The treatment of panel paintings has always been and still is one of the most serious and baffling problems in the whole field of conservation. . . . We recognize that they are inherently vulnerable to damage and must be treated rather like invalids in delicate health, likely to contract a mortal illness with any change in climate.
—Richard D. Buck, 1963

The painting in treatment must make the rules. Use the system you know best until you have mastered the intricacies of another.
—Caroline Keck, 1977

When Man with a Sheet of Music, signed and dated by Rembrandt van Rijn in 1633, entered the collection of the National Gallery of Art in 2014, the painting was not put on display because of its compromised condition, specifically, open joins in the wood support. Previous attempts to stabilize the panel did not withstand the test of time, and it is undergoing treatment again to address the structural issues as well as aesthetic ones. This essay discusses the changing approaches to the structural treatment of paintings on wood, reviews previous interventions on this painting, and describes the treatment that is in progress.

Various treatments for paintings on wood, or panel paintings, have been developed over the past two and a half centuries to deal with damage stemming from dimensional changes in the support as a result of wide fluctuations in the relative humidity of the local environment. As wood is an anisotropic material, the dimensional changes that occur are unequal, depending on the grain direction, with greater shrinkage occurring across or perpendicular to the grain and very little shrinkage in the direction of the grain. When one side of the panel is painted and the reverse left bare, the unpainted side is more reactive to changes in humidity than the painted side. After repeated cycles of dry and humid conditions, the exposed wood cells shrink and swell in tandem, leading to permanently crushed and deformed cells. The result of this compression shrinkage is a convex warp of the support, which is apparent when viewing the painted side.

Returning the painting to a flat state not only for aesthetic reasons but also to arrest further movement of the wood support, which could lead to paint loss, often involved removing a certain quantity of the support to make the painting more malleable and controllable and therefore easier to flatten. This thinning of the panel, along with the attachment of an additional structure, an auxiliary support, to the reverse to immobilize it and keep the painting flat, was a standard treatment until the late twentieth century. But it was found that if a warped panel is forced flat, the paint layers can experience compression and release the stress by cupping and flaking. It was later discovered that the restraint of the auxiliary supports when subjected to unstable environments can cause cracks to develop. Though these structural interventions were done with the best intentions and knowledge at the time, they could cause more problems than they solved. 

An example of a painting that underwent an extreme version of this process in the collection of the National Gallery is Raphael’s arresting portrait of Bindo Altoviti (c. 1515), which had a history of blisters of insecure paint forming (fig. 1). In this case, instead of thinning the panel, all the wood down to the gesso layer was removed to eliminate any possibility of separation occurring between the ground and paint layers (fig. 2). ^[1] This procedure was carried out by hand with chisels and planes, but first a protective layer of paper, or facing, was temporarily glued to the paint surface. At the end of this procedure, only the paint layers, with a scant amount of ground, remained; these were then attached to a new, more stable support. This technique, known as a transfer, was not an uncommon way to treat panel paintings that were proving difficult to preserve because of continued instability in the paint layers, such as lifting and flaking. As treatments have evolved to be less invasive and with the acceptance of deformations that have developed over time if they do not adversely affect the stability of the paint layers and support and the aesthetic reading of the painting, today every attempt is made to preserve the original support and its invaluable data. ^[2]

Another painting on wood in the National Gallery’s collection that seemed destined to the same fate as the portrait by Raphael is Rembrandt’s Man with a Sheet of Music (fig. 3). The bust-length portrait depicts a man with a moustache and goatee, wearing a wide-brimmed hat and holding a rolled sheet of music in his left hand, against a slightly modulated cool brown background where it is signed and dated by the artist. Though there has been disagreement about the attribution of this painting, an in-depth discussion of which is beyond the scope of this essay, the National Gallery currently accepts it as an autograph work by Rembrandt. ^[3] The artist painted on both panel and canvas, but the choice of an oak panel support for this portrait is characteristic of the early part of Rembrandt’s career, which mirrors a period of transition from the smooth surfaces of wood to textured canvas supports in seventeenth-century northern Europe. ^[4] The reasons for this were undoubtedly practical, driven by dwindling supplies of quality oak, as well as the possibilities that canvas offered for lighter and larger paintings.

