successful loss-compensation campaigns for decorative marbles using tinted and bulked acrylic resin-based fillers (Riccardelli et al. 2014). This development is most welcome, as acrylic fills retain all the best properties of epoxy-based fills (i.e., translucency, infinite modification) with none of the drawbacks (i.e., excessive hardness, toxicity, difficulty in reversibility). However, in the experience of this author, acrylic resin mixtures offer little advantage in terms of efficiency and can be, in fact, more time-consuming to prepare, apply, control, refine, and store between uses than epoxy-based fills.

5.4 The Solution It became increasingly apparent that no single material would do the job and that it would be beneficial and expeditious to employ a combination of materials to create a core fill overlaid by another fill to simulate the marble.

For the core, six materials were evaluated for their appearance, ease of application and compatibility with the marble: acrylic resin bulked with paper pulp, strips of mat board, epoxy putty, separately-cast plaster

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forms, acrylic resin bulked with stone fillers, and foam. These were evaluated in combination with five candidates for the outer fill: acrylic putty, plaster, epoxy-based filler, bulked acrylic resin, and wax.

In the end, 9-pound, closed-cell polyethylene foam was chosen for use as the core fill. Despite having a cooler tonality than the stone, it could be shaped very easily and efficiently. Its opacity is greater than what would have been achieved with a resin bulked with paper pulp or stone fillers but less than that of plaster or acrylic putty.

Because the outer fill was designed to be relatively thin and the inner fill somewhat amorphous, the hardness of an epoxy presented a particular advantage in conferring stability and durability. For this, AKEPOX 5010 was chosen—a two-part, high-viscosity (knife-grade) epoxy resin. In addition to structural strength, other properties of this particular epoxy made it very advantageous in terms of efficiency: the 2:1 mixing ratio is easy to apportion by eye; it has a working time of 20 minutes; and it reaches full cure in approximately 12 hours. None of these parameters could have been achieved with an acrylic resin.

AKEPOX 5010 is semi-translucent with a slightly bluish-white tone. Therefore, strategic bulking using a wide array of fillers and pigments would be necessary not only to match the surrounding stone but also to counter the contrasting tonalities of both the resin and the underlying foam. Because the foam would occupy the majority of the void, the overlying epoxy and filler combination would have to be more translucent than normal to offset the opacity of the foam—an additional challenge.

6. TREATMENT

The entire surface and the abundant drill holes that punctuate the contours of the high relief areas were extremely dirty and covered uniformly with loose particulate matter alongside various stains and accretions (fig. 7). Prior to beginning the campaign of loss compensation, surface dirt and debris were

Fig. 7. The drill work delineating the contours of the relief (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

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removed from the object using compressed air. Insufficient time was available to attempt solvent cleaning or poulticing of the iron staining. However, given the viewing distance and planned ambient lighting in the gallery, such measures were ultimately not deemed necessary.

The interiors of the cavities of the two largest losses were liberally consolidated with acrylic resin (10% w/v in acetone) to act as a barrier layer between the stone and the fill, and to seal the iron cramp, inhibiting moisture ingress and further oxidation. Two square planks of foam were cut to roughly the same dimensions as the large voids. With a blade, the backs of the foam planks were whittled to shape according to the profile of the interior of the cavity. The exposed surfaces of the planks were trimmed and carved until they were set back approximately 6 mm below the stone. The foam was bonded to the interior of the cavity using acrylic resin (40% w/v in acetone) bulked with glass beads to promote adhesion and to bridge any voids between the foam and the underlying stone. The foam was clamped in position until the resin reached full cure (fig. 8).

After the foam was securely bonded in position, the clamps were removed and the stone immediately surrounding the losses was coated with acrylic resin (10% w/v in acetone) to protect the surface during the subsequent phases of treatment. As much epoxy, pigment, and filler as could be adequately handled, prepared, and matched within the 20-minute curing window was applied over the foam with a spatula. No hard and fast rule governed which combinations of pigments and fillers worked best for any given area. At times, different, contrasting mixtures were deliberately applied in close proximity to one another to mimic veins and faults, enhancing the naturalism. No test coupons were prepared. Instead, these decisions as to texture, density, and color were made on the spot, guided by the author’s previous experience.

Fig. 8. Ethafoam core fill bonded in position on the proper right side (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

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Generally speaking, successful combinations are achieved by first selecting the aggregate or filler that produces the best textural match to the stone. Aluminum oxide and aluminum hydroxide perform particularly well in emulating dense, highly polished stones. Coarser aggregates such as marble powders and sands work well for weathered or sugary stones. The majority of these fillers tend to impart a greenish cast to the resins. This effect is best offset with pigments, with the proviso that many pigments, titanium white in particular, are potent colorants with opacifying properties all their own and a less-is-more approach definitely wins the day.

