ART CONSERVATOR
DIGITAL
A PUBLICATION OF THE WILLIAMSTOWN + ATLANTA ART CONSERVATION CENTER
VOLUME 16. NO.1
SEEING THROUGH A MATRIX | UTILIZING X-RAYS AND RTI TO EXAMINE THORNTON DIAL’S METHODS OF CONSTRUCTION
KRISTEN GILLETTE | ASSOCIATE OBJECTS CONSERVATOR
Thornton Dial’s use of found materials and often bold choice of color allows for expressive storytelling that produces a unique aesthetic within the spectrum of self-taught and contemporary artists. Dial’s three-dimensional assemblages and sculptures display a network of layered materials camouflaged by modeling epoxy putty and paint (fig.1). Tin, steel, copper, and aluminum, old wood, carpet, rope, old clothes, sand, rocks, wire, screen, toys, tree limbs, and roots—his methods of working with these materials are drawn from his life experience working in various industries such as metalworking, painting, pipe fitting, bricklaying and carpentry [1]. Many of the objects he worked with were salvaged materials that were already aged or deteriorating when Dial used them in his art, which has resulted in condition issues that require conservation. On his working processes, Dial states that:
“I like to work on that surface, rub it, scratch it, smear it. I beat on it sometimes, knock holes in it. I have even set fire to it.” —thornton dial [2]
FIGURE 1. Detail of Driving to the End of the World (Silver) where Dial has manipulated multiple pieces of metal car parts to express his discontent with the oil industry.
The result is an impressive matrix of his artistic ideas and social commentary; however, from a conservation perspective, the complex construction and the inherent qualities of degradation in his choice of materials can complicate examination and monitoring.
After a significant acquisition of thirteen artworks by Thornton Dial in 2017 from the Souls Grown Deep Foundation, the High Museum of Art has made it their objective to be stewards of his work to ensure the preservation and appreciation of Dial’s art for future generations. Both the amalgamation of aged and found materials and various types of construction present a difficult task for collection management: each facet of his work may respond differently to environmental changes, vibrations, and associated degradation components may even interact with each other. The High’s mission to preserve his artworks resulted in the extensive survey funded by the Bank of America and carried out by the Atlanta and Williamstown Art Conservation Centers.
The initial survey and inspection allowed us to delineate general materials usage and pinpoint areas of concern; however, it was constrained to what was visually accessible through observation, only a part of the art piece's story of construction and condition. Understanding an artist’s materials and techniques is essential to developing the appropriate conservation methodology—since Dial’s construction complicates visual identification of condition concerns, conservators at the Centers needed to integrate imaging techniques to elucidate his complex construction. Underlying condition issues would require additional forms of visual investigation; in this case, both digital X-Radiography and Reflectance Transformation Imaging (RTI) were implemented as analytical tools.
IMAGING METHODOLOGY
There are several ‘knowns’ in Dial’s artistic methods which provide a foundation for our understanding of the assemblages’ construction. For example, we know that he made significant use of canvases stretched over plywood and attached materials to these surfaces mechanically with nails and staples (visible on both recto and verso on several pieces). However, we were only able to hypothesize particular material relationships, i.e., how is Dial attaching certain materials to each other? When is he choosing an epoxy putty over nails? Is he explicitly using any items structurally rather than aesthetically? Many of these relationships are obscured by layering and the application or patterning of paint.
X-Radiography The x-radiography component was utilized primarily to evaluate our material and construction hypotheses as well as discover relationships that were optically unobservable. X-rays can reveal differences in construction due to variances in elemental density: for instance, lead is a heavy, dense element that displays as a white material, while organic materials such as cloth or wood would be more transparent in an x-ray. This would allow our team to see beyond some of the organic found objects Dial used to visualize points of adhesion or construction that were not readily visible. Applied Technical Services, Inc. conducted the x-radiography in the High Museum’s facilities with a Fuji Series V HR digital x-ray scanning system and Dynam IX HR, a processing software typically used in the medical field. This digital x-ray system did not require the film to be developed in a dark room; instead, an instantaneous digital x-ray is available minutes after it is captured. The size of the x-ray plates was limited to 14” x 17”, requiring multiple x-rays to be taken and stitched together to create a unified x-ray for one of Dial’s assemblages.
