Summer 2002

Things Fall Apart: An Interview with George Scherer

The impermanence of stone

Jeffrey Kastner and George Scherer

A professor in the Department of Civil and Environmental Engineering at Princeton University, George Scherer is a materials scientist and a leading researcher in the field of stone conservation. Educated at the Massachusetts Institute of Technology, Scherer began his career in industry, working for companies including Corning and DuPont, where he specialized in sol-gel processing, a low-temperature method for producing ceramics with consumer applications including scratch-resistant coatings for eyeglasses. In the late 1980s, Scherer attended a conference where George Wheeler, a conservator at New York’s Metropolitan Museum of Art, discussed the use of various treatments, including sol-gel technology, in the preservation of art objects and architecture made from stone. After arriving at Princeton in 1996, Scherer began a research program into the effects of various environmental factors on ­stone artworks and monuments and potential ways to counteract them. Speaking on the phone with Cabinet editor Jeffrey Kastner, Scherer discussed the threats posed to stone by water, salt, wind, and pollution; the utilization of predictive modeling in forecasting their impact on various types of stone; and the potential application of such models in speculating about the long-term fate of objects ranging from marble sculpture to the face of Mount Rushmore.

Cabinet: What are the main environmental threats to stone?

­George Scherer: Water is basically the problem. The conservator at the Cloisters says that 90% of art conservation is controlling the flow of water, and that’s right. Many times you can do a lot by just fixing the roof and the downspouts and that kind of thing. One main mechanism of deterioration is freeze-thaw damage and another is salt crystallization. You can get salt into materials from a variety of directions—sometimes it’s in the groundwater, sometimes you can leach it out of the mortar, and sometimes it forms directly by chemical reaction with air pollution. And once those salt crystals start to grow, they can cause a lot of damage.

Are most treatments designed primarily to protect stone from harmful environmental effects or to stop deterioration that’s already started? Are these two very different kinds of processes?

They are different kinds of processes, and we may try to do one or the other or both. For instance, if you have an object that’s going to remain outdoors and it’s beginning to deteriorate, you would like to stop whatever’s doing the harm and restore some of its strength. Sometimes you can stop the problem and sometimes you can’t. Suppose I have a sculpture out in the middle of a plaza—there’s nothing I can do to stop rain from hitting it, so it’s going to get wet and it may not even be possible to prevent water from getting into it. You could imagine putting a water-repellent coating on the top surface, but if it’s standing out in the open, there’s a good chance that water will rise up from the ground by capillary action, the way it rises into a sponge. And if you can’t stop water from getting into it you can’t stop frost damage. So maybe the only thing you can do is to try to restore some strength to the stone and so you’re just treating the symptom and not the cause. There are other cases where you can treat the cause. So if you have small object sitting on a pedestal, you can take it inside and put a water-repellent coating on all of the surfaces so that water can’t get into it from any direction. And then you can eliminate the cause of the problem.

Are there treatments that do both things?

There are treatments that do both and treatments that only do one. For instance, putting a water-repellent coating on the material doesn’t restore any integrity to the internal structure. On the other hand, there are things you can soak into the stone, because most stones are surprisingly porous and will soak up material that will provide, say, water repellency and at the same time act like a glue to restore strength to the material.

We’ve been talking a lot about water repellency, but generally speaking that’s something that conservators are wary of. You can put a coating on a wall of a building you’re trying to conserve, but you can’t necessarily stop water from getting in through a leaky roof or rising up from the foundation. And if you let water get into a wall that has a sealed surface, then the water can’t get back out, and a freezing event can destroy the whole surface.

So you have to be careful what you’re trapping inside?

Exactly. The tendency is to use breathable coatings at the least, if not to avoid water repellency altogether, because it’s dangerous to trap moisture inside.

One main area of your current research deals with salt damage, with a process that works to reduce the destructive tendencies of salt crystals.

Yes. What I particularly like about that is that we’re attacking the mechanism and not just dealing with the consequence. Most of the things that people do to defend against salt have to do with restoring strength or controlling the flow of water, but what we’re trying to do is to deal with cases where you can’t prevent the water from getting in. For instance, you have a cathedral that’s too immense to treat and you may not be able to prevent the water from getting in, but this treatment would allow the crystals to come and go without doing any harm.

You know, the Sphinx is being damaged and there are reports by Egyptian experts who claim that the damage is mostly being done by salt, and that it has to do with the rise and fall of the Nile water table. When the water rises, it brings salty water up into the Sphinx and then when it dries out, the salts precipitate and do harm. And this is a case where you can’t get underneath the object and stop the water from rising up. Now it’s also possible that there’s an important issue of thermal shock, because the temperature changes quite a lot. And there was a recent paper that said you could go out in the morning and hear the Sphinx popping. It claimed that that was because the salts were precipitating; but you could argue that maybe it was the temperature going up.

Another important issue, of course, is acid rain. Limestone, the material the Sphinx is made from, is relatively soluble in acid and so is marble, whereas sandstones are not. Just the natural acidity of rainwater will dissolve marble and limestone away at a rate such that medieval marble sculpture would be showing serious distress. As the acid runs down it converts the calcium carbonate into a soluble salt that washes off. Natural rainwater in the most pristine environment is slightly acidic because of the CO2 that’s naturally in the air—a pH of 7 is neutral and you bring it down to 5.6 with the natural CO2 concentration. Incidentally, the “greenhouse effect,” the increase in CO2 in the air, has almost no impact on that, because you could double the amount of CO2 in the air and it would make very little change in the acidity. What really changes the acidity is other kinds of air pollution—so if you’re in New York City, the pH of the water could be down to 4 because you’ve got sulfates and nitrates and other things from burning fossil fuels. And those things bring down the water pH so much that they grossly accelerate the dissolution.