Initially, the portrait arrived in the United States without any notable condition issues except for a small crack in the panel. The painting had been acquired in Europe in 1912 and entered the collection of Montana senator William A. Clark, who was then living in New York and on his death in 1925 bequeathed it to the Corcoran Gallery of Art. ^[5] Shortly thereafter, the first condition and treatment record of the painting was made: “June 1927 H.E Thompson, MFA Boston cleaned the painting. Noted that crack appearing near upper right corner.” ^[6] In ensuing years, Man with a Sheet of Music would undergo a series of structural interventions, each one progressively more complicated.

As the Corcoran Gallery of Art had a dedicated conservator for its collection, Russell Quandt, the condition of the artworks could be carefully monitored over time. Man with a Sheet of Music was one work that required Quandt’s repeated attention. The first of its structural treatments took place in 1951. Quandt, who had been hired the year before, ^[7] consolidated the paint cleavage with a wax-resin adhesive, after which the painting was cleaned and retouched. In an attempt to reduce further movement of the support, which would in turn keep the paint layers from lifting and detaching, a layer of linen was adhered to the reverse of the panel, also with wax-resin adhesive, to act as a moisture barrier. ^[8] Serious research on moisture as a key factor to consider in the preservation of panel paintings was begun in the 1930s by the influential conservator Richard Buck, based then at the Fogg Museum of Art at Harvard University. ^[9] The Fogg Museum was the first American museum to establish a laboratory dedicated to the conservation and scientific research of cultural heritage and became the de facto training center for the first generation of conservators in the United States. ^[10] A year before the Rembrandt was treated, in 1950, one of the conservators at the Fogg Museum, Morton C. Bradley, published The Treatment of Pictures. ^[11] At the time, it was an important practical reference manual “for competent conservators and curators and for students working under their directions”; it was not intended to be used as a do-it-yourself guide. ^[12] Among the large number of procedures presented in the manual, “moisture barrier for a wooden panel that did not require reinforcement” is described in the first chapter. It outlines the attachment of a linen fabric using a wax-resin adhesive to the reverse and edges of the panel “to serve as a tension member and as a moisture barrier.” ^[13] Quandt, who trained under Sheldon Keck, who had himself apprenticed at the Fogg Museum, ^[14] had an extensive professional network and must have been familiar with Buck’s research and Bradley’s publication to have decided to reinforce the structure and protect the panel from fluctuations in relative humidity according to Bradley’s instructions. 

An unexpected turn of events led Quandt to intervene again on Man with a Sheet of Music not long after the application of the moisture barrier. In November 1959, there was an attempt to cut the painting out of its frame, but as its support was wood, not canvas, the would-be thief was unsuccessful. ^[15] The painting was left with deep incisions in the lower right corner from the knife that was used (fig. 4). ^[16] This unfortunate incident prompted an overall reassessment of the painting’s condition. Writing in 1959, Russell Quandt, conservator at the Corcoran, noted, “Examination showed paint should be transferred.” ^[17] Buckling of the support on the right side of the panel was observed, as well as some paint cleavage in the upper half of the painting. ^[18] The absence of a controlled and well-regulated climate in the gallery was taking a toll on the painting. ^[19]

Quandt suggested two options for treating the recent damage, one that would only address the damage caused by the attempted robbery and one that was more comprehensive. “The treatment of 1951 has obviously not been entirely effective. This failure is manifested by the recurrence of the old flaking and cleavages of the paint layers. . . . Therefore, the alternative is to remove the existing support from the paint and transfer it to a new one,” wrote Quandt. Given the two choices, the transfer was approved. In a memo, Quandt emphasized, “As this is an important + valuable [painting], might as well do the full job.” ^[20]

A December 1959 Washington Post interview with Quandt reported, “Quandt will use power tools to cut into the oak base until he has reached about a 16th of an inch from the painting’s surface. Then he will plane the base down with hand tools until only the film of the painting remains” (fig. 5). ^[21] Ultimately, however, Man with a Sheet of Music was not transferred. In the time between the interview by the journalist and the start of the treatment, Quandt changed his mind. Though Bradley advocates removing the original wood support and replacing it with a more stable layer in his Treatment of Pictures, Quandt chose to preserve the original oak support. ^[22] A 1960 document states that Quandt again consolidated the flaking paint with a wax adhesive. He did reduce the thickness of the panel to approximately one-sixteenth inch (2 mm) but then impregnated the exposed wood with PVA AYAB, a synthetic resin, ^[23] and attached an auxiliary support made of masonite hardboard, ^[24] redwood strips, and aluminum crossbars to the reverse of the painting. ^[25]

