Glass disease, weeping glass, glass deterioration, funky glass* (*author’s description)–just a few of the many names used to describe the degradation of glass beads that museums have observed as a white precipitate/cloudy appearance and/or cracking and splitting. If you’ve observed this in your collection, take notice- Mellon Fellow in Objects Conservation, Robin O’Hern, is on the case.
O’Hern has taken advantage of the history of glass disease detection at the National Museum of the American Indian (NMAI) and begun evaluating how the different cleaning methods have fared over the years. In 1999, Kelly McHugh (research supervisor and co-author) and Scott Carrlee performed a condition survey of the NMAI collection. The collection was moved into a state-of-the-art storage facility after the survey, where the RH has remained constant, but at a higher level than recommended for glass pieces. (The beads are present on composite pieces with hide, bone, shell, feather, hair, etc. and therefore the environmental controls must address as many materials as possible, not just glass.) Some of the pieces were treated at that time, and others have been treated in the interim years. Using the museum database, O’Hern found that 25% of the condition records that list glass beads as a material also list glass disease. O’Hern has performed another survey, this time seeking to observe condition changes over the past 15 years in a selection of objects from the 1999 survey, to assess treatment technique (ie, which solvents worked best to reduce glass disease), and to discover susceptibility trends (which beads are the worst culprits).
To understand the beads, O’Hern provided background on history of use and manufacture.
- Glass beads arrived after contact with Europeans in 1492
- Pony beads were introduced after 1675
- Wound beads were introduced after the late 17th century
- Seed beads were introduced 1710-1840
- Red beads were colored from copper in the 17th century, ruby red in the early 18th century, and selenium in the 1890’s
- Blue beads were colored from copper or cobalt, but from 1640-1700, they were tin-rich
- Beads can be made by pulling the heated glass, called “drawn,” or by winding heated glass around a rod, called “wound”
- Glass is made from silica, alkali (to lower the melting point, but also makes it water soluble), and calcium carbonate (that turns to lime- it’s added to help stabilize the glass after the alkali)
There are several explanations for the cause of glass disease. Too little or too much of the lime (part of the bead’s composition) may cause water to leach out of the glass matrix as ions that then form salt on the surface of the bead. The environmental conditions, such as fluctuations in RH, or materials in proximity, such as semi-tanned hide, may accelerate glass disease. As seen from the list above, the beads were manufactured over a range of time, in different ways, and in different places.
As you can tell, there are many factors to research when evaluating glass disease. O’Hern addressed as many as possible while still managing the scope of the project.
Condition Change: By comparing condition of the beads today to past condition/treatment reports, 16% of the beads have more deterioration now than in 1999. Measuring pH was used in addition to visual examination to determine condition. Some beads that did not look bad had a higher pH (above 7), signaling glass disease. Some beads that looked hazy did not have a higher pH, meaning no glass disease (perhaps hazy from manufacture).
Differing Manufacturing Techniques: Wound beads have it worse than drawn beads–95% of wound beads have glass disease. This could be because they have a compositional percentage of lime that is less stable.
Differing Colors: Black, red, and blue are the most disease-ridden. O’Hern looked through the museum database and found that the entries with the most “glass disease” indicated had blue beads. Blue beads are very clearly the “winner” of the glass disease competition, followed by red and black.
Treatment Techniques: Here’s where it gets even more interesting. The conservation literature and posts on the Objects Specialty Group list serve debate the use of three solvents to remove the salts on glass disease: water alone, ethanol alone, and a 1:1 water:ethanol mix. By comparing the 1999 survey to her own results, O’Hern capitalized on real-time aging to observe how each solvent mixture fares over time. Water-cleaned beads had a 50% rate of glass disease return; water:ethanol-cleaned had a slightly higher than 50% rate of return; ethanol-cleaned had the least amount of return at just under 50%. However, when looking at the beads cleaned with ethanol over the same time period as those cleaned with 1:1 water:ethanol (removing the very oldest treatments), the rate of return for glass disease falls to 40%.
(Note: Acetone has also been listed as a solvent for cleaning glass beads, but since the NMAI doesn’t use acetone, it was not included in this research.)
1. Measuring pH is essential because beads may look like they don’t have glass disease, but are actually more alkaline. Measuring pH is also quick and easy- cut your pH strip to a small piece, slightly dampen it in deionized water, press it onto the bead for 3 seconds, and then determine any color change in the strip.
2. The most affected beads were those sewn onto hide, but the disease was present when beads were in contact with many other materials as well.
3. Although cleaning with ethanol is a better choice for long-term disease prevention, the solvent chosen should still depend on the substrate around the bead.
Advice from O’Hern:
1. Record treatment materials when removing glass disease.
2. Take BT and AT details of beads so you can easily compare for condition changes in the future.
3. Measure the pH of the beads… and RECORD THE RESULTS.
4. Have consistent monitoring of glass disease.
As an audience member, it’s always exciting to see a project that has results, especially on a topic that is not studied as extensively as it persists. This is definitely a postprint worth visiting for more details and results.
For other examples (and some “good” photographic examples), visit Ellen Carrlee’s project “What’s that White Stuff?” that she and (then WUDPAC graduate intern) Christa Pack reported on in Ellen’s blog: http://alaskawhitestuffid.wordpress.com/2011/08/09/glass/