45th Annual Meeting – Paintings Specialty Session, June 1, “The Conservation of Alexander Calder’s Last Work Mexico #3: The cross-disciplinary treatment supported by SEM and TEM paint cross section analysis using focus ion beam (FIB) sample preparation,” by Sara Wohler and Ralph Weigandt

Author Sara Wohler discussed the fascinating history of Alexander Calder’s airplane model, Mexico #3, the last work he completed before his death, and then presented the conservation treatment of the model. Author Ralph Weigandt then discussed the technical analysis of the paint film on the airplane.  This presentation served as a fun continuation of the painted airplane theme, following Lauren Horelick’s May 30th talk “When an Airplane Acts like a Painting: Applying Established Conservation Methodologies to Ephemeral Aircraft Materials.”


Wohler described the beginning of Alexander Calder’s airplane-making career: In 1972, New York advertiser George Gordon approached Calder with the idea of painting an full-scale airplane.  Calder loved the idea, as it would combined his experience in kinetic art and his background in engineering.  Gordon paired Calder with Braniff International Airways, and Calder created the designs for two airplanes: Flying Colors of South America and Flying Colors of the United States. These were both tremendous public successes.

^Braniff International Airways employee ceremony, 1975, with Flying Colors of the United States.

The author then described the process in which Calder painted the planes: He began by experimenting with designs on several 1/25-scale Westway Aircraft Models.  The chosen design from the model was then scaled up using graph paper that was attached to the full-size airplane.  Calder and his team then used pounce wheels to poke holes through the design on the graph paper, and black spray paint was applied through the pounce holes.  The graph paper was removed, and the paint colors were spray applied by a Braniff team.  Calder supervised the entire process, and hand-painted the engine necelles during the spray process.

Then the author described the artistic process for the model Mexico #3. In 1976, Braniff commissioned a third plane from Calder, this one to celebrate the great relationship between the U.S. and Mexico.  The author provided amazing historic film footage of Calder painting the Mexico #3 model plane.  She noted that the plane itself was made of fiberglass, and Calder created his design using gouache. On November 11, 1976, Calder completed and signed the work, and tragically, passed away later that evening.  Although the design was completed, Mexico #3 was not transferred to an airplane, as Calder was no longer alive to approve of the final result.

^Calder painting the Mexico #3 model.


The model airplane was brought to Kuneij Berry Associates, Chicago, for conservation treatment. Through examination, the author found that the fiberglass model airplane had two priming layers, blue and grey, and a final, even, white coating. Calder painted onto the proprietary white surface using gouache, possibly that he made himself. While the airplane was quite dirty and structurally had sustained a few losses, the treatment was relatively straightforward.

The plane was in poor aesthetic condition; it had previously been displayed in a planter with dirt and plants around it, exposing it to both dirt and moisture. Fortunately, the gouache paint layer was generally in good condition and intact, aside from a few abrasions.  The synthetic varnish layer, which had protected the gouache layer, was covered in surface dirt and grime.  The plane was first surface cleaned with deionized water and PVOH sponges, but a lot of the dirt remained embedded in the varnish.  The synthetic varnish was removed with aromatic solvents.  Care was taken to only thin the varnish on top of the gouache paints, as the paints were sensitive to aromatic solvents.

^Detail of the varnish removal, cleaned (left) and with remaining varnish (right).

Structurally, the plane had suffered a few chips to its wings and there were a few areas of flaking paint. The flaking paint was consolidated with Paraloid B72. To recreate the tips of the wings that had been chipped away, molds were made of Elastosil M4600 A/B and cast using Milliputti. The cast pieces were sanded and adhered to the wings using Paraloid B72.

Shallow losses in the white priming layer were filled and inpainted simultaneously with Golden MSA colors. Losses in the gouache colors were then inpainted with QoR watercolors.  The model was then sprayed with a few, light, protective layers of RegalRez 1094. After the successful treatment, it was recommended that the painting be displayed in a new, more environmentally stable location.

^Sara Wohler inpainting Mexico #3.

