Metox: Metal oxalates in 15-16th century Netherlandish oil paintings

Many Southern Netherlandish 15th and 16th paintings by Jan and Hubert van Eyck, Robert Campin, Rogier van der Weyden, Hans Memling, Dirk Bouts, Bernard van Orley etc., in the federal collections of the Royal Museums of Fine Arts of Belgium, Brussels (RMFAB), are among to the most important masterpieces in their category. They are an inestimable source of information for scholars in art history, history and conservation science. It is essential, for the next generations, to keep the visual aspect of these artworks as close as possible to the original. It is well known that over time, the paint materials, pigments and binders are inevitably undergoing spontaneous chemical transformations; these unwanted reactions are influenced by multiple environmental factors such as light and humidity but also by repeated cleaning, varnishing and/or retouching. These transformations are sometimes very slow and insidious and during many years may remain not or hardly visible to the naked eye. Therefore, sensitive chemical analyses are needed to detect them at a very early stage in order to stop or at least slow down the on-going processes. This is one of the roles of material and conservation science. The project will focus on one class of alteration products: metal-oxalates - which environmental factors stimulate or inhibit their spontaneous formation. Metal-oxalates are formed either within the paint layers or precipitate on the surface as an optically disturbing crust that is very problematic to remove without damaging the original paint film. The presence of calcium oxalates in altered paint layers has been demonstrated in several previous studies directed by the project partners on Southern Netherlandish paintings such as van Eyck's the Adoration of the Mystic Lamb or Memling's Christ with singing and music-making Angels. Often, the oxalates were detected in the blue areas painted with natural ultramarine, the glazes and some varnishes; in many cases the layers were rich in (oil- and resin-based) organic binder. It is crucial to expand in a systematic manner our understanding of this phenomenon, its origin and its progress, in order to be able to control the chemical and physical oxalate formation process. The ability to stop or slow down the degradation process will be very useful for making conservation-restoration strategies more effective, and will thus contribute to the better preservation of 15th and 16th Southern Netherlandish oil paintings in the federal collections. The research project proposes to develop and apply an analytical methodology to provide the information that leads to a full understanding of the formation of metal-oxalates and their accumulation on (and below) the paint surface. The following issues will be addressed: (a) how degraded is the original layer, (b) which types of oxalate-related damage is present and (c) what are the causes of the chemical alteration ? In particular, the discoloration of transparent red and blue glaze layers caused by the formation of an opaque metal-oxalates surface coating will be studied. Our research will be carried out with advanced analytical techniques that operate at different length-scales: i.e., at macro-, micro- and nano-level.