By Tomas Markevicius, Nina Olsson, Chiara Chillé, Maddalena Magnani, Paula Serra Sánchez, Elizabeth Wicks, Lorenzo Conti, Arianna Acciai

The mid-20th century quote “We shape our tools, and then our tools shape us” resonates in the art conservation field where the innovation of instrumentation and equipment is of fundamental importance for refining our means of understanding and treating artworks. The technology and treatments using flexible silicone mats for low-energy precision heat transfer, discussed here, show how a new mobile accurate temperature (MAT) management instrument has begun transforming approaches in the conservation of paintings, paper, photographs, plastics and modern materials.

The connection between innovation in technology and transformation in treatment approaches is particularly evident in the structural treatment of paintings, where there is a storied and evolving continuum of how to address the persistent problem of poorly controlled heat and its adverse effects. Generations of restorers, re-liners and conservators have sought to improve control over the heat transfer with technological advances from the hot sand Pietro Edwards used around 1800 to the development of heating tables in the mid-20th century and, in the 1990s, with multipurpose low-pressure tables. Excessive heat is one of the original sins of numerous past structural treatments, evident in many objects that we re-treat today. However, we inevitably depend on heat, humidity and pressure to successfully resolve structural treatment challenges in paintings and other works of art; without those factors at our disposal, or if they cannot be applied safely, our treatment options become quite limited. The real issue is not the heat, moisture or pressure, but the poor control over these physical factors due to our limited ability to prevent fluctuating and excessively high temperatures. Inconsistent or immoderate heating exacerbates the effects of moisture and pressure and, when compounded, has been the main source of stress on constituent painting materials in treatments of the past.

Flexible heat transfer mats have already influenced the way conservators employ heat in diverse areas of specialization. While the first experimental use of silicone heating mats was conducted in 2003, research was advanced during the EU-funded IMAT Project (2011-2014), coordinated by the University of Florence which developed a proof-of-concept technology based on carbon nanotubes. IMAT mats were composed of a smart textile with nanocarbon yarns, electrically conductive at an ultra-low voltage (36VDC) and laminated with clear silicone. This mat was connected to a prototype proportional-integral-derivate (PID) controller which self-corrects up to 40 times per second providing an even and steady heating pattern, which was plotted in real-time on the touch-screen display (Figure 1).

While IMAT remains at the prototype stage, N. Olsson, and T. Markevicius designed the alternative MAT system to allow the uptake of precision low-energy heat transfer technology in the field. The system is composed of a MAT-lab console with a PID controller, a temperature sensor and flexible mats in standardized sizes to allow a range of localized to modular treatments. While the IMATs were transparent and thinner, running on 36VDC, the MAT mats are slightly thicker, opaque and run on standard domestic wall power of 120VAC or 240VAC (Figure 2). The MAT PID controller is optimized with “fuzzy logic”, a form of AI that enables self-training of the device for improved accuracy, tailored for the user’s application.

Since their introduction, flexible mats have been used in structural treatments of paintings, photographs, paper and cellulose acetate sheets, pressure sensitive tape removal and enzymatic gels. This technology is opening the door to new targeted “low and slow” temperature-based approaches and inspiring further experimentation in reshaping treatment designs.

MAT was presented to conservators at the Getty’s Conserving Canvas Symposium (2019) and has been used by emerging conservators in five master's theses by M. Magnani at Università degli Studi di Torino (2019) on the conservation of modern materials, by Y. Liu at Technical University of Munich (2018) on bound manuscripts and paper objects, by M. Paganin at the Accademia delle Belle Arti di Bologna (2019) on photographic materials, by A. Acciai on the consolidation treatment of 19th-century paintings on canvas at the Opificio delle Pietre Dure (OPD) in Florence (2023) and by I. Pigueras whose thesis is currently in progress at the University of Lisbon.

Flexible mats may be used to line paintings of any size in the lab or on-site using easily portable equipment. For example, the 2017 treatment of a painting by Orazio and Artemisia Gentileschi from 1611 at the National Gallery of Norway, Oslo, included the replacement of a failed 19th-century glue-paste lining. The new lining was carried out using a pre-stretched lining canvas in a low-pressure envelope and modular heat transfer using a transparent IMAT mat (60x90cm) to activate the Beva 371 film in sections. The larger format Mat5 or Mat6 from the MAT system could be used in the same way.

Of particular note is MAT’s improved accuracy of heat transfer in the low temperature range of 25-45oC, which was previously inaccessible. For the treatment of certain categories of works, heat transfer in the low temperature range is critical; such works include those composed using acrylic paint or recent oil paint (which have a lower Tg than aged/crosslinked oil paints) and works previously treated with wax resin as well as when using temperature sensitive treatment materials like enzymes, hydrogels or sublimating consolidants. A. Acciai experimented with menthol at the OPD as a temporary consolidation media, and IMAT was used to control its diffusion within the canvas (Figure 3).

