Consolidation of a Glass Pitcher

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.

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.

Parthian Vessel Fill, Part II : Microballoons

Filling a small gap in the Parthian vessel with plaster was a good experience, so I decided to try to gap fill with glass microballoons, or glass bubbles, to see the difference. I picked another small gap on the side of the large Parthian vessel and created a barrier by pressing dental wax on the inside of the vessel wall. The wax is used to hold up the fill and create a smooth interior surface, but it is not always necessary with glass bubbles, unlike plaster. When thick and set enough, the glass microballoons can be molded and pressed on without worrying about the fill material dripping or being unstable. The fill material looks much like plaster. It is a white powder which, when mixed with liquid, becomes thick and workable before drying hard. The plaster was mixed with water, but glass bubbles are mixed with adhesive. I slowly added the powder to a solution of 40% Paraloid B-72 in acetone, stirring it in until a paste formed. I had to add a lot more powder than I thought, but unlike plaster, you add and mix until it is the right consistency, rather than adding it all and then stirring, hoping it is the right ratio.

 Left: Microballoon powder, before mixing. Right: Wax barrier, from interior.

The bulked glass bubble mixture was thick, but it remained wet and workable longer than the plaster. I pushed the mixture into the gap with a spatula, taking care to get the sides filled entirely. I tried to get it as smooth and level to the surrounding ceramic as possible. Unlike plaster, microballoon fills are not carved down after drying. This fill shrinks less. As I was working, a film formed on top the fill. The top was dry and set, but still flexible enough due to the wet fill beneath it. It felt squishy and I could use my finger to press the fill into the proper shape.

Glass bubble gapfill, before setting.

After drying, the glass microballoon fill was hard and rough. The surface felt like soft sandpaper. Microballoon fills are often used because they match the feel and look of ceramic so well. I could see a tinted fill blending in perfectly with the texture of many ceramics. In my case, it was decided that the fill should be polished down. Paraloid B-72 is soluble in acetone, and, as the base of the fill mixture, so is the glass bubble fill. To smooth down the rough parts, I dipped a spatula in acetone. The acetone softened the fill, and I was able to slowly push the fill around slightly. Using this method of steady strokes over the surface with a spatula dipped in acetone, I was able to smooth the fill, flatten the imperfections, and create lines along the edges to simulate material loss like the other cracks. At the end, the fill felt much like the plaster one.

After polishing. The dark smudges are trapped dirt particles from the ceramic.

After polishing, I painted the fill. Using the same water-based acrylic paints as the plaster fill, I tried to match the buff ceramic color. Once again, I was not exact. It is much harder than it looks to match, especially since the paint looks different once it dries. This time, I tried to add a bit of a subtle spattered effect, like the ceramic, instead of a single color field. In theory, this break up of the color makes it look more natural and blend better.

Left: Close up of fill in process, with yellow and brown speckles. Right: The finished product.

Overall, I quite liked the glass microballoon fill. I thought that it was fairly easy to work with and looks good with the ceramic. For rougher pieces, leaving it unpolished would make the fill look quite natural. The glass bubbles fill was good over a small area, but could be more difficult to mold into the proper shape if the gap is too large, complex, or curved. I found mixing and working with plaster harder, but it would work better for a large gap.