We study the use of microfading testing (MFT) to predict the light sensitivity of white (or whitish) paper, an important substrate in virtually all cultural heritage collections. White paper is found in prints, documents, drawings and books, all of which are regularly seen in exhibitions. While MFT is the common tool for determining the light sensitivity of media on paper, little attention has been paid in MFT research to blank paper, even though it contributes significantly to the aesthetic appreciation of paper objects. However, as we show, even pre-aged historical rag paper, which is said to be highly durable, is prone to harmful light-induced color changes. In the collection, we provide a better understanding of the light sensitivity of the major historical white paper classes and show the extent to which MFT can predict the future color change of paper exposed to different lighting conditions – whether MFT reflects real-life color change, which is one of the most-discussed topics in the MFT community. The color change study that compares MFT and real-time data highlights uncertainties associated with MFT and increases confidence in the technique.
We distinguish three focus areas: (1) MFT functionality: After optimization of the instrumental error or drift of the MFT devices, the authors compare two MFT devices that are widely used in material heritage collections and feature either xenon (Oriel Fading Test System, VIS and VIS/UVA) or LED sources (Instytut Fotonowy, VIS). The UV amount of the Xe-MFT is measured at the point of measurement, and in some tests, UV was introduced to evaluate the effects of radiation on the light sensitivity of the papers. The interpretation of the color change of some relevant papers is given by examining the individual color coordinates. (2) Reliability of MFT in predicting the expected color change of white paper in exhibition situations: The authors present the results of a comparative study in which both the Xe-MFT and the LED-MFT were used on thirty-eight white paper samples prepared in the laboratory. They represent early handmade papers, early industrial papers and modern papers with optical brighteners, and also include samples doped with typical chromophores produced by paper. MFT results are compared to the color changes that occurred in sets of these paper samples during exposure to accelerated aging and exposures that mimic real-life light-dark cycles in a museum (VIS) and in a gallery (VIS/UV-A) setting. The white paper samples are also discussed to show the effects of material composition on the directions of color change such as fading, darkening and color shifts in a range of yellow to reddish-brown tonalities captured in CIELab and spectral curves. The authors show how, beyond lignin as the known flagship of photosensitivity, compositional parameters can be used to refine the prediction of color change in white papers for light-budgeting strategies in collections. (3) Bringing the research into museum practice: The authors discuss the evolution of museum lighting guidelines, they explain the testing of light sources in connection with lighting standards such as DIN EN 16163. Using exemplary studies, they show how MFT can be integrated into museum practice and which questions can be answered with this method. They will discuss the advantages and disadvantages of using MFT to budget lighting exposure in exhibitions with light-sensitive objects as an alternative or supplement to the application of existing lighting guidelines.
Overall, we hope to present a comprehensive overview of the relationship between MFT and white paper in the preservation of paper objects in museums and other collection institutions. We intend this collection to open up several areas of interest that will be useful to both practicing conservators and researchers interested in the study of historic paper, MFT technology, or lighting guidelines in collections.
