In preparation for the current exhibition, Life on a String, a group of Venetian chevron beads was brought to the Museum from our off-site storage facility. The vibrations from handling and transport caused the old repair of one of the beads to fail. This isn’t that unusual because adhesives degrade over time and there was no record of the beads being treated since coming into the Museum’s collection in 1962 which meant that the adhesive was at least 50 years old. What was interesting about this particular bead was the clear evidence that this bead and the old repair had survived the 1972 flood.
It’s rare to find an object that survived the flood with its repairs intact. Partly because some of the adhesives and fill materials used in the past soften or even dissolve in water (such as animal glue and plaster) and partly because glass is so smooth that adhesives don’t bond that well to it and water can seep in between the glue and the glass causing the repair to fail.
As you can see in the before treatment images, there was a very fine mud (typical of mud from the flood) in the center of the bead and in the break. Since the mud is only in the areas where there is no glue and not under the glue at all, we know that the glue must have been in place when the mud got there.
Looking at the remnants of the old adhesive we can get a pretty good idea of what kind of adhesive it is. The air bubbles trapped in the set adhesive indicate that it is a solvent based adhesive. A solvent based adhesive is a resin that is turned into an adhesive by dissolving it in a solvent (or multiple solvents). As it sets the solvent evaporates and the resin becomes hard again. We can also see that the adhesive is yellow. When it was originally applied it was most likely colorless and has yellowed as it aged. The adhesive film was very brittle, another way in which it has aged. The adhesive is most likely a cellulose nitrate, like Duco cement. In the past, cellulose nitrate adhesives were often used in the conservation of glass and ceramics. However, they are not used much anymore because of how they change when they age, most notably yellowing and becoming brittle, and because a better alternative has been developed.
The bead was put back together with an adhesive called Paraloid B-72. It is also a solvent based adhesive, but it is much more stable as it ages than cellulose nitrate.