Seeking cures for glass disease
5/11/2002Wash good crystal in a dishwasher repeatedly, and you'll start the deterioration.
Put stained-glass windows in a church and wait a few centuries. The sickness takes hold.
Seal the Declaration of Independence in an airtight case, solder the edges with lead, and, against all odds, the glass will break down.
And, in the Toledo Museum of Art, or in any collection of glass objects, just wait. Glass disease, an illness without a cure, is almost inevitable.
Davira S. Taragin, director of the Center of Glass at the museum, points to a glass sculpture in solitary confinement. It's a work by a modern Italian artist, Ettore Sottsass. And it's sick.
It's displayed in its own small case, not due to any risk of contagion. Despite the term glass “disease,'' it's not catching. Rather, the work is sequestered to keep its environment under control. A sack of silica gel hidden in the display helps stabilize relative humidity within the case.
About 100 glass objects in the Toledo museum's collection have glass disease, says Cliff Craine, the conservator from Daedalus Labs of Boston whose job is to watch for such developments and reduce their ravages when possible. Another 200 suffer from another form of glass corrosion common to antiquities.
To assess the full scope of the damage, the museum will soon close its Art in Glass Gallery, home to about 2,000 of the museum's 9,000-piece glass collection.
“The gallery will be closed so we can do what we need to those pieces,'' Ms. Taragin said. It's still not clear whether the Glass Study Room will remain open, Ms. Taragin said. That will be determined only after further assessment of the room's humidity.
Some of the glass could ultimately end up in a “sick bay'' when a new $25 million Center for Glass building is completed in 2004. A thorough glass inspection will dictate the size of such a sick bay. It also could reveal whether the glass is healthy enough for the sick bay to be open to the fluctuating humidity created by crowds of visitors.
To the casual observer, the early symptoms of disease are almost impossible to detect. While glass “weeps” in the early stage of the disease, it's tough to see even a teardrop on the Sottsass sculpture. Atop the object, just to the left of its center, Ms. Taragin says, the glass is slick and moist. Take her word for it. Even experts can miss this stuff.
Glass disease starts so simply and subtly that it takes no more than steadily high humidity to begin the destruction of glass. Glass only appears impervious to water. In truth, water changes things.
The presence of water sets molecules in motion. Water shuttles molecules of hydrogen into the glass, inducing the glass to swap its own molecules of sodium or potassium - depending on the glass - in exchange for the hydrogen.
It's nothing much more complicated than that grade-school experiment where water dyed red flows into clear water until everything is equally pink.
Equilibrium happens.
In the case of water on glass, the equilibrium is between concentrations of alkali that makes up the glass - the sodium or potassium - and hydrogen in the water outside the glass. Nature demands equilibrium, and this exchange of alkali and hydrogen accomplishes that, says Dr. Blythe McCarthy, a Toledo native who is a conservation scientist specializing in glass and ceramics at the Smithsonian's Freer Gallery.
But now look what's happened.
Equilibrium satisfied, the water drop is altered. Instead of just plain water, a more-or-less neutral liquid, the sodium (or potassium) increased the water's pH. The water is now basic - meaning it has a pH of greater than 7 (pH of less then 7 is acid). Basic solutions are bad for glass. As the pH rises to 9 or higher, the slow process that began with a single drop of water accelerates. Passive deterioration becomes active destruction.
Now, instead of exchanging bits of potassium and hydrogen, the basic solution goes after the stuff that makes up two-thirds of the glass structure: silica.
“At high humidity, this happens faster,'' says Stephen Koob, conservator at the world's largest glass collection, the Corning Museum of Glass, in Corning, N.Y. “Anything above 55 percent relative humidity - a nice muggy summer day - and water is on the surface of the glass as a molecule. You can't see it, but if it's left there, it will actually leach out some'' of the sodium or potassium, also called alkali.
Once the process is in full swing “the glass will actually drip little rivulets of liquid alkali weeping down the glass. This will occur until the alkali (the sodium or potassium) dries out.''
Weeping glass cannot tolerate low humidity, Mr. Koob says. Should humidity drop too low, the water that displaced the alkali evaporates. The glass cracks in an effect called crizzling as water escapes. From here, it's all downhill, and glass will disintegrate with little encouragement if conditions don't improve.
“The stress has to go somewhere, so the glass cracks,'' says Mr. Koob. “It's microscopic at first. It just happens at the surface. You don't even see it. Eventually, you see a hazy cloudiness. You wash the glass, it looks like it's gone - well, the glass is wet. When the glass is dry again, it's back. It's something you can't clean off.''
“The drop to low humidity is extremely dangerous. The cracking can happen overnight if the humidity drops quickly,'' he said. All glass faces such risks, Mr. Koob added. At least twice a month he hears from people wondering why their dishwasher-cleaned crystal lost its luster.
“You're taking water and hot alkali, which is what soap is,'' he says.
“Your glasses are going to deteriorate much quicker if you run them through the dishwasher.''
The effect is also apparent in older double-pane windows, where trapped humidity eventually degrades the glass at the corners.
Even the state-of-the-art cases that sealed the Declaration of Independence and the Bill of Rights in 1952 suffered the ravages of trapped humidity. (See related article on Page 1.)
While the encasements were a marvelously advanced technology for their time, they couldn't stop the natural aging of glass and water, said Chris Barry of Pilkington Inc., the Toledo company that made the original encasements. Pilkington's German manufacturing site is making glass for the new encasement as well.
The Corning museum, with 40,000 pieces in its collection, attempts to prevent glass disease by keeping relative humidity between 40 and 55 percent museum-wide. Glass that's already crizzled - between 600 and 800 pieces in the Corning collection - should be kept as close to 42 percent humidity as possible, Mr. Koob said. Plus, the museum puts fans in cases with damaged glass in order to provide airflow.
There is no universal agreement on the perfect humidity for glass. Ms. Taragin said she hopes to stabilize diseased glass in the Toledo collection at 38 percent relative humidity.
But all seem to agree the real devil isn't a few percentage points of humidity, it's keeping the humidity stable.
“It fluctuates, and that's the issue,'' she said.
In essence, glass curators are merely living with the after-effects of artists perfecting a craft. Certain eras, and certain areas, are often associated with glass disease. Venetian glass artists in the 16th and 19th centuries, for instance, wanted to increase glass clarity. To do that, they tinkered with the glass-making formula. Venetian glass is often subject to glass disease.
Typically, glass is 65 percent to 70 percent silicon. Another 20 percent is alkali - either sodium or potassium. Alkali lowers the melting point of silicon. Without it, only modern technology would produce the temperatures necessary to melt pure silicon. Finally, the recipe included about 10 percent lime, which stabilizes glass. Due to its higher proportion of positive ions, lime actually prevents the exchange of negative alkali molecules with the positive hydrogen molecules in water. In other words, it tends to repel water.
But lime has a downside. It reduces glass clarity. Venetian cut the lime in their efforts to make clearest glass possible. It's no surprise, then, that Venetian glass is some of the most prone to crizzling. In addition, glassmakers who used potassium rather than sodium in order to reduce the melt-point also created a glass more prone to glass disease.
Modern artists, such as Ettore Sottsass, may also tinker with glass formulas to produce a certain effect.
“With the whole studio glass movement, the artists are always pushing the materials. They're making the glass do what it doesn't want to do,'' says Ms. Taragin. “My responsibility is not to say to the artist, `You can't do that.' My responsibility is to say, `How do we preserve it?'''