This Roman period glass bowl is made of translucent, light green glass. The bowl would have been formed by blowing a bubble of glass into a mold, creating the ribs on the vessel’s exterior and leaving the areas between the ribs extremely thin. Despite its age, this glass object is intact with the exception of one small, oblong loss in the thinnest part of the wall. Conservators do not always fill every loss in an object, especially when an object comes from an ancient and/or archaeological context, or when the loss is small; however, in the case of this bowl, the glass surrounding the area of loss is so thin that filling the loss actually improves the object’s stability by protecting the edges of the loss from further damage.
The loss in the bowl’s wall was filled with a thermoplastic acrylic resin called Paraloid® B-72. Paraloid® B-72 is a favorite material in conservation, used as an adhesive, a consolidant, a fill material, and a coating. B-72 is versatile as it is soluble in a variety of solvents in a range of concentrations, and particularly well suited to conservation as the resin is chemically stable, reversible and manipulable with solvents and heat, structurally strong, optically clear, and bonds well to many materials. Paraloid® B-72 finds use on a wide range of materials, but because the resin sets through solvent evaporation, tiny bubbles visually disrupt the resin film, making B-72 less aesthetically appropriate for glass. To help conservators use this excellent repair material in glass conservation, conservators at the Corning Museum of Glass developed a way to cast Paraloid® B-72 resin films without bubbles.
To make the fill for the bowl, B-72 resin was tinted with dyes and dry pigments, poured into silicone rubber molds, and cast into thin sheets. The molds were placed into polyethylene bags (left) to allow the solvent to evaporate slowly, reducing the formation of bubbles in the resin film. Once set (right), the film was cut to shape and adhered to the loss by applying a tiny bit of solvent to the edges of the fill; because B-72 is itself an adhesive, no additional adhesive is necessary.
Through exposure to its archaeological burial environment, the surface of the bowl has developed a layer of iridescence often referred to as “weathering products.” Unlike accretions that have become adhered to the surface of the glass, weathering products are actually the original surface of the glass that has delaminated, or split, into many layers. As light passes through these extremely thin layers of glass that are separated by small pockets of air, the light bends or refracts, the optical effect creating iridescent colors like oil on water or the colors of some butterfly wings.
To make the Paraloid® B-72 fill resemble the weathered glass, a layer of goldbeater’s skin painted with acrylic iridescent paints was added. Traditionally used in the beating of gold sheet into gold leaf, goldbeater’s skin is thin, strong, translucent, and has a satiny sheen similar to the glass weathering products. This bowl will be part of the upcoming reinstallation of the Brooklyn Museum’s Asian and Islamic Galleries.
Posted by Victoria Schussler