Technical Analysis

The technical analysis of Calder’s gouache paint was carried out by Ralph Weigandt, who is currently the primary researcher on the collaborative National Science Foundation (NSF-SCIART) grant with the University of Rochester’s Integrated Nanotechnology Center to advance the scientific understanding and preservation of daguerreotypes. The authors carried out technical analysis of the gouache paint in order to better understand Calder’s materials and techniques, potentially inform the conservation treatment, and to pioneer the use of Focus Ion Beam (FIB) milling for SEM-EDX analysis and PLM examination on paint films.  Through Transmission Electron Microscopy, SEM-FIB allows for the elemental analysis of paint layers at the nanometer scale!

Weigandt explained in depth about the sample preparation process, the Focus Ion Beam milling of the larger sample into the much smaller (~12 um x 0.5 um) cross-section, the comparison between traditional SEM-EDX spectroscopic elemental analysis and mapping vs. the Transmission Electron Microscopy and associated SEM-EDX elemental analysis and mapping capabilities.  In essence, the FIB milling and TEM allows for highly precise, high resolution elemental analysis and mapping, allowing scientists and conservators to see the inorganic composition of individual pigment particles.  A poster from University of Rochester graduate student So Youn Kim outlines the project with excellent photographs and illustrations.

In the end, the elemental analysis did not contribute greatly to the decision-making process of the treatment, but did provide excellent information about Calder’s painting techniques and materials for Mexico #3, which can inform a discussion about his art-making process for this piece and his art in general.  It is clear that this Focus Ion Beam technique coupled with Transmission Electron Microscopy and SEM-EDX elemental analysis is an exciting analytical technique that will be extremely useful in the precise identification of inorganic pigments, fillers, etc., in paint films. Furthermore, it is great to see yet another example of private conservators working with scientific departments at universities (or elsewhere) to investigate materials of cultural heritage objects!

45th Annual Meeting – Paintings + Research & Technical Studies, May 30, “Pioneering Solutions for Treating Water Stains on Acrylic Paintings: Case Study of Composition, 1963, by Justin Knowles” by Maggie Barkovic and Olympia Diamond

Maggie Barkovic and Olympia Diamond presented a case study that outlined the decision-making process that lead to the successful treatment of darkened, dirt-infused water stains on the bare canvas portion of a large-scale acrylic dispersion painting: Composition, 1963, by Justin Knowles.  The authors attributed the treatment’s success to the combination of extensive evaluation of Knowles’ materials and aesthetic aims and the understanding of new, innovative cleaning techniques designed for acrylic dispersion paintings (with the help of Brownyn Ormsby, TATE, and Maureen Cross, Courtauld Institute of Art).  This presentation served an excellent compliment to Jay Kruger’s presentation Color Field Paintings and Sun-Bleaching: An approach for removing stains in unprimed canvas, which discussed the treatment of acrylic solution and oil paintings on bare canvas.

Composition is a privately-owned work that was brought to the Conservation and Technology Department at the Courtauld Institute of Art for treatment in 2013.  The large-format work is a two-dimensional acrylic painting with brightly colored geometric forms juxtaposed against an unpigmented acrylic sized canvas.  The painting had sustained disfiguring water stains along the top and bottom edges which disrupted the aesthetic reading of the image, rendering it unexhibitable.


In the first step of the conservation process, Barkovic and Diamond assessed how the water stain affected the aesthetic interpretation of the painting.  They explored where this painting fit into the artist’s oeuvre: it was part of a series of early, pivotal works where Knowles explored his initial ideas of spatial tension using non-illusionistic geometric compositions that incorporate negative space in the form of unpainted canvas. The authors carried out technical examinations of four other paintings from this early stage in his career, finding that Composition was painted in a comparable manner to his other early works: a fine linen canvas was stretched on a wooden stretcher and then sized with an unpigmented (pEA/MMA) acrylic dispersion coating.  Then, Knowles used pencil and pressure-sensitive tape to demarcate where he would paint the geometric forms with acrylic dispersion paints.  Though he applied a transparent acrylic “size” layer over the linen/negative space, he still considered the visible canvas “raw” and unprimed. Through the examinations and research on Justin Knowles’ personal notes, the authors assessed that the characteristics and color of the linen canvas were equally important to the interpretation of the work as the paint colors.  As such, the canvas should be treated and the water stains removed if at all possible.


