Author: Erik Farrell, QAR Conservator
When we think of glass today, we mostly think of something like clear, flat window glass – impervious to water and unchanging across the years. Sure, if someone hits a baseball through it the window will break, but there’s not much you can do to chemically damage the glass itself… right?
Glass is made of three major components: silica (from sand), a flux (potassium or sodium compounds), and a stabilizer (calcium, magnesium, and lead compounds).
When underwater, glass will slowly lose sodium and/or potassium ions, which are replaced by hydrogen from the water. Glass which has decayed in this manner is often referred to as “hydrogen glass” as a result. Hydrogen glass will take on a pearly, iridescent sheen (almost like an oil slick), become opaque, and eventually start peeling apart in thin, flaky layers. Salt can accelerate this process, and can physically aid in peeling apart layers of glass if allowed to dry and re-crystallize.
But wait… I wash my glass dishes. It rains on my windshield. And what about my fishtank?!
Modern glass is very, very well made, and even in poor quality glass the loss of flux material is extremely slow. Because of its quality, modern glass might be able to remain fully immersed in water for hundreds of years before showing even the slightest hint of decay. Glass in the early eighteenth century? That’s a little more variable.
Queen Anne’s Revenge had several types of glass on board: French flacon glass, English forest glass, and English leaded glass are some of the major ones. French flacon glass was made with a sodium flux (like most modern glass), and is very stable. English leaded glass used a lead-based stabilizer; it might not be great to drink from, but it’s very stable. English forest glass used a potassium flux, and is generally in terrible condition.
So how do conservators deal with unstable glass? For a familiar first step, we desalinate the material, removing salts left behind by seawater. This prevents the salt from causing physical damage after drying, and allows us to begin treating the underlying conditions.
Short of melting it all down and starting over, we can’t replace the lost potassium and sodium inside the chemical structure of the glass. We can, however, treat the physical symptoms; we can limit the damage and risk involved in those flaky layers of hydrogen glass.
Imagine the glass like layers of flaky pastry; there are air gaps in between the layers, and if you press on the outside they all collapse into each other and break. Treating the symptoms of that condition is fundamentally pretty simple: we fill the gaps between the layers with another material, bonding them together. This would be like taking your flaky, layered pastry and soaking it in glue. In addition to making you a remarkably unpopular dinner guest, the glue will fill all those air gaps, harden, and allow you to handle the material without the layers collapsing.
In glass, we usually bond the layers together with a clear acrylic, or sometimes a silicon-based material. Most of the time we will dehydrate the material in successive baths of different solvents before eventually soaking in a mixture of acetone and a consolidating material. Occasionally we apply consolidating materials to fill those gaps in water-based emulsions. Thin glass in good condition might be in this consolidating bath for a few days, thick glass in poor condition for a few weeks. Once the layers are consolidated we able to safely dry, handle, and re-assemble broken objects before sending them to the museum.
Images:
-Bottle base from the site, showing deterioration of glass. Image by NC Department of Natural and Cultural Resources.
-Different types of glass from the site. Image by NC Department of Natural and Cultural Resources.
-Fragment of flacon glass, before and after being repaired. Image by NC Department of Natural and Cultural Resources.