The type of auxiliary support design described is an example of one of the first of the so-called reconstruction systems that were developed at the Fogg Museum in the 1930s. ^[26] In this method, most of the original wood support was replaced with a new, composite wood support. According to Buck, “Its particular merit is that it is almost completely unresponsive to atmospheric variations.” ^[27] In theory, this would stop further movement and warping of the panel and potential damage to the paint layers. The earliest diagram of this composite support dates to 1937 and shows alternating strips of redwood and balsa wood on top of a series of sheet materials between the reverse of the panel and the strips (fig. 6). ^[28]

This new type of solid support with overall continuous contact on the reverse of the panel was considered an improvement over cradling. Developed in Italy in the eighteenth century, cradling was a structural treatment that spread to other countries in Europe, notably France, Germany, and England, and eventually to the United States. Cradles can be found on many of the panel paintings in the National Gallery’s collection (fig. 7). In this type of intervention, strips of wood are glued in the same grain direction as the wood support and hold sliding crosspieces, creating a grid-work construction on the reverse. The design took into consideration the natural movement of wood in reaction to its surrounding environment, swelling when humid and shrinking when dry. With time, even allowing for some movement, if the environment is not stable, the sliding members can become locked, and the cradle will no longer function as it was intended, leading to unwanted consequences such as further surface deformations and cracks. ^[29]

By 1967, the cracks in Man with a Sheet of Music were noted as worsening, though no issues with the paint layers were mentioned. ^[30] Up to this time, just one crack had ever been mentioned in the file, the small one in the upper right corner. No cracks were visible in a photograph from the last treatment, seven years earlier (see fig. 4a). The only visual record we have of the panel’s condition from around the time of the noted cracks is a black-and-white photograph of the reverse, “probably taken during the 1969 treatment,” ^[31] after the 1959/1960 auxiliary support was removed (fig. 8). In this photograph, various score marks are apparent on the panel that were probably made to aid in adhering the previous “reconstruction” support. There is also a translucent material present in patches that is peeling away, which could be the PVA AYAB that had been applied as a moisture barrier. The print is annotated with measurements that appear to indicate the widths of the individual planks that make up the panel: three planks of oak, a wide central board flanked by two narrower ones. ^[32] Between the central board and the board on the right, the join appears to be partially open away from the edges. One could assume the comment about the worsening cracks refers to the right join that opened. For almost three centuries, until it moved to the United States, the painting existed in a good state of preservation. In the span of about forty years the painting went from having only a small crack to at least one long crack that developed in less than a decade after the previous treatment was supposed to have stabilized the painting’s structure. ^[33]

In addition to the reopened join, the panel had warped. ^[34] The direction of the warp, either convex or concave when viewed from the front, was not specified in the treatment report. The warp in this case was probably concave as the auxiliary support would have introduced compression at the surface and not at the back of the panel. ^[35] A convex warp tends to form when the panel is relatively free from restraint but also depends on the cut of wood and its orientation in relation to the painted surface. 

Then began the third structural intervention since the painting arrived in the United States. In 1969, the painting was “placed in humidity chamber until warp reversed,” which can be interpreted to mean until the panel became flat, after which the auxiliary support from the 1960 treatment was removed and the new auxiliary support consisting of balsa blocks was attached. ^[36]