FIGURE 2.
Hanging apparatus constructed for the x-ray and RTI portion of the project using a textile storage rack. The rails were replaced with a modular wooden structure that would be less visible in the x-rays.
To facilitate the imaging process, a hanging system for the artworks was built to fit onto an empty rolling textile storage rail (fig.2 ). It was able to accommodate cleats or d-rings and be adjusted for height as necessary. It was constructed of wood to minimize its visibility on the x-ray images (wood is a less dense object when compared to metal). Numbered tapes were placed on the reverse of the artworks to establish a grid (fig. 3); this would ensure that there were no gaps in the final image once all of the x-rays were stitched together. The image plates were attached to the reverse, exposed to the x-radiation, processed, and saved to the computer; the plates were relocated along the taped grid until the entire artwork was x-rayed. The x-radiographs were shot at varying durations (35 to 90 seconds) and exposure power (35 to 90 kV) according to the mix of material densities present in the assemblage. Utilizing a digital system for x-radiography allowed for real-time adjustment of exposure settings and processing to find the best image for each artwork. The numbered tape is visible in the x-rays and allowed the separate images to be stitched into a composite image x-ray of the artwork (fig.3).
FIGURE 3.
Left: Tapes with x-ray visible numbers were placed over the back of the hanging structure to allow for stitching multiple exposure images together in post-processing. Right: Detail from a final x-ray image with the numbers visible at the joins.
Reflectance Transformation Imaging Reflectance Transformation Imaging (RTI) is a method for capturing the surface information of an object or artwork by allowing it to be digitally re-lit from varying directions. It is similar to a raking light image in that the topographical image of surface roughness can be produced --- whether it be the texture of an incongruous surface, lifting paint, or flocked fibers. There are several outcomes of RTI which are valuable to the conservation examination process. The completed images are compilations of hundreds of images and, therefore, are a far higher resolution than a standard photograph. The RTI image building software (RTIBuilder) uses this high-resolution information to extrapolate surface data, essentially generating a topographical map of the artwork. The viewing software alters the visual interpretation of the RTI image by subtracting color (particularly useful with the bold applications of pattern and color on the Dial assemblages) or increasing specularity (fig.4). Moreover, it allows for the simple dissemination of information for visual investigation and produces a high-quality record for observing condition change over time.
FIGURE 4.
Examples of visualizations possible with the completed RTI files using details from Heckle & Jeckle: Pleasure for the People. Left to Right, from Top to Bottom: 1) Detail with full saturation 2) Reduced color saturation 3) Reduced saturation with increased specularity 4) Normals (i.e. extracted topographical data).
The capture sequences for Dial’s assemblages were taken with the Highlight Method which requires the use of reflective spheres (in this case, billiard balls), which are used by the RTI image building software to compute the direction of the light for each image (figs. 5-6). This method is useful for capturing larger artworks due to its flexibility when used in environments limited by space.
FIGURE 5.
RTI setup with two spheres and a black curtain to diminish incident light.
The creation of an RTI image begins with the capture process. The capture process involves taking a series of photographs with a light source at varying positions. The ideal dispersion of light positions will form a figurative half-sphere or dome around the object. The artwork is the lateral plane of the dome or equator (0°) if it were a whole sphere. The camera is placed in a fixed position at the pole of the dome directly facing the center of the artwork (90° perpendicular). A light source is positioned to face the center of the artwork as a photo is taken. The light is then moved, and the process is repeated. The light’s position is always equidistant from the artwork, which can be achieved by using a premeasured string or, in our case, marking the positions on the floor. There is a range of angles from the 0° plane of the artwork, which are ideal for this process (15°-65°) [3] . Images taken with a light position lower than 15° can be underlit and the reflecting spheres may be unable to capture a highlight. Images taken at higher angles above 65° can overexpose the final RTI, and do not provide as much relevant surface information as a standard raking light photo.
FIGURE 6.
This is a sphere blend of all of the light positions which is produced by the RTI file creation process. The increased number of positions towards the edges indicate the additional raking light captures taken to compensate for the limitations of space.