Second, the authors explained that they needed to identify the components of the water stain (with no prior knowledge of water-staining incident) in order to test cleaning methods.  Replicas were made using linen and the same unpigmented acrylic polymer that Knowles most likely used. The replicas were then stained with dirty water. Using XRF spectroscopy and empirical testing as a guide, a visually accurate and equally tenacious water stain was made with iron, calcium, and organic “dirt” components from aged linen.  The test replicas were aged in a light box for two years to allow the stain to photo-oxidize and bond with the fabric and size layers.


Third, the authors needed to determine how to treat the water stain with the presence of the unpigmented acrylic dispersion size layer, which swelled in water and was affected using polar solvents. Their goal was to remove the stain or reduce the appearance of the stain to make successful inpainting possible.  The authors looked to successful textile and paper conservation treatments for possible methods.  The initial cleaning and/or retouching tests included the use of solutions with various pH values, conductivities, chelating agents, surfactants, bleach (sodium borohydride), the application of toasted cellulose powder, and pastel retouching.

The authors thoroughly explained the various test groups, but a recapitulation of all of these various solutions is outside of the scope of this blog post.  In general, higher pH values (around 8) and higher conductivity values (above 2.5 uS) allowed for better cleaning efficacy.  Perhaps more notably, the chelating agent DTPA (diethylene triamine pentaacetic acid) greatly outperformed TAC in cleaning efficacy.  This is likely because DPTA is a much stronger chelator that is much more suitable for sequestering iron and calcium (which XRF showed to be present in the stain).  DPTA could be used safely because the acrylic size layer was unpigmented.  Finally, the use of agar (rather than free solution) was found to be useful in the reduction of the stain.  The agar gel allowed for greater control of the solution distribution onto the stain and dirt absorption into the gel.  The most effective cleaning agent, which was eventually used to clean the painting, was made from a higher concentration of agar gel at 5% (w/v), using Boric Acid 0.5% (w/v), DTPA 0.5% (w/v), TEA, at pH 8, 2.4 mS.

Evaluation of Successful Treatment

While a successful treatment methodology was developed through empirical testing, an investigation into the effects on the surface morphology of an unpigmented acrylic dispersion size layer was thought necessary due to the different absorbencies among the test canvases, observed differences in retention times for the agar gel, and concerns about the higher pH required to reduce the stain.  The lack of pigmentation and hard surface features made changes caused by cleaning more difficult to perceive, measure and contextualize, so changes in surface gloss and stain reduction were evaluated with a spectrophotometer and subjective observations by conservators. The impact of the cleaning methodology on the surface of the size layer and canvas fibers were examined with dynamic Atomic Force Microscopy (AFM) and high resolution digital microscopy. A preliminary investigation into possible residues from cleaning was also investigated using FTIR-ATR spectroscopy.

The number of samples for AFM was too small to draw concrete conclusions without more testing and utilizing additional analysis such as FTIR-ATR; however, a general trend was observed that an increase in the gel concentration from 2.5% (w/v) to 5% (w/v) appeared to reduce the time in which fiber flattening occurred.  In addition, FTIR-ATR showed a decrease or complete removal of migrated surfactant from the acrylic size layer surface in all treated samples regardless of the agar concentration in the gel, and along with the swelling of the acrylic layer, was considered by the authors an acceptable risk with this treatment.  IR bands corresponding to agar or the additives  in the cleaning solutions were not detected.

Final Treatment

As mentioned previously, the cleaning agent that was eventually used to clean the painting was made from a higher concentration of agar gel at 5% (w/v), using Boric Acid 0.5% (w/v), DTPA 0.5% (w/v), TEA, at pH 8, 2.4 mS. The agar was hand-cut to perfectly align with the stain patterns on the canvas and weighted with sandbags to increase the gel-canvas contact.  Using this method, stains were greatly reduced.  However, a few, minor discolorations remained after the cleaning.  Further tests were carried out to determine the best inpainting method for these residual discolorations. Dry pigment with Lascaux Jun Funori, Aquazol 50, Aquazol 200,  watercolour and gum arabic and Paraloid B72 were all tested for optical effects, handling properties, and reversibility. The Aquazol 50 series was found to be the most effective overall and was used to inpaint the remaining discolorations.


The authors concluded by restating that the success of the treatment would not have been possible without the combination of art historical and material understanding of Knowles’ work and research into new cleaning methodologies for acrylic dispersion paint films.  They thanked their project advisors Maureen Cross, Courtauld Institute, and Bronwyn Ormsby, Tate, and many others for their generous support and guidance throughout the project.