New Optical Instruments have Transformed Conservation and Curatorial Practices of Dating, Displaying and Conserving Priceless and Irreplaceable Cultural Heritage Assets

NTU has developed novel optical instruments and has examined irreplaceable and fragile cultural assets, revealing crucial details whilst avoiding traditional damaging invasive sampling methods. The NTU team assisted the Louvre’s successful 2016 restoration of Leonardo da Vinci’s “Saint John the Baptist” and changed conservation practices which impact all museums in France.

NTU’s analysis informed the public display strategy of rare Daguerreotype photographs of Charles Darwin’s family at Down House, Royal Bed Hangings at Audley End House, and of “Acanthes” in the “Henri Matisse: The Cut-Outs” exhibition at the Tate Modern, the most successful Tate exhibition which attract 562,622 visitors, and at the Museum of Modern Art in New York. NTU’s mobile optical imaging and remote sensing methods transformed understanding about the Buddhist wall paintings at the Mogao Caves UNESCO World Heritage Site in Dunhuang, China and changed dating analysis of wall paintings along the Silk Road.

Research conducted by NTU’s ISAAC team has led to the development of a new versatile, modular and mobile analytical platform that consists of imaging and spectroscopy systems, that operate from stand-off distances of <3m (close range) to tens of metres (long range), for material identification and monitoring of degradation/corrosion.

NTU ISAAC’s research on a holistic approach to the non-invasive analysis of artworks by combining OCT with a complementary set of spectroscopic/imaging techniques including the minimally micro-invasive microfade spectrometry has defined the analysis strategy of the ISAAC Mobile Lab which has been used in situ on a range of historical artefacts in museums and archives from paintings, manuscripts to enamels and historical sites around the world from the UK to China and the USA.

Research by the Imaging and Sensing for Archaeology, Art History and Conservation (ISAAC) Lab at Nottingham Trent University (NTU) led by Professor Haida Liang has focused on the development of a complementary suite of cutting-edge non-invasive imaging and spectroscopy techniques including Optical Coherence Tomography (OCT), Microfade Spectrometry, and ground based remote spectral imaging (PRISMS), as well as their application to conservation and historical understanding of cultural heritage.

NTU ISAAC group’s PRISMS non-invasive remote spectral imaging system for cultural heritage, funded by EPSRC, is the first such system that allows simultaneous reflectance spectral imaging at sub-millimetre resolution combined with 3D mapping, at stand-off distances of tens of metres from a fixed position on the ground. This system was developed specifically for automated high-resolution spectral imaging of large paintings, painted surfaces and wall paintings in churches, temples, palaces and monuments to identify materials and record the state of preservation of entire architectural interiors. It has been adopted by the IPERION HS European Research Infrastructure for Heritage Science as part of the pan-European Mobile lab offerings.

Liang was one of the first to show that the medical imaging technique OCT could be applied to the non-invasive analysis of artworks. OCT, an optical technique based on the Michelson interferometer, is normally used for non-invasive 3D volume imaging of the eye. The study, selected as an editors’ choice by the journal Science, showed how OCT could visualise the sub-surface microstructure of paintings. Through cross-disciplinary collaborative funding (£1M), including from the Leverhulme Trust and EPSRC/AHRC, the NTU ISAAC team developed a range of cutting edge mobile OCT systems tailored to the special needs of the cultural heritage sector, applying these techniques in museums and publishing over 30 papers.

Traditional commercial biomedical OCT systems had significant resolution and depth of penetration limitations and were restricted to wavelengths of 800nm-1300nm. An NTU-led AHRC/ESPRC funded research collaboration developed the world’s first “Long Wavelength OCT” at 1960nm with the best depth of penetration in materials with low water content, partnering with the National Gallery, English Heritage, Southampton University, and optical component manufacturer Gooch & Housego PLC. This collaboration also developed “Ultra High-Resolution OCT” that provides subsurface microstructure in real-time non-invasively whilst matching or exceeding the resolution and contrast of conventional invasive paintings analysis techniques. These cutting-edge OCT systems were successfully applied to a 16th century copy of a Raphael painting at the National Gallery, and their efficacy at conservation monitoring and archaeology applications were demonstrated for a range of cultural assets including water transport in sandstone (relevant to Neolithic rock art in the care of Historic England), and the British Museum’s collection of ancient Egyptian faience, ancient Jade and Limoges enamels.