Balsa-block backings originated at the Fogg Museum, where Richard Buck and his colleagues in the 1930s and 1940s developed the reconstruction treatments for panel paintings. Redwood was initially used for its lightness, uniform grain, and adequate strength, ^[37] then was used in combination with balsa wood, which has similar characteristics but is even lighter and became easier to source as redwood is a threatened species. These treatments evolved into the balsa-block design, which was refined during Buck’s directorship at the regional conservation center of the Intermuseum Conservation Association (ICA), in Oberlin, Ohio, starting in 1952. ^[38] The materials and procedures of this system are well documented by Spurlock and Horns, ^[39] so only a summary is provided here. The cleaned reverse of the panel is coated with Saran F-310, a synthetic resin with low permeability, to serve as a moisture barrier. ^[40] Then an open-weave fiberglass marquisette cloth is applied to the back of the panel and a second coat of Saran F-310 is brushed through it to adhere the cloth to the panel. End grain balsa blocks, rectangular in shape, are cut to the desired thickness, ranging from 1 to 5 centimeters depending on the panel, and are adhered cross grain to the support “to ensure the greatest strength.” ^[41] The blocks are arranged in a staggered brick pattern with a wax-resin adhesive bulked with wood flour and kaolin, called a mortar. Strips of pine can also be added across the grain of the panel for extra reinforcement. This treatment was carried out as a last resort on previously thinned panels where the existing auxiliary support, usually a cradle, was failing. A balsa-block backing was considered appropriate for paintings on wood supports that would continue to be exposed to a poorly controlled environment, not to restrict the movement of the panel, but to isolate the panel from changes in humidity to prevent any movement in the first place. ^[42]

Despite the three attempts in 1951, 1960, and 1969 to stabilize Man with a Sheet of Music by isolating the wood support from the environment to maintain its stability and planarity, the panel continued to warp and crack. In photographs dated June 1984, both joins in the panel are partially open. When the painting entered the National Gallery’s collection in 2014, the condition seems to have remained unchanged from 1984, except that the right open join appears to have extended farther into the background. In addition to the open joins, there was microcupping of the background paint in the same area where it had been documented previously due to the concave warp of the support that had reoccurred (fig. 9). ^[43] In this case the painted side has become more reactive than the back side due to the various additions on the reverse that shielded it from the environment. This has caused the panel to warp concavely. All the above conditions are the cumulative adverse effects of continued compression stress on the paint layers and the panel by the restraint of the auxiliary support.

To repair the panel by closing the open joins and reversing the concave warp, the stress from the auxiliary support had to be eliminated. The only way to achieve this was to dismantle the balsa-block backing. Because the panel is documented as being very thin, it can no longer support itself, so the old auxiliary support must be replaced with one of a new design that allows for some natural movement of the original panel. 

The removal of existing auxiliary supports that no longer function and are creating additional problems must be carefully evaluated before proceeding. ^[44] For extremely thin panels such as those of Man with a Sheet of Music, a primary concern was how the panel will react when it is no longer under the restraint of the balsa-block backing. A prediction about the movement must made. An initial visual assessment was difficult when the end grain was not clean, and there was no access to the unpainted reverse of the support to examine the wood grain to understand how stable the cut of wood is. Reading the painted surface, the raised, closely spaced wood grain, it can be inferred that the planks that make up the panel are radially cut, which is the most stable cut of wood and the least prone to dimensional changes. ^[45] When Rembrandt painted on panels early in his career they were typically constructed of radially cut, or quartersawn, oak. Depending on the size of the painting, the support could be a single plank or two or three planks with butt-joins for a larger format up to about 0.39 inch (1 cm) thick (fig. 10). ^[46] For portraits the size of Man with a Sheet of Music, a wide central plank flanked by two narrower boards is an ideal structure that avoids positioning a join through the center of the sitter. Another example of a painting by Rembrandt with a similar support constructed of three planks completed just a few years after Man with a Sheet of Music is a Self-Portrait from 1636–1638 belonging to the Norton Simon Foundation. As the image side of this Self-Portrait also had a concave warp, the paint layers suffered from compression stress. Fortunately, a successful structural treatment reversed the warp by releasing the tension caused by attached strips around the perimeter on the reverse. ^[47] Though the support of Self-Portrait retains most of its original thickness, in contrast to Man with a Sheet of Music, its treatment nonetheless lends confidence to a similar positive outcome from the removal of the balsa-block backing (figs. 11, 12).