For the treatment of Jim Schull's Malheur Series #12, 1978, an acrylic painting on a cotton duck, mild heat was applied over a sustained period in combination with controlled humidity for the remediation of severe planar deformations caused by the vertical stretcher bar. The flexibility of the mats permitted the treatment to be conducted without removing the canvas from the stretcher (Figure 4).

Several recent treatments have incorporated precision heat transfer into the project design, such as the 2023 structural treatment of Artemisia Gentileschi’s Allegory of Inclination, 1616, a painting on canvas at the Casa Buonarroti in Florence, Italy. The canvas and the paint layers presented severe areas of lifting and cupping. MAT was used to thoroughly warm the thick and brittle paint layers in a controlled manner during the consolidation process to reduce the pronounced surface distortions, to adhere delaminated paint and ground layers, and to activate the thermoplastic resin used for consolidation while the loomed painting was placed inside a low-pressure envelope system (Figure 5).

Other important treatments of paintings on canvas are in progress at the National Gallery of Ireland as part of the international Getty Foundation Conserving Canvas Initiative (Figure 6). The oil painting Saint Joseph with the Christ Child (1637-1638) by Giovanni Francesco Barbieri--known as Guercino-presented several conservation issues, the most concerning of which was the delamination of a wax-resin lining, performed in the late 1960s, a few decades after a previous glue-paste lining. Upon assessment, the team deduced that a reforming treatment of the wax-resin lining was not feasible, and therefore the de-lining of the painting was the best approach, to be followed by relevant structural treatments. The customary de-lining procedure through the reactivation of the wax-resin with solvents or heat was not considered suitable because of the possible interactions with the well-preserved varnish layer. Several approaches to removing the lining canvas were evaluated, and low-energy heat transfer to the verso was found to be the most effective and least invasive. MAT ensured uniform and precise heat transfer in the range of 25-50 °C, which was essential for the treatment.

The advantage of using the MAT system derived from the small size of the mats which allowed the de-lining work to
be carried out in sections without repeatedly exposing the entire surface of the painting to heat. The steady and uniform mild heating of the painting verso, kept below 50 °C, allowed softening of the wax resin while staying below the melting point and ensured that the wax remained at the interface between the two canvases, permitting the effective removal of the residues. The correct temperature settings and time of application were determined based on the knowledge of the materials composing the adhesive layer and experimentally by increasing the temperature gradually from ambient to the minimum necessary to soften the wax resin. Further applications with Precision Mat are being considered for another painting in the project: a mixed-media collage, Carafe, cups and glasses (1913-1914) by the Spanish cubist artist José Victoriano—known as Juan Gris—to support the consolidation of the brittle, finely cracked, cupped and lifted paint layer. Mild heat transfer may also help reduce deformations while ensuring a safer and more effective consolidation process.

Heat, humidity, pressure and time are effective in successful structural treatments due to the inherent viscoelastic nature of paintings and paper materials but cannot be fully exploited without precision conservation-grade instruments. The new “low and slow” approach, enabled by the MAT innovation, represents a shift towards the low-energy approaches where heat transfer is targeted, low-stress and safer. MAT is an innovation by conservators for conservators, seeking to resolve the old problem of heat and provide alternatives to formulate safer and more effective treatments. True to the adage, “We shape our tools, and then our tools shape us”, MAT is coming to conservation and is already shaping us—conservators—towards safer, more efficient, greener and sustainable approaches.

AUTHOR BIOS

Tomas Markevicius, Conservator of Paintings, Senior Research Scholar, Co-founder of Precision Mat, LLC.

Nina Olsson, Conservator of Paintings in private practice, Portland, OR, USA; President Heritage Conservation Group; Senior Researcher MOXY Project; Co-founder of Precision Mat, LLC.

Chiara Chillé, Conservator of Paintings, National Gallery of Ireland; Getty Foundation Conserving Canvas Initiative.

Maddalena Magnani, Conservator of Paintings, National Gallery of Ireland; Getty Foundation Conserving Canvas Initiative.

Paula Serra Sánchez, Conservator of Paintings, National Gallery of Ireland; Getty Foundation Conserving Canvas Initiative.

Elizabeth Wicks, Conservator of Paintings in private practice, Florence, Italy; Lead Conservator Artemisia UpClose Project.

Lorenzo Conti, Conservator of Paintings in private practice, Florence, Italy; Senior Conservator Artemisia UpClose Project.

Arianna Acciai, Conservator of Paintings, 2023 Graduate Opifico delle Pietre Dure di Firenze.

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(Read the article in the June-July 2023 "News in Conservation" Issue 96, p. 20-25)