The light source can be a flash or a continuous light but must always be oriented to the center of the artwork or object. A continuous light is advantageous as its orientation can be more accurately observed, and the camera can automatically adjust its exposure by prioritizing the aperture. It also illuminates the otherwise dark space while moving from position to position safely, especially important when working on ladders. As mentioned, the Highlight capture method collects light position data via red or black reflecting spheres placed next to the artwork and uses the reflection of the light source (an illuminated dot on the black billiard ball) to calculate the angle (fig.6). The images are compiled and processed through RTI image building software, a free program provided by Cultural Heritage Imaging [4], a non-profit resource for RTI and other imaging techniques. The RTI software compiles the images by calibrating the light source captured by the reflecting sphere and utilizing an algorithm that predicts lighting at an angle that was not captured by the camera (i.e. 90.47°). The early foundational algorithm for RTI was Polynomial Texture Mapping (PTMs). This project used the Hemispherical Harmonics (HSH) algorithm fitter, an updated algorithm that is purported to map three-dimensional objects better. The compiled RTI image is viewed in the RTIViewer application where the user can “virtually” move light around the object to capture different topographical information for analysis:
The compiled RTI image for Struggling Tiger is viewed in the RTIViewer application where the user can “virtually” move light around the object. See the user move the light source around on the digital black sphere. To the left of the sphere is an image capture button, allowing the user to take a screenshot of the painting at the desired light angle.
The algorithm’s precision for predicting light cast on the object, is weighted on the number of images taken; smaller objects may need a minimum of twenty photographs taken at various angles whilst larger ones like Dial’s assemblages make take more than one-hundred photographs at various angles to create the composite RTI image.
The RTI capture process used the same hanging apparatus constructed for the x-ray portion of the project. The key to capturing usable images for RTI is to minimize incident light as much as possible. This required the construction of a black curtain hung around the artwork during the capture sequence (fig. 5). Because the artworks were hung vertically (as opposed to shooting the object from overhead), the ideal ‘sphere of light positions’ suggested for RTI was constricted by space and truncated by the floor and overhead height limits. This could be counteracted by increasing the number of images and light positions closest to the artwork, i.e., those creating raking light. As opposed to direct light, these angular positions will prevent overexposure and will create a better final image with more variance when digitally redirecting the light source in the RTIViewer software. Once the artwork was hung and the curtain adjusted to obstruct any reflecting surfaces on the hanging rack, the spheres were attached to threaded rods secured by tripods placed on either side of the object. The floor was marked with tape in 12 points that were 165” equidistant from the center of the artwork forming a half-circle demarcating the directional light angles between 15° and 65°. A remote shutter allowed the entirety of the capture process to be conducted by this author without other assistance. Using the remote shutter trigger and a large continuous LED light, the process began at a marked point closest to the artwork and a series of images were taken with the light directed at the center of the artwork from different heights. In this case, images were taken at 4 to 6 height positions from the ground up and 6 or more positions from the top down (a minimum of 12 images at each marked angle point on the floor. Image capture was facilitated with the use of a ladder and an extension pole for the LED light. This was repeated at each point marked on the floor establishing a minimum of 120 light positions and images for processing per each artwork (fig. 7). Additional light positions were added according to the size of the assemblages. For example, Surviving the Frost, as one of the tallest pieces, had 210 light positions. Once the capture sequences were complete, the photos were examined for quality, batch edited, converted to JPEG files, and processed with the RTIBuilder software. The RTIBuilder only extrapolates light position data from one of the two reflecting spheres, which can be chosen based on accuracy and quantity of points during the building process.
FIGURE 7.
A table listing the capture sequence data. The number of light positions represents the number of images taken for each painting that will be compiled utilizing the RTI Builder Software.
IMAGING RESULTS
Ten assemblages were chosen for x-radiography and RTI. Visual analysis with both x-radiographs and RTI images enabled us to determine Thornton Dial’s methods of construction and make hypotheses concerning condition issues visible during preliminary examinations of the artworks. For example, we may determine if physical damage or changes due to the use of specific materials (i.e. Splash Zone epoxy, corroded metals, plastics) were isolated or related to how they were attached to the assemblage. Similarly, we can hope to better understand the future stability of these additions based on their method or quality of attachment.
