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.