With any structural treatment, it is essential to track the changes in the shape of the panel during the removal of the old auxiliary support to look for the release of stress through an improvement in its shape and to understand its range of movement, which informs the choice and design of the new auxiliary support. Traditionally, the movement of the panel is monitored by taking profile tracings of the end grain edges at different times during the treatment, recording the temperature and relative humidity, and measuring the difference in deflection between the tracings to gauge how much the panel moved. While this is useful data, this method gives information only on what is happening at the edges of the painting and not what is happening in the center of the support. Warping may not always occur in a perfect cylindrical shape parallel to the grain axis, which would be mirrored by the end grain; there may also be a twist. In the case of Man with a Sheet of Music a concave warp is located in the right background away from the edges, so this deformation is not captured by profile tracings. To document the overall surface of the painting and monitor the changes in the panel’s shape during the removal of the balsa-block backing, the more comprehensive techniques of reflectance transformation imaging (RTI) and photogrammetry are being employed in collaboration with imaging specialists. ^[48] These techniques for studying the movement of paintings, first introduced to the cultural heritage sector in the 1990s, have vastly improved sampling resolution and do not require any contact with the surface. ^[49]

The balsa-block backing on Man with a Sheet of Music varies in interesting ways from the method previously outlined. Since the back of the painting is covered by a sheet of masonite hardboard, x-radiography was performed to reveal the construction of the auxiliary support before the treatment was started (fig. 13). ^[50] The blocks are almost square instead of rectangular, except for two columns on the right side that are narrower vertical rectangles oriented in the same vertical grain direction as the panel. All the blocks are laid out in a staggered pattern, according to the normal procedure. The choice of block shapes and their placement seems deliberate. In theory, square blocks would cause less tension than rectangular ones oriented across the grain. The column of rectangular blocks ensures that the right join was centered for maximum support. The left join is somewhat off-center over the column of square blocks, but there is adequate overlap on either side of it. Since most of the blocks vary a bit in size, almost none of the horizontal seams are aligned, which would aid in distributing those forces more evenly across the surface and avoid the concentration of any stresses or weaknesses along a single axis. All these choices are thoughtful considerations in attempting to eliminate to the extent possible the introduction of new stresses in the panel from the new auxiliary support.

To protect the paint layers during the removal of the balsa-block backing, when the painting would rest facedown on a cushioned surface, a facing of Japanese tissue was adhered to the front of the painting with a low-water-content adhesive that can be peeled off when dry. ^[51] Reducing the panel’s exposure to moisture as much as possible, in this case eliminating the need to reactivate the adhesive to remove the facing, mitigates a possible adverse reaction from the panel. Once the canvas strips that were wrapped around the edges of the panel hiding the balsa construction were removed, it was reassuring to have visual confirmation that the paint layers were not transferred and that indeed the wood support was only thinned to approximately one-sixteenth inch (2 mm), as was recommended for “reconstruction” treatments. It was surprising to find that the thick waxy adhesive present was not soluble in the usual range of chemicals as one would expect with a wax-resin-whiting mortar that was cited in the treatment record. ^[52] Analysis carried out to characterize the materials present and aid in determining an appropriate removal method returned unexpected results. There was no evidence of a resin component but rather a mixture of bleached carnauba wax, beeswax, and calcium carbonate. ^[53] In the earliest reinforcement treatments of 1950 at the Fogg Museum that included balsa wood, a beeswax-paraffin mixture was used as the adhesive, ^[54] but afterward only wax-resin mixtures have been documented as part of the balsa-block system, sometimes with the addition of an organic bulking agent such as sawdust or cork to serve as gap fillers (see Appendix). Wax-resin adhesives were developed in the Netherlands in the eighteenth century to structurally reinforce paintings on canvas by attaching a new canvas to the original, a treatment known as a lining. ^[55] Later, wax-resin was adopted to line canvases onto stiffer materials such as aluminum panels. 

It seems that the exclusion of resin was a deliberate choice, as Quandt explained in a letter of 1965 why he does not include resin in his lining adhesive for paintings on canvas: “There are no resinous components to become brittle (and therefore weak and ineffectual) upon exposure to the atmosphere.” He used to “combine it with damar and beeswax but . . . since found the addition destroys the properties of the Bareco [wax] as far as its ability to stick to wood.” ^[56] It appears that Quandt mistakenly included resin in his treatment report.