Backing Board Supports As discussed in the introduction to Thornton Dial’s assemblages, his backing board supports played an imperative role in both Dial’s approach to construction and the stability of his artwork. X-rays in particular were an important piece of visual analysis used to identify what type of support was used in each assemblage, and therefore, the risks and concerns for its preservation. His art dealer, Bill Arnett, streamlined the construction of substrates for Dial’s assemblages by providing him with prefabricated backing boards. The prefabricated backing boards typically consisted of a piece of canvas stretched and attached to a large sheet of plywood with staples. Dial would then attach his found objects, utilizing various methods of adhesion, to the backing board. Backing boards for some of the larger artworks had additional wood batten gridding on the verso (fig. 8) to prevent flexing in the support. As opposed to a typical stretched canvas used for painting, these structures were able to bear more weight and provide a secure means for physical attachment. While many of the artworks examined in this survey utilized these substrates, there are several examples where Thornton Dial used found materials or raw, unprepared surfaces for his supports. A carpeted cubicle partition wall was used for the backing board in Driving to the End of the World: Gold, while an unprepared piece of plywood was used in Driving to the End of the World: Silver. In general, there is less visible damage to assemblages created with the prefabricated backing boards due to their consistency in strength and stability.
FIGURE 8.
The verso of Surviving the Frost provides a clear example of the substrate construction common with many of Dial’s assemblages. Clearly visible is the plywood foundation and structural battens. The canvas is pulled over the recto surface and stapled to the outer edge battens. Also note the number of nail holes and staple ends apparent from attaching elements to the front.
Methods of Attachment The materials present in Thornton Dial’s assemblages were often associated with specific methods of attachment that Dial borrowed from his experience in various industrial jobs. X-radiography showed that in addition to Splash Zone A788, Thornton Dial relied heavily on staples and various types of nails or screws. Dial would use mechanical fastening techniques relative to the weight of the material being applied to the substrate. For example, an x-ray of Old Projects (fig.9) showed that the braided rope element was secured with staples, whereas wood and metal components were attached with nails; in Surviving the Frost (fig.10), plastic sheeting and long grasses are attached with staples. X-rays also helped to differentiate between the types of nails used to construct the backing boards and those used for securing objects or elements in place. Wire was also used to connect items together. Looking Out the Windows (fig.11) is an excellent example where wire is used both to connect substrate layered elements such as mesh and metal gridding as well as individual toys to the piece. The majority of the toys are less dense and less visible in the x-ray, but their wire attachment points are visible. Similarly, the fish in Heckle & Jeckle: Pleasure for the People is connected to the ropes underlaying with twisted wire.
FIGURE 9.
X-ray of Old Projects. The rope is semi-transparent and is held with staples while nails have been used to attached the more dense/opaque pieces of metal.
FIGURE 10.
Staples have been used to secure the pieces of plastic (semi-transparent) in Surviving the Frost.
FIGURE 11.
The toys and metal mesh in Looking out the Windows have been attached using metal wire.
Splash Zone A788 epoxy compound was used in two capacities: physical attachment and sculpting. Often Dial used Splash Zone to blend seams between the edges of two different materials, sculpt three-dimensional features, add surface texture, or adhere objects to each other. Its use as an adhesive, rather than a sculpting putty, is associated with points of detachment between elements in the assemblages. It was important to utilize both X-rays and RTI images to identify these fail points where they may not be readily visible. These concealed areas could be indicative of future failures or weaknesses which should be monitored. They can also allow us to make informed decisions on the ability of these pieces to travel for loan or how they could safely be transported. Although there are examples of Splash Zone A788 being used to attach elements directly to the substrate, the epoxy putty was generally used to attach elements directly to each other. This is visible on the Driving to the End of the World series, Looking Out the Windows, and, unexpectedly, on Birmingham News where it joins newspaper and fabric to wood supports on the front of the artwork which forms the curtain-like rows. We were aware of the presence of Splash Zone used with Birmingham News only when the fabric layers were gently lifted. With the x-ray, the extent of its use is very apparent as there are many ‘globs’ pressed onto the rows, boards, and edges of the textiles. There is more tension created between textile and Splash Zone than attachments with nails or staples, which allow for some flexibility of movement. It is important to be aware of this tension as the synthetic fabrics continue to deteriorate and become brittle over time. It is the physical joins between heavy materials where we see the limitations of Splash Zone for bearing weight or movement. Driving to the End of the World: Gold has numerous points where the Splash Zone has failed or broken due to embrittlement or unprepared substrates (i.e., corroded metal, textiles) (fig. 12). These have occurred primarily along wire junctures holding other heavier metal elements in specific orientations. As the material becomes embrittled over time, there are risks to breaking and failure of the adhesive when heavy objects vibrate during handling and transportation. The reasoning for adhesive failure is complex due to the ways Thornton mixed and applied the epoxy compound; further research into the means by which Splash Zone degrades and hypotheses concerning its interactions with the variety of materials has been described in a supporting article on Splash Zone A788 Compound.
