There will be an exhibition of Islamic glass objects this fall. All the objects need to be examined before they are put on display. This includes writing or updating the object's condition report and photographs. I was given a small glass pitcher from Egypt to check and treat.
The pitcher is small and narrow, made from undecorated glass, tinted yellow-ish green. It was blown, as there is a little pontil mark visible on the underside of the base where it was attached. Bubbles are also evident in the glass, as are some manufacturing lines. The rim and spot are made from a wide disk pinched together. The handle is made from a separate, thick piece of glass, which is slightly darker and greener than the body. It twists slightly, connecting to the body further to the right than where it joins the rim. The handle is folded under and doubled up underneath the rim. There are ridges for decoration around the edge of the rim, in the middle of the top, and where the handle meets the body. The latter ring is mostly missing. The bottom is concave, with a thick central pontil mark.
Although originally green and transparent, much of the glass has turned brown, iridescent, and opaque. The worst of it is on the right side, where it covers most of the exterior. The left side is not as deteriorated, with some clear, transparent spots. The handle is the least weathered, with the interior and the left side of the handle clean, smooth, and transparent. Rainbow iridescence occurs when very thin deterioration layers are superimposed on one another, parallel to the original surface, that cause a distortion in the transmittance of light. Under ultraviolet light, some of the weathering crust faintly fluoresced yellow-white. The spots are seen mainly on the bottom and handle of the pitcher. It could be from microcracks in the glass, or the pitcher could have been consolidated in 1975, though no record of treatment remains.
The glass is heavily weathered from burial, with a beige enamel-like weathering crust over much of one side. In burial conditions, the alkali in the glass gets leached out and replaced with water. This process produces degradation layers with a different composition than the rest of the glass. In the best cases, this creates a weathering crust which protects the bulk of the glass. In the worst cases, the glass is completely dissolved. The discoloring weathering crust is not a layer of corrosion products on top of the glass, but rather the original glass after some of the chemicals have been replaced. Therefore, the weathering crust is never taken off. In removing the weathering, the original surface is lost and the glass is more vulnerable to chemical and liquid attacks. In this case, the crust is unstable and flaking off, particularly on the lower body. The glass is rough and iridescent where the patina layer has flaked off. In the object's old conservation file, there is a single slide from 1975 that shows that although this deterioration and delamination was happening then, it has since gotten worse. Some of the flakes have been retained in a bag, and could be analyzed later.
The worst flaking edges of the weathering crust were consolidated with a 10% solution of Paraloid B-72 in acetone. Glass is usually not consolidated or coated because it often seals water in the glass, which leads to deterioration. Paraloid B-72 is more permeable by air and water than other consolidants, and looks much like the glass. The Paraloid B-72 was applied with a small, soft brush along the laminated edge of the weathering layers on the right side and bottom of the pitcher. The adhesive was quite thin and runny. This allowed it to be brought into the weathering layers by capillary action. When the acetone dried, the consolidant tightened the layers and secured them to the rest of the glass. The excess Paraloid B-72 was removed with acetone so the consolidant is not visible on the surface. This should protect the crust and prevent further deterioration or delamination.
The pitcher is small and narrow, made from undecorated glass, tinted yellow-ish green. It was blown, as there is a little pontil mark visible on the underside of the base where it was attached. Bubbles are also evident in the glass, as are some manufacturing lines. The rim and spot are made from a wide disk pinched together. The handle is made from a separate, thick piece of glass, which is slightly darker and greener than the body. It twists slightly, connecting to the body further to the right than where it joins the rim. The handle is folded under and doubled up underneath the rim. There are ridges for decoration around the edge of the rim, in the middle of the top, and where the handle meets the body. The latter ring is mostly missing. The bottom is concave, with a thick central pontil mark.
Glass pitcher, before treatment.
Although originally green and transparent, much of the glass has turned brown, iridescent, and opaque. The worst of it is on the right side, where it covers most of the exterior. The left side is not as deteriorated, with some clear, transparent spots. The handle is the least weathered, with the interior and the left side of the handle clean, smooth, and transparent. Rainbow iridescence occurs when very thin deterioration layers are superimposed on one another, parallel to the original surface, that cause a distortion in the transmittance of light. Under ultraviolet light, some of the weathering crust faintly fluoresced yellow-white. The spots are seen mainly on the bottom and handle of the pitcher. It could be from microcracks in the glass, or the pitcher could have been consolidated in 1975, though no record of treatment remains.
The glass is heavily weathered from burial, with a beige enamel-like weathering crust over much of one side. In burial conditions, the alkali in the glass gets leached out and replaced with water. This process produces degradation layers with a different composition than the rest of the glass. In the best cases, this creates a weathering crust which protects the bulk of the glass. In the worst cases, the glass is completely dissolved. The discoloring weathering crust is not a layer of corrosion products on top of the glass, but rather the original glass after some of the chemicals have been replaced. Therefore, the weathering crust is never taken off. In removing the weathering, the original surface is lost and the glass is more vulnerable to chemical and liquid attacks. In this case, the crust is unstable and flaking off, particularly on the lower body. The glass is rough and iridescent where the patina layer has flaked off. In the object's old conservation file, there is a single slide from 1975 that shows that although this deterioration and delamination was happening then, it has since gotten worse. Some of the flakes have been retained in a bag, and could be analyzed later.
The worst flaking edges of the weathering crust were consolidated with a 10% solution of Paraloid B-72 in acetone. Glass is usually not consolidated or coated because it often seals water in the glass, which leads to deterioration. Paraloid B-72 is more permeable by air and water than other consolidants, and looks much like the glass. The Paraloid B-72 was applied with a small, soft brush along the laminated edge of the weathering layers on the right side and bottom of the pitcher. The adhesive was quite thin and runny. This allowed it to be brought into the weathering layers by capillary action. When the acetone dried, the consolidant tightened the layers and secured them to the rest of the glass. The excess Paraloid B-72 was removed with acetone so the consolidant is not visible on the surface. This should protect the crust and prevent further deterioration or delamination.
Top left: before treatment. Top right: applying the consolidant. Bottom: after treatment.
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ReplyDeletesmall glass pitcher