After mechanical removal of the masonite hardboard, balsa blocks, and wax adhesive, a shocking pink textile with an open weave, directly adhered to the reverse of the panel, was uncovered as the last layer of the backing system (fig. 14). Though fiberglass was published as one of the materials used in balsa-block backings, the treatment report documents a jute fabric, which was corroborated by microscopic analysis. ^[57] We know that Quandt recommended fiberglass with some caveats for lining canvas paintings. In 1960, he wrote, “The material seems to be wonderful for lining purposes; above all, it does not move!” ^[58] As the nonreactive nature of fiberglass is ideal for use in the treatment of paintings on wood, it is not clear why Quandt chose not to use it for building up the auxiliary support. ^[59] The textile has a coarser weave and seems thicker and less flexible than what was normally used, but perhaps this was simply what was available in that moment. Regardless, the inclusion of a fabric interleaf follows the normal balsa-block backing procedure, serving not only as a moisture barrier but also acting as a separation layer that would release if internal stress became too strong. ^[60]

Buck would not have been disturbed by Quandt’s departure from the usual balsa-block backing methods. He acknowledged that details of the system can be varied without compromising the general principles of providing a moisture barrier layer with some mechanical restraint through an overall dimensionally stable support. ^[61] As evidence of this, Buck’s 1963 publication indicates the materials and procedures of the balsa backing without giving precise recommendations for measurements or quantities of materials, serving as guidelines rather than strict instructions. 
Finally, the textile interleaf was removed, revealing only the panel support covered by a thin, patchy layer of the wax adhesive. It appears that the wax adhesive mixture was applied hot through the textile while in contact with the reverse of the painting. ^[62] Since analysis provided evidence for the presence of turpentine, ^[63] it seems likely that the wax adhesive was in a paste form or of a less viscous consistency, which would have aided in infusing it in the textile. The patchy pattern of the wax adhesive on the panel—it does not completely cover it—and the side of the fabric interleaf in contact with the panel is in contrast to the complete saturation of the fabric’s other side, which is a sign that the adhesive did not completely penetrate, probably due to cooling (fig. 15). The pattern also strongly suggests that the wax was applied in several passes.

An interesting feature of the oak support that had not been documented previously but can now be seen is light-colored bands, vertical with the grain, in the center board. Found on the edges and in the middle with indistinct borders, these appear to be Mondrings, “moon rings.” ^[64] Also known as included sapwood, moon rings consist of sapwood that did not convert to heartwood due to extreme cold. ^[65] And like sapwood, moon rings are structurally weaker than heartwood, are more reactive to the environment, and are more susceptible to insect attack. For these reasons, it was against seventeenth-century guild regulations to include sapwood in panel construction. Nevertheless, this defect is often found on one side of seventeenth-century panels due to increased demand and scarcity of quality supplies. ^[66] The presence of moon rings along the joins in Man with a Sheet of Music created an innately structurally weak support, which may well explain the continuing problems with the joins before and after the panel was thinned.

The mid-twentieth century, when Quandt’s first structural treatments took place, was a period of great change in the field of art conservation that was increasingly informed by new scientific knowledge. The term “restorer,” which had been used for centuries, was transitioning to “conservator,” recognizing that a trade rooted in craft was evolving to include not just aesthetic work but also science to preserve our cultural heritage. Quandt expressly stated in an interview that he preferred the term “conservation” over “restoration.” ^[67] An important example of this shift was the appointment of the organic chemist Robert Feller in 1950 as the Senior Fellow at the Mellon Institute for the National Gallery of Art research project on new, more stable materials to use in conservation. One important outcome was the adoption of Paraloid B-72, a synthetic resin, which is ubiquitous today. ^[68]

Another example of evolution in the field was the formalization of conservation education with the establishment of the first graduate program in conservation in 1961. ^[69] Previously, in the United States the skills of a conservator were acquired in traditional apprenticeships such as those offered at the Fogg Museum. The International Institute for Conservation (IIC) was incorporated in 1950 and through its publication, Studies in Conservation, contributed to wider dissemination of conservation knowledge. ^[70] Quandt was very active professionally beyond the walls of the Corcoran Gallery of Art. He was a peer reviewer for Feller’s seminal work, On Picture Varnishes and Their Solvents, ^[71] a founding member of the American Institute for Conservation (AIC), ^[72] and a fellow of IIC. ^[73] Buck, who was a founding member of IIC, presented a seminal work on the conservation of panel paintings at IIC’s inaugural international conference held in Rome in 1961; the paper was published in 1963. The 1960 treatment that followed the attempted theft demonstrated Quandt’s familiarity with the reconstruction treatments carried out at the Fogg Museum, staying abreast of the developments in the structural conservation of wood supports through publications and through colleagues such as his mentor, Sheldon Keck. ^[74]

