Practical applications of microfadometry in museum lighting, by Malin Borin

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Workshop participants. Photo Gothenburg Museum

Some highly valued watercolours in the collections of Gothenburg Museum of Art are exposed to constant exhibition demands. Meeting up to these demands is seen as essential to the museums attraction and to the relationship to other institutions.
Conservators at the museum wish to bring more fact-based information on the sensitivity of the watercolours into the discussion. A project was initiated to use Microfading testing (MFT) for the purpose. In this context the museum decided to host a workshop that took place in February 2016 in collaboration with Jacob Thomas, conservation scientist at the University of Gothenburg.
MFT is a spot test that fades a microscopic area to a maximum degree that is not perceptible to the eye. A software processes the data to visualize colour change and to rate the relative vulnerability equivalent to the Blue Wool Scale. The speakers introduced us to the development of MFT technique towards a more user friendly equipment. Several speakers emphasized the use of MFT as a powerful tool, among several tools needed, in lighting policy work.
Some highly valued watercolours in the collections of Gothenburg Museum of Art are exposed to constant exhibition demands. Meeting up to these demands is seen as essential to the museums attraction and to the relationship to other institutions.
Conservators at the museum wish to bring more fact-based information on the sensitivity of the watercolours into the discussion. A project was initiated to use Microfading testing (MFT) for the purpose. In this context the museum decided to host a workshop that took place in February 2016 in collaboration with Jacob Thomas, conservation scientist at the University of Gothenburg.
MFT is a spot test that fades a microscopic area to a maximum degree that is not perceptible to the eye. A software processes the data to visualize colour change and to rate the relative vulnerability equivalent to the Blue Wool Scale. The speakers introduced us to the development of MFT technique towards a more user friendly equipment. Several speakers emphasized the use of MFT as a powerful tool, among several tools needed, in lighting policy work.
The equipment is also easily transported. The disadvantage of the contact probe is of course that objects may have sensitive surfaces which do not tolerate touching. Another drawback could be that the light beam only focuses on the contacted surface. In some situations one might want to focus slightly further down in the material, for example through a transparent laminate.
Tomasz Lojewski, head of Paper Degradation Laboratory at the Faculty of Chemistry of the Jagiellonian University in Krakov, presented a newly developed MFT equipment. It’s a user friendly non-contact probe that aims to solve the issues associated with this type of tool; focusing of the light beam and disturbance by vibrations. The equipment is built in a stabilizing chassis with automatic focusing. A drawback is that it’s more difficult to transport.
How reliable are the results?
One issue discussed during the seminar was the reliability of the analysis results. Is a very strong light exposure during a short period equivalent to low light exposure over a long period of time? The direct relationship between exposure time and light intensity is called reciprocity. Reciprocity is recognized as valid within moderate light levels, this makes it possible for us to count lux levels when exposing objects. At extreme light levels, as used in MFT, a direct conversion to low light levels is questionable. The seminar speakers suggested that MFT results should only be used to determine relative differences in light sensitivity. For this purpose the technique is reliable. Participants at the seminar, now using MFT in their work at the Royal Library of Copenhagen, confirmed the usability.
Case study
In connection to the seminar, a case study was based on a test performed on a watercolour by Carl Larsson from the collection of Gothenburg Museum of Art.
The study underpinned an issue in analysing the data from microfading tests. To have an accurate result one has to test several spots within the area of one colourant. The test points are microscopic and the exposures in this case did hit locations of rather irregular character.
In a watercolour the paint layer is often thin and the pigment particle distribution is uneven on a microscopic level. The occurrence of pigment particles may be very sparse in one point and considerably denser in an adjunct spot. The resistance to colour change is much larger in a dense area since the overlaying pigments will protect the underlying. For this reason we had results differing from Blue Wool 1 to Blue Wool 3 within one colourant.
Should we then, as suggested during the seminar, consider the object as sensitive as the most sensitive test point? We came to the conclusion that we need more test points to have better statistics on the sensitivity of the tested area.
MFT at the National Heritage Board
To further investigate the Carl Larsson watercolours, the Gothenburg Museum of Art is taking part in a collaboration project at the National Heritage Board in Sweden. They regularly host colleagues providing them with analytical equipment and expertise. Magnus Mårtensson, Conservation Science Advisor, introduced us to the guest colleague programme idea and how the use of MFT may help develop guidelines for lighting.

Malin Borin is a paintings conservator at Gothenburg Museum of Art.
She graduated in 2004 with an MSc in conservation at Gothenburg University.
Malin has also worked in Studio West Swedish Conservation (SVK) and is
regularly lecturing at the Department of Conservation, Gothenburg University.