Daly Conservation

I am glad to welcome the guest blogger Carrie Roberts, for this posting.  Carrie and I have been neighbors for the past two years at the Winterthur/University of Delaware Program in Art Conservation.  She is spending her summer as an intern at English Heritage in London.  Carrie will summarize the morning session of the technical imaging conference at the British Museum.  Thanks Carrie for all your work!

L. Pezzati et al. (CNR-INOA, Italy): Multi-band scanner for in situ imaging of paintings.

This talk presented a relatively new method of infrared imaging using a multi-band scanner. While infrared reflectography was developed in the 1960’s and has been used as scanning technique since the 1990’s, multispectral infrared emerged only last year, in 2009. The technique uses band pass filters to irradiate the surface of objects with multiple, narrow bands up to 2265nm across the infrared spectrum, and allows for color imaging with three separate RGB channels.

L. Pezzati explained multispectral technology in a very clear, concise way, and offered a glimpse how he and his team are working to expand the current 16 channel instrument to one capable of imaging at 36 infrared bands! The advantages of scanning at multiple, narrow bands include clearer images and a wider range of materials absorbing across the infrared spectrum; what one might miss in the near infrared may be picked up in the mid-infrared, and vice-versa. Certainly one of the most innovative presentations of the day.

K. Martinez et al. (University of Southampton, UK): The use of multi-light imaging in recording archaeological artefacts

K. Martinez offered a useful introduction to an increasingly popular imaging technique called polynomial (or multi-light) imaging. Mel Wachoviak referred to it as reflectance transformation imaging in the talk he gave at Winterthur last year, and they are in essence the same technique.

The multi-lighting imaging involves capturing a series of images of an artifact under a ‘dome’ of lights. 60 to 80 lights, or points of light are generally needed. A flash unit at the end of the string will also work as long as the distance from the object is the same at all points. A shiny sphere – usually a black marble – is used in the image space to allow the software to locate where the light is at each point, and to integrate all the images into one that contains surface information from every angle of light captured.

Multi-light imaging has been cited as a more affordable alternative to laser scanning, but only as a surface technique. RTI is good at capturing surface texture, but does not record spatial information the way lasers can. The two techniques have been used together to produce 3D images with highly detailed surface information. You can download another article by Martinez about polynomial surface mapping. He also has posted online about the mapping of Roman paintings.

F.P. Rutland and A.M. La Pensée: Non-contact 3D laser scanning as a tool to aid identification and interpretation of archaeological artefacts; the case of Middle Bronze Age (MBA II) Hittite Mold

This excellent talk featured contributions from F.P. Rutland, an archaeologist working at National Museums Liverpool, and A.M. La Pensée, scientist at the National Conservation Centre at Liverpool, a world center for laser scanning technology. Their collaborative project involved the scanning of a Hittite stone mold of unknown use.

A 3D digital model of the object was made using a triangulation-based non-invasive laser. Once this was done, the model was digitally flipped to form a mirror-image of the original, revealing the object as one half of a two-part stone mold for a bronze, fenestrated axe head. This type of weapon tip can be found in a number of UK collections including the British Museum. Both the mold and the axe-head positive were reproduced to scale using 3D printing. This type of work has been done before, but the presentation demonstrates the value of collaboration between conservation science and research fields.

The next installment will be written by another guest blogger, Sagita Mirjam Sunara from Croatia.

This summary was provided by Sagita Mirjam Sunara.  Sagita is based in Croatia, she traveled to London for this conference and has provided me with a summary of three of the afternoon lectures.

Visible-Induced Luminescence Imaging Applied to Ancient Stone Sculpture

Giovanni Verri (The British Museum, Department of Conservation and Scientific Research)

Just like wall paintings and ceramics, ancient sculptures and architecture were frequently painted. In most cases, original coloration has been lost due to weathering, unfavorable microclimatic conditions, and un-sympathetic cleaning procedures. Scientific investigation and imaging technologies can reveal the presence of color on ancient objects, thus opening new insights in the understanding of ancient art.

Giovanni Verri presented one of the techniques that can be applied for characterization of materials in archaeological museum objects: visible-induced luminescence imaging in the near infrared range (800-1700 nm). This is a non-invasive, low-tech imaging technique, very similar to UV induced luminescence imaging, which is often used by conservators and conservation scientists. Verri and his team used this technique for identification of Egyptian blue in several objects in the collection of the British Museum. When irradiated with visible light, Egyptian blue emits infrared radiation. Furthermore, Egyptian blue is highest IR emitter at a molecular level and an extremely long half-life. Visible-induced luminescence imaging allowed detection of single particles of this pigment on examined objects, even when they were covered by layers of discolored binding media, varnishes and inorganic patina.