The ISAAC team at NTU developed a Microfade spectrometry instrument to measure the light sensitivity of museum collections for preventive conservation. A trade-off, of vital importance to museum, gallery and library curators and conservation professionals, is that whilst artworks are sensitive to light-induced damage, they need to be properly illuminated when on public display to provide best visitor experience as part of the museum and gallery’s public missions. The Tate co-funded the NTU ISAAC team’s development of the first automated portable and flexible Microfade spectrometry that is capable of determining the light sensitivity of watercolour paintings required to determine optimum exhibition conditions.

Enabling precision restoration of an internationally important painting, in collaboration with Louvre Museum professionals, leading to adoption of improved conservation practices across France

The NTU ISAAC team’s “Ultra-High Resolution OCT” directly guided the Louvre Museum (Paris, France) conservator’s varnish cleaning restoration strategy of Leonardo da Vinci’s “Saint John the Baptist”. The restoration was in preparation for the 2019 exhibition Leonardo da Vinci at the Louvre to mark the 500-year anniversary of the death of the artist in France. According to the exhibition programme, “Saint John the Baptist” is considered to be one of da Vinci’s masterpieces and is one of the 3 paintings that he was working on at the end of his life (there are only 15 paintings securely attributed to him world-wide).

The painting had not been cleaned since 1802 and its surface had darkened because many layers of varnish applied over the centuries had considerably degraded. Cleaning this irreplaceable and fragile painting took place under immense scrutiny, since the previous restoration of da Vinci’s “The Virgin and Child with Saint Anne” was criticized for modifying the painting due to “over-cleaning”, prompting staff resignations (Jean-Pierre Cuzin, Le Monde, 2012). NTU’s ISAAC team undertook OCT imaging of the layers of varnish on “Saint John the Baptist”, working alongside The Louvre’s restorer and conservation scientists. Informed by the OCT images, nearly half of the original 15 layers of varnish were meticulously removed without damaging the underlying painting.

The success of The Louvre and NTU ISAAC collaborative restoration work led directly to the Centre for Research and Restoration of the Museums of France (C2RMF) incorporating OCT analysis (since 2018) “into our conservation practice for routine cleaning of varnished paintings”. The reach of this impact is over 1,200 museums in France.

Optimising art galleries’ and museums’ display policies to enable greater visitor access and enhanced public experience, whilst ensuring that exhibition doesn’t damage or degrade irreplaceable cultural artefacts

The NTU ISAAC team worked, under commission, with the Beyeler Foundation in Switzerland to evaluate the light sensitivity of Henri Matisse’s “Acanthes”, which is significant as a major work in the artist’s series of large-format papiers découpés. Chromatic modifications and fading of coloured art pieces can change their colour impression from what was originally intended by the artist.

The Chief Conservator at the Beyeler Foundation has confirmed that the NTU Microfade study gave the Foundation the confidence to loan “Acanthes” to the Tate Modern for a key role in its exhibition “Henri Matisse: The Cut-Outs” from April to September 2014, with concurrent broadcast “Matisse Live” to 15,000 cinema goers, and to loan the piece to continue the tour to the Museum of Modern Art, New York, where it was displayed from October 2014 to February 2015.

The NTU ISAAC team, working with English Heritage, used NTU’s OCT techniques to elucidate the degradation processes of cover glasses of a unique series of Daguerreotype photographs of Charles Darwin’s family. This included the only known image of Darwin with another person (according to the Darwin Correspondence Project, Cambridge University Library). A Senior Conservation Scientist at English Heritage, testified to this work’s impact: “Professor Liang’s ground breaking ultra-high resolution optical coherence tomography equipment was instrumental in understanding the decay phenomena and making the decision to replace the glass and remediate the showcase environments to allow this unique cultural heritage to continue to be appreciated by the 71,000 visitors [to Down house] per annum.”

The NTU ISAAC team undertook Microfade spectrometry surveys of light sensitivity of historically significant “Royal bed hangings” (1786) and carpets which is part of the visitors guided tour of Audley End House, one of the largest country houses of the Jacobean era, and one of the few fully furnished houses in English Heritage’s portfolio. The State Bed of embroidered Chinese silk, commissioned by Sir John Griffin for an anticipated visit by George III, is an important surviving example of a late18th century bed and was made by the London firm Chipchase and Lambert in 1786 at a vast cost. NTU’s automated microfade spectrometer permitted accurate on-site analysis.