The missing angel’s wing and feathers were not replicated precisely, barb by barb. Instead, the outermost edges of each vane were built up sharply enough to be legible to the viewer, but the material between the resultant hard edges was modeled to be suggestive of the weathered surfaces visible elsewhere on the sculpture (fig. 9).

The square notch at the top proper right corner was left unfilled to accommodate a new mount, but the disfiguring void was camouflaged by a thin skin of the same epoxy-based fill material. To retain the square shape, a plug of polyethylene foam was inserted into the cavity and the fill material was skimmed over the top. Before the resin cured completely, the foam was extracted from the cavity (fig. 10).

Smaller voids and cracks were filled with the same epoxy-based fill material, tinted and bulked to best match the surrounding area. All fills, both major and minor, were smoothed and brought level with the surface using a rotary flex-shaft tool fitted with various silicone-rubber polishing wheels and points. At the close of treatment, the protective acrylic resin barrier layer was removed from the surface using acetone on cotton wool swabs. The fills were further refined using acrylic paints, colored pencils,

Fig. 9. Detail of the relief after complete loss compensation (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

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Fig. 10. The square notch at the top proper right corner before treatment (left) and after treatment with space left to accommodate a mount (right). (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

and/or pastels. Areas of minor superficial staining were toned out with gouache and refined with colored pencils and/or pastels following the application of a barrier layer of cellulose ether (3% w/v in distilled water; fig. 11). In this particular application, the use of cellulose ether as a barrier layer provides a robust separation between the stone and the overlying retouching media with a minimum of interference. Relative to other classes of materials, it does not rest heavily on the surface; impose a gloss or sheen; or inhibit the retouching media from sitting, feathering, or blending convincingly with the original.

6.1 Cramp Gaps and Texture Pops To maintain the visual effect of the 3-mm gap around the cramps, strips of blotter board stacked to the correct height were faced with sealing film and secured to the top of the cramp. The epoxy was then applied up to and against the strips. After the epoxy had cured, the blotters could be easily extracted thanks to the sealing film that served as a release agent. Once removed, the gap required only a single pass with a file to round the edge (fig. 12).

To replicate the texture of the marble around the fills, small quantities of vinyl polysiloxane impression material were prepared and placed against the stone following application of a protective coat of acrylic resin. Simultaneously, the impression material was built upwards around wooden stirrers, which served as handles. These so-called “texture pops” were used to impress the texture onto the semi-cured epoxy putty during the latter stage of curing. A light sprinkling of one of the fillers (dependent on the color and morphology of the surrounding crystal matrix) served as a release agent for the mold and as a matting agent for the epoxy, which cured to a relatively high gloss (fig. 13).

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Fig. 11. The relief fragment installed in the gallery at the close of treatment (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

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Fig. 12. Side view of the relief after treatment showing the 3-mm gap around the cramp and the mount engaged in the notch (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago)

7. RETREATABILITY

Should it be necessary to remove the epoxy due to aging, yellowing, and/or failure or to address overarching structural issues that warrant the wholesale removal of the repair, it is easily reversed in stages. The conservator need merely coat the perimeter of the fill with acrylic resin to protect the

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Fig. 13. Detail showing the “texture pops” in the foreground and the blotter board spacers in the background (Photograph by Rachel C. Sabino, courtesy of the Art Institute of Chicago) surrounding stone and apply methylene chloride–based paint stripper to the surface of the epoxy skin. The epoxy covering is sufficiently thin that it will swell rapidly, producing a distinct cleavage plane between it and the underlying foam. Once swelled, the compromised epoxy fill can be levered

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out with a long, flat implement, such as a palette knife, spatula, or blade introduced through a breach in the epoxy. The exposed underlayer of foam can be cleaned and sanitized with ethanol or acetone, trimmed and consolidated again if necessary, and a new fill, either done in the same material or a different one, can be applied over the top. Alternately, to continue removing the fill, the foam is easily detached from the stone with acetone, as is any residual adhesive and/or glass beads.

ACKNOWLEDGMENTS

For this and many other treatments, I am grateful to have the deep reservoir of Tony Sigel’s experience for ideas, and I appreciate his willingness to be a sounding board and to brainstorm with me during the early phases of this treatment. Many thanks to Jann Trujillo for ordering an endless stream of materials.

REFERENCES

Riccardelli, Carolyn, Michael Morris, George Wheeler, Jack Soultanian, Lawrence Becker, and Ronald Street. 2014. “The Treatment of Tullio Lombardo’s Adam: A New Approach to the Conservation of Monumental Marble Sculpture.” Metropolitan Museum Journal 49: 49–116. Wolfe, Julie, and Talitha O’Connor. 2005. “Properties of Fillers in Putties Based on Acryloid B-72.” Objects Specialty Group Postprints, Volume Twelve, 2005. Washington, DC: American Institute for Conservation (AIC). 12: 91–117.