When Quandt carried out the balsa backing treatment in 1969, he relied on the best practice at the time. It was known that auxiliary supports that were too strong would cause thinned panels to continue to crack, hence the evolution of the reconstruction supports. ^[75] Even with the improved reconstruction system design of the balsa backing, Buck recognized there was still a risk of further mechanical damage to the wood support. ^[76] Though he thought this risk was low, his prediction that they could fail despite the dimensional stability provided by a rigid support became an unfortunate reality for Rembrandt’s Man with a Sheet of Music. Another reason for the balsa backing design was the prevailing philosophy that a painting conservator should be able to treat all types of paintings. ^[77]

Balsa-block backings fell out of favor in the 1980s and were replaced by increasingly flexible auxiliary support designs that allow freer movement of the wood support while still maintaining some mechanical restraint. ^[78] These new designs incorporate springs and flexible battens (figs. 16, 17). ^[79] In 1997, Al Brewer and Colin Forno suggested that for some thinner panels, unattached, uniform minimal restraint such as a cushioned tray or other flexible design could be considered. ^[80]

So far, RTI and photogrammetry models have demonstrated that the removal of the balsa-block backing has been beneficial for the painting. The panel was imaged four times: before treatment; when the masonite hardboard and half the thickness of the balsa wood were sawn off; after the wood was removed, leaving the textile interleaf; and when the textile was removed. A comparison of these four moments during the treatment shows the panel gradually flattening out as the stress imposed by each layer is released. It is useful to have a record of how much of an influence these disparate materials had on the panel to help us understand which materials and methods would or would not be appropriate to use. At each stage the concave warp of the panel becomes slightly convex as it relaxes and finds a new equilibrium with its environment. As the treatment continues, the painting will be imaged three more times: after the removal of the wax adhesive, after repairing the open joins, and after treatment with the new auxiliary support. This last measurement will serve as the baseline model to which the panel can be compared to aid in the detection of any movement of the panel in the future. With the new auxiliary support, which will be discussed in a future article, either attached or unattached, the panel will be held in its natural and neutral position, so it will not necessarily be flat after the treatment. This imperfect aesthetic is now accepted because we have a much better understanding of how wood continually reacts to its surrounding environment and therefore should not be subjected to absolute restraint. Most importantly, the overall structure should remain stable and intact.

Wax-Resin Recipes from the Twentieth Century

Wax-resin adhesive ^[81] 
Bleached beeswax, 7 parts; Singapore dammar resin, 2 parts; gum elemi, 1 part

Wax adhesive ^[82] 
Plenderleith relining adhesive 600 g; chalk, 150 g; gypsum, 50 g; zinc oxide, 25 g; titanium oxide, 75 g

Wax mortar ^[83] 
75% Plenderleith relining adhesive, formula #13; 10% pulverized cork; 3% jute fiber; 2% rotten stone; 0.5% burnt umber pigment; 9.5% kaolin

Plenderleith wax resin formula No. 13 ^[84] 
4:3:2:1 ratio of unbleached beeswax, paraffin, dammar, and gum elemi

Wax-resin mixture ^[85] 
3 parts synthetic microcrystalline wax Bareco Victory White to 1 part Piccolyte resin S 80 (Piccolyte is a natural terpene resin. This product is now known as Dercolyte A 80.)

Wax-resin adhesive, wax paste, and wax filler ^[86] 
Unbleached beeswax, 2000 g; dammar resin, 1000 g; paraffin, melting point 56–58℃, 1500 g; gum elemi, 500 g

Hot wax mortar ^[87] 
Relining wax, similar to formula 13 published by Plenderleith; chalk and wood flour

Wax cement ^[88] 
Wax, dammar resin, and sawdust

Wax-resin ^[89] 
50–50 mixture of beeswax and Lascaux 443-95 adhesive wax