Transformation to curation, understanding and interpretation, of artworks, leading to lasting adoption of improved analysis practices

The Mogao Cave temple complex near Dunhuang, China is a UNESCO World Heritage Site along the ancient Silk Road at cross-roads of trade, religion, technology and cultural influences. There are 492 cave temples painted ceiling to wall with masterpieces of Buddhist art from 4th to 14th century of a total of 45,000 square meters of murals and are strongly linked to the history of transcontinental relations and of the spread of Buddhism throughout Asia. Cave 465 at the northern end of the site, has wall paintings in a unique Indo-Tibetan tantric Buddhist style with the full range of Mahāyoga tantric Buddhist imagery. These paintings are culturally important because there are few extant pre-14th century Tibetan or Indian/Nepalese tantric Buddhist paintings. The time of construction of this cave was unknown before the work of the NTU ISAAC team.

The NTU ISAAC team’s expertise allowed accurate recording of not only high-resolution colour images but more importantly the painting materials identified through their characteristic spectra and the 3D surface of the wall paintings, all under safe imaging conditions. This was done through the in situ use of mobile complementary imaging and spectroscopy instruments, including novel optical instruments developed specifically for cultural heritage such as the PRISMS ground based remote spectral imaging system for automated survey of large wall/ceiling paintings, OCT for revealing preparatory sketches and subsurface cracks, and microfade spectrometry to ensure the illumination required for imaging does not cause damage to the paintings.

The NTU study dated the Buddhist wall paintings in Mogao Cave 465 to be from the late 12th to 13th century using the evidence collected by the suite of non-invasive instruments. The Director of  Dunhuang Research Academy stated that, “This conclusion is significant and long awaited by scholars working on Buddhism in Eastern Central Asia, because it settles a long dispute, over 50 years, amongst experts of this unique cave temple and the era over which major Buddhist caves were constructed in Dunhuang. This collaboration with Professor Liang’s team provides important new knowledge, tracing the spread of Buddhism from India to Tibet, the Tangut realm and to China”. Inspired by the study of Mogao Cave 465, Dunhuang Research Academy assisted Professor Liang’s team to survey 7 more caves at the site.

• Liang H., Lucian A., Lange R., Cheung C., Su B., “Remote spectral imaging with simultaneous extraction of 3D topography for historical wall paintings”, ISPRS Journal of Photogrammetry and Remote Sensing 95, 13-22 (2014). https://doi.org/10.1016/j.isprsjprs.2014.05.011
• Liang H., Gomez Cid M., Cucu R.G., Dobre G.M., Podoleanu A.Gh., Pedro J.,Saunders D., “En-face Optical Coherence Tomography - a novel application of non-invasive imaging to art conservation”, Optics Express 13, 6133-6144 (2005). https://doi.org/10.1364/OPEX.13.006133
• Cheung C. S., Daniel J., Tokurakawa M., Clarkson W. A., Liang H., “High resolution Fourier domain optical coherence tomography in the 2mm wavelength range using a broadband supercontinuum source”, Optics Express 23(3), 1992-2001 (2015) https://doi.org/10.1364/OE.23.001992
• Cheung C. S., Spring M., Liang H., “Ultra-high resolution Fourier domain optical coherence tomography for old master paintings”, Optics Express 23(8), 10145-10157 (2015) https://doi.org/10.1364/OE.23.010145
• Lerwill A., Brookes A., Townsend J., Hackney S., Liang H., “Micro-fading spectrometry: investigating the wavelength specificity of fading”, Applied Physics A 118, 457-463 (2014) https://doi.org/10.1007/s00339-014-8645-3
• Kogou S., Shahtahmassebi G., Lucian A., Liang H., Shui B., Zhang W., Su B., van Schaik S., “From remote sensing and machine learning to the history of the Silk Road: large scale material identification on wall paintings”, Scientific Reports 10, Article number 19312 (2020) https://www.nature.com/articles/s41598-020-76457-9
• Li Y., Cheung C.S., Kogou S., Liggins F., Liang H., “Standoff Raman spectroscopy for architectural interiors from 3-15 m distances," Optics Express 27, 31338-31347 (2019) https://opg.optica.org/oe/fulltext.cfm?uri=oe-27-22-31338&id=422303