FIGURE 12.
These are 3 examples of breakages or detachments of elements on Driving to the End of the World: Gold where Splash Zone putty is the primary bonding agent.
Paint Surface + Inclusions RTI images were useful to identify delamination, wrinkles, and creases in the paint that Dial applied to both his objects and Splash Zone putty. Several paint samples from his assemblages were analyzed with FTIR and identified as alkyd-based paint, commonly used in both house or spray paint mixtures. Areas of delamination identified with RTI, will need to be consolidated as part of the approach to conserving Thornton Dial’s assemblages. Lifting paint and associated loss were most commonly observed when the substrate was unprepared or unsuitable for the type of paint used. The plastic sheeting used in Surviving the Frost has significant areas of paint loss, simply due to lack of adhesion. Similarly, losses on metal surfaces can be contributed to corrosion likely present prior to its addition to the assemblages. Wrinkling and creases in the paint are related to the method of application (i.e., quantity of paint and drying rate). Thornton used objects found in the woods and junkyards. It was expected that these objects would not only show previous “wear and tear” but would show traces of the places he found them. RTI images showed clear evidence of the environment in which he found his materials: inclusions in the paint film included insects, grass or other organic material including a large spider embedded in the paint of Struggling Tiger Know His Way Out where it is evident that Dial included it in the artwork as black paint was applied overtop (fig.13). While surface dirt and grime are not desired since it facilitates degradation, it was essential to Thornton Dial’s intended aesthetic:
FIGURE 13.
A large spider is embedded in the surface of Struggling Tiger Know His Way Out.
“ I only want materials that have been used by people, the works of the United States, that have did people some good but once they got the service out of them they throwed them away. So I pick it up and make something new out of it. That’s why we pick up these things. Negroes done learned how to pick up old things and make them brand-new. They had to learn them things to survive, and they done got wiser for doing, wiser by looking at the things and taking them into the mind. You call that 'smart.' ” —thornton dial [5]
Visual identification of dirt, grime, and other organic inclusions is crucial for a conservator’s ability to assess the risks and concerns for cleaning his artworks. The soiling of fabrics or corrosion of metal pieces becomes as important as a brushstroke. This does not imply that the work is necessarily ephemeral and will be left to decay over a long period of time; however, it impresses that the decision-making to clean an artwork for continued preservation may not be straightforward. In some cases, material specialists may be consulted or asked for assistance (i.e., a textile and/or paper conservator will be required for deeper assessment of Birmingham News). A general approach to cleaning the assemblages (dry cleaning with brush, low suction vacuum, and soft sponges) followed by a detailed aqueous cleaning will continue to inform our understanding of the materials and their condition. Cleaning itself is another method for examination.
DISCUSSION
X-radiography is not a perfect solution for analyzing every assemblage. The mix of materials with higher densities, such as metal, and low density, such as cloth or Splash Zone, creates a contrast within the x-ray image where some elements are less defined than others despite best efforts to find a suitable middle ground of exposure. Similarly, RTI is not ideal for artworks where the application of three-dimensional items casts numerous shadows over the surface of the object. A good example of both of these concerns is Looking Out the Windows. The mix of object layering and material densities made it difficult to fully demarcate each item or group of items through x-radiography. RTI was able, however, to give a general sense of physical construction through wires, nails, and some hazy outlines of Splash Zone; this is informative about Dial’s construction even if many relationships could not be exactly defined. RTI imaging was not done for Looking Out the Windows due to its dimensionality, which would have cast excessive shadows making it difficult to accurately capture the surface. Two further artworks were unsuitable for RTI: Driving to the End of the World: Silver & Driving to the End of the World: Gold. An additional Dial artwork, Proud Cats Made to Climb, was substituted.