How important is geography?

A marble sculpture sitting in a pristine environment will decay slowly, and the same sculpture sitting in New York might decay 40 times faster—that’s the difference between a pH of 4 and of 5.6. So if you have a medieval sculpture sitting in a little village in Italy where it’s quite clean, it would show distress. A sculpture in Rome would have been fine up until about 1850, but in the 150 years since it might have lost all the features of its face. You can find dramatic photographs that show a sculpture photographed in the 19th century and it looks perfectly okay—it might be a little dirty, but all its features are there—and 60 or 100 years later it has no face.

So the Sphinx might also be showing the effects of the Cairo metropolitan pollution?

Yes. You’d also want to find out which way the wind was blowing.

Prevailing winds carrying pollution are an issue. What about wind erosion in general?

I think that’s a pretty minor effect. From what I’ve read about this, the sand grains aren’t easily picked off the ground by the wind. Typically the grains that are relatively heavy don’t get picked up more than about a meter—and the Sphinx, you’ll recall, is sunk in the ground, in a pit. In fact, that pit is large enough that the sand grains that come across fall into the pit and don’t hit the sculpture. But in the past, you know, it was buried in the sand and in that case it was being scoured by the sand that was actually able to come right up to the surface and hit it. And at various times over these thousands of years, the levels of the dunes varied. So apparently there’s a considerable amount of damage that was done by erosion in past centuries, when it was buried, and the damage would only occur right at the line where the tops of the sand dunes were.

So there are a lot of different things we need to be thinking about–wind, geography, the relation of geography to pollution patterns.

In the city, the sheltering of neighboring buildings and the direction of the wind can also have a big effect. And it’s not unusual to find that one wall or one corner of a building is deteriorating much faster than another, because it gets more wet, or whatever.

Are there models that can predict the effects of these various conditions on different objects? Could you take the David, for instance, and put it in Florence and turn the time machine to 100 or 200 years from now and get a sense of what might be happening to it based on these kinds of variables?

In some cases there are, and the easiest one is acid dissolution. Because you can measure how fast a marble or a limestone dissolves in acid. And if you know that it will take off a millimeter in 50 years, then you can extrapolate how long it will be before the nose is gone. For that kind of thing, you would need to know the level of pollution and the pH of the rainwater and how much rain falls in a year in that location.

But is the decay regular enough so you can say a millimeter is coming off the surface of the whole thing? Or is it conceivable that a millimeter lost in a certain place could then weaken some feature and cause it to fall off?

It’s a very good point. It wouldn’t lose surface uniformly all over—the rain for instance would probably hit it from one direction or another. Moreover, as the water runs down the cliff, the acid in the water is consumed by reaction with stone, so it becomes more dilute, and the resulting damage is not uniform over the surface.

Could we play this game with Mt. Rushmore? It’s not necessarily the case that the decay would be consistent over a large feature-rich environment like that.

I don’t know offhand what kind of stone it is, but in a place like that, I would guess that air pollution isn’t a big problem. Freeze-thaw damage certainly should be. You could imagine that there might be a weak layer at the base of the nose—you don’t have to wear away the nose from the tip back, you could just snap it off. So the heterogeneity of the cliff is an issue. If there’s a weak layer somewhere, then that’s where the damage will happen and everything in front of it could come off.

What about vegetation?

Things like lichen dissolve the stone. The way they get a foothold is that they exude acids and they can chew away at the surface and then send their roots down. There are some pretty dramatic photos of roots embedded in various kinds of stones—they’re capable of chewing up anything.

Everything from bacteria to algae to mosses and higher plants are capable of invading stone. The little guys will dissolve away the surface and get a foothold, but the rate at which they do damage is probably not so great. But as the higher plants move in, they’re capable of sending their roots down and really doing macroscopic damage. You’ve seen seedlings growing up through asphalt—they can sink their roots in and their roots swell and they can generate enormous pressures.

Like when the tree in front of your house flips your sidewalk up?

Exactly. So you could easily imagine that happening on Mt. Rushmore. You can use biocides to prevent plants from invading. I don’t know if they actually send people out to climb around and pull out saplings, but that could be a major cause of deterioration—if a plant opens a crack then the water gets in. Or it could work the other way around, where you have a fairly nice surface of stone that soaks up moisture and then in a sudden freeze, you get cracking from the freezing and that opens up an opportunity for plants to get in.

Is there a way to use models to forecast the long-term future of a large-scale formation like Mt. Rushmore? Presumably the weakening of the wrong area could lead to a catastrophic case of decay that can’t be forecast because you can’t know where it’s going to get weak.

Honestly, I doubt you could very accurately. You can do a predication like that under certain particular circumstances that you’ve touched on already—if the material’s homogeneous and it decays away at a uniform predictable rate and it doesn’t have any veins in it. But in the case of a mountain, that’s not likely to be the case. There are going to be faults in it and there are going to be places where moisture or plants or roots can get in and then you can have chunks coming off at a time. I mean you could have a whole ear or a whole nose fall off. In principle, you should be able to look around at the structure of the cliff and figure out where the danger spots are—if you can find porous layers or pockets where water could get trapped. If you were to show me a vein that was different from the bordering material, I could say, “This looks like a place we should divert water away from,” but I wouldn’t be in a position to say, “It’s going to fail in 50 years.” I would be surprised if you could really project forward 500 or 1,000 years.

George Scherer is a materials scientist and a leading researcher in the field of stone conservation. He teaches in the Department of Civil and Environmental Engineering at Princeton University.

Jeffrey Kastner is a New York-based writer and an editor of Cabinet.

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