By Guest Bloggers:
Golya Mirderikvand (Art Conservation, Queen’s University) and George Bevan (Classics, Queen’s University)
In the fall of 2010 a Master of Art Conservation Student, Paintings Stream, at Queen’s University, Golya Mirderikvand was confronted with a unique challenge: how to document a reverse glass painting before beginning treatment. From the recto the painting is convex with the paint being applied on the concave verso. Because the flaking and delamination of the paint was on the concave side, it was difficult to produce raking-light shots that would fully reveal the variation in texture without creating internal shadows that obscured the surface. Reflectance Transformation Imaging (RTI), which had already been extensively used by George Bevan in Classics, was then employed as the ideal way to document such a work.
First some background on the type of work in question. Reverse glass paintings, or more commonly known by their German name Hinterglasmalerei, refer to the decoration of glass by painting or engraving metallic foil on the back of a glass panel. Reverse glass paintings have a rich history with distinct variations in styles and techniques apparent from different periods. Although their exact origins in Europe are uncertain, it is generally agreed upon that they were popularized in the 14th century, with the oldest surviving reverse glass paintings dating from the second half of the 13th century.
Paintings executed on glass panels are inherently fragile and susceptible to deterioration since they are not fired to allow the paint to become fused with the glass. The primary mechanism of deterioration of reverse glass paintings is the detachment of paint films to the non-porous glass substrate. It has been speculated that a lack of proper preparation of the glass support in order to provide a better tooth, or a lack of application of a priming layer, or a combination of both, can lead to the delamination of the paint layers. Other possible reasons of detachment could be as a result of condensation of moisture on the surface of glass, causing the paint films to release. Internal and external factors contributing to the creation of interlayer shear forces can also lead to paint delamination. This can commonly occur as a result of any backings or adhesives in direct contact with the paint layer from the back, or changes in relative humidity causing contraction and expansion within the paint layers.