On several of the pieces, specific areas were identified for detailed examination, in some cases with additional x-ray images centered on the locations in question. These may be areas where the construction materials and methods are unclear, such as determining the underlying structure, or isolating particular visual elements such as faces or heads which may be separated between two separate exposures during the overall x-ray capture process. A ‘deer head’ on the top right corner of Heading for the Higher Paying Jobs was one example (fig.14). Small unpainted areas revealed the presence of Splash Zone as a component of its construction, but the composition of the form around which the epoxy putty was sculpted was unclear. The x-ray displayed areas of higher and lower density within the head without an identifiable form. By comparing the head with the exposure level of known nearby items, certain materials could be eliminated as possibilities. For example, metal strips are visible through the head on the x-ray and are bright and denser than the material that forms the head (fig.14). Similarly, a coiled wire in the nose provides a direct contrast to the other materials inside the head. As such, a general hypothesis can be made that the interior of the head is composed of a lighter material (cloth, mesh, paper), which was compressed or bundled to become the armature to which the Splash Zone was applied.
FIGURE 14.
The ‘deer head’ on Heading for the Higher Paying Jobs as visible through photograph and x-ray.
Heading for the Higher Paying Jobs is also an excellent example of the use of RTI to visualize topographical information. The application of color by Dial is not inhibited by the use of materials and the dimensionality of the assembled materials are difficult to discern through the bold hues. By desaturating the color from the RTI image with the RTIViewer software, details such as layering of materials and attachment points are more easily observable (fig.15).
FIGURE 15.
Once desaturated, the dimensionality of the piece is far more discernable when moving the light virtual in RTIViewer. Heading for the Higher Paying Jobs is an example of the desaturation process.
CONCLUSION
This article demonstrates that the dense matrix of materials in Thornton Dial’s assemblages can be as complicated as the sociopolitical issues Dial sought to convey. In this phase of the High Museum of Art’s initiative, the Atlanta-Williamstown Art Conservation Center team needed to assess the artworks' condition to formulate a holistic approach to the conservation and treatment of the assemblages. Points of adhesion and adhesive failure, were often obfuscated by the layers of materials. By pursuing additional methods for visual examination via Reflectance Transformation Imaging and X-Radiography, we were able to see into Dial’s matrix to understand his construction and pinpoint areas of instability. Both methods are time-consuming and costly; however, the size and complexity of the objects warranted both methods of investigation. X-rays exposed and delineated methods of construction, while RTI modified the visual perspective begetting new observations. It is a technological advantage to have a quality digital record that can be shared for research and archived for continued observation of condition over time. The data from this visual analysis will be used to monitor condition, tailor treatment methodology, and set up criteria for long-term exhibition, display, and transportation.
Acknowledgement
Funding for the conservation of this artwork was generously provided through a grant from the Bank of America Art Conservation Project.
Photography Credit | Kristen Gillette, Matthew Hamilton, Maggie Barkovic, and the High Museum of Art.
REFERENCES
[1] Souls Grown Deep Foundation. “Thornton Dial: Taken from interviews with Thornton Dial by William Arnett in 1995 and 1996.”, soulsgrowndeep.org, www.soulsgrowndeep.org/artists/thornton-dial. Accessed 14 Sept 2021.
[2] Ibid.
[3] “RTI: Guide to Highlight Image Capture.” Cultural Heritage Imaging http://culturalheritageimaging.org/What_We_Offer/Downloads/RTI_Hlt_Capture_Guide_v2_0.pdf
[4] Ibid.
[5] Souls Grown Deep Foundation. “Thornton Dial: Taken from interviews with Thornton Dial by William Arnett in 1995 and 1996.”, soulsgrowndeep.org, . Accessed 14 Sept 2021.

[6] Gillette-Woodard, Hélène, Gillette, Kristen, Gunhee Kim, Sally and Puza, Christine; Thornton dial: an examination of 17 assemblages and 1 sculpture at the High Museum of Art. Bank of America Conservation Grant, April 2021.