This technique can provide information on distribution of blue material with respect of the construction of the object. It is useful to inform sampling strategies, and may help in authentication of objects and in the identification of appropriate conservation treatments.

Multispectral Image-Based Paint Formulation Software for Restorative Inpainting

Marissa I. Haddock, Roy S. Berns (Munsell Color Science Laboratory, Center for Imaging Science, Rochester Institute of Technology)

Marissa Haddock developed a computer program that provides a color match and paint recipe for restorative inpainting. She took a technique traditionally used in industrial paint and plastics formulation and applied it to art conservation. Her research focuses on characterizing and developing a spectral database of the optical properties of the Gamblin Conservation Colors, designing and creating an application color matching software tailored to the needs and practices of  art conservators and investigating the use of multispectral imaging as an additional measurement tool for color matching formulation.

Perceived color is a function of the object, the light and of the observer. Mixing paint for retouching requires good knowledge of pigments and color matching. However, if one of the above-mentioned factors changes, the color changes too. Computational techniques can be used to create a color mixture that will match the original paint under a range of viewing and illumination conditions. Multispectral Image-Based Paint Formulation Software performs both image-based color matching, and color matching based on direct spectrophotometric measurements. After taking the image data or contact measurement, paint formula is devised. Pigments that most closely match the mixture are selected, and their concentration in the mixture calculated.

Non-Invasive Color Restoration of Faded Paintings Using Light From a Digital Projector - the topic of an article in the Wall Street Journal

S. Cuellar (Straus Center for Conservation and Technical Studies, Harvard Art Museum; MIT Media Lab), J. Stenger (Straus Center for Conservation and Technical Studies, Harvard Art Museum; Center for the Technical Study of Modern Art, Harvard Art Museum), R. Gschwind (Imaging and Media Lab, University of Basel), A. Mohan (MIT Media Lab), Y. Mukaigawa (MIT Media Lab; Institute of Scientific and Industrial Research, Osaka University), R. Ruskar (MIT Media Lab), K. Eremin (Straus Center for Conservation and Technical Studies), N. Khandekar (Straus Center for Conservation and Technical Studies)

Jens Stenger of Harvard Art Museum gave a presentation of a conservation treatment that does not change the paint surface physically.

In 1963 Mark Rothko donated five paintings on canvas to Harvard University. These paintings are today known as Harvard Murals. Due to the fugitive nature of the pigment he used (Lithol Red) and overexposure to light, paintings had faded greatly and were taken down in 1979. They have been in storage ever since.

In order to establish the original appearance of the paintings, Ektachrome photographs of the paintings were consulted. The photographs had faded as well, and had to be restored. Once the original appearance of the paintings was known, it was necessary to decide how the original color can be brought back. Since inpainting  would be unacceptable, alternative methods were examined.

The concept of bringing back the color by using colored light and illumination was first formulated in a paper by Raymond H. Lafontaine  “Seeing Through a Yellow Varnish: A Compensating Illumination System” (1986). Lafontaine used a combination of neutral light and blue light to ‘look through’ the yellowed varnish, and two slide projectors to ‘restore’ the original color of the painting. The same concept was used for Rothko’s paintings. An image of the current state of the painting was taken and compared with the target image in the computer. A compensation image was created and projected on the painting, recreating its original color. Ambient light had to be adjusted. As Rothko’s paintings are still sensitive to light, light levels have to be adjusted to a safe level.

Thanks to Sagita and Carrie for your summaries of this exciting and interesting conference.  I hope you both had a lovely time and a nice cup of tea.

The Nasher Sculpture Center has a fantastic buildingdesigned by Renzo Piano with large windows so you can be indoors but feel outdoors at the same time. Since the collection mainly consists of materials that are not very light sensitive like metals and stone, having natural light in the space creates a beautiful space that is also a good environment for the display of modern sculpture. The ceiling is made of glass with a white cast aluminum sunscreen over the roof. Light levels in the galleries can be reduced by placing scrims across the ceiling to limit the amount of light entering the space. In the picture below you can see how the scrims on the left side of the gallery ceiling has prevented any natural light from entering the space, and on the right the ceiling is open to allow natural light into the space.

We are now looking for a material that would allow 25-foot-candles of light into the space, a compromise between the two halves you see above. To audition for new scrim fabric the materials are being tested to measure exactly how much light is coming through the scrims into the galleries. For this we have a mock-up where the scrim fabric can be stretched over a frame, and then a light meter can be inserted into the box to measure the amount of light penetrating the scrim.

I have decided to take one day and record the light measured in this apparatus every two hours, to have a complete picture of the amount of light that will be entering the room. Ideally, it will be a bright and sunny day next Monday when the museum is closed to the public and I will be able to measure the light levels in the galleries.

If anyone has experience with this type of testing I would be really eager to speak to more people about it, I searched for more data online about scrims and light levels but I have not found anything quantitative.