Deterioration of Marble Structures The Role of Acid Rain Roger J. Cheng Atmospheric Sciences Research Center State University of New York Albany, N.Y. 12222
Jih Ru Hwu Jung T. Kim Show-Mei Leu Department of Chemistry The Johns Hopkins University Baltimore, Md. 21218
The city hall building in Schenectady, N.Y., a registered historical building, was built in the 1930s of the finest Ver mont marble, comparable to the Carra ra marble of Italy. It has already fallen victim to destruction by acid rain. The structure of the building has been weakened by the conversion of marble to gypsum. Even old structures that have survived several centuries have begun to deteriorate at much more no
which give rain an acid ic character. The sul fates then convert the calcium carbonate, an insoluble component of marble, into a soluble material known as gyp sum. The nitrates con vert t h e calcium carbonate into calcium nitrate. To slow down the formation of gyp sum, t h e destructive material must be identi fied and its origin deter mined. Analysis We used modern optical and classical analytical techniques to identify the destructive materi al. Optical microscopes
ANALYTICAL APPROACH ticeable rates. In Peking, China, there are marble monuments that are 500 years old (Figure 1) that contain the history of the empire. Until 40 years ago the inscriptions were legible; now they are unreadable. This clearly indi cates that most of the damage to the marble structures has occurred recent ly. The acceleration in destruction has created an interest in discovering how the damage occurs and generated con cern about the role of acid rain in this destruction. Increased emission of sulfur dioxides and nitric oxides by the industrial and private sectors has resulted in an in crease in acid rain formation and its destructive effects on marble. These materials, which are emitted from in dustrial smokestacks, are converted to sulfates and nitrates, respectively,
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Figure 1. A 500-year-old marble monument in Peking, China Forty years ago the inscriptions were legible, as shown in the rubbing (bottom). Now, they are unreadable (top)
with a photomicrographic system and dispersive staining capabilities were used to magnify the material several hundred times. Using this technique, we discovered that gypsum was always present in samples of deteriorated mar ble. For example, significant deteriora tion was found on the protected area of the balustrade from a 50-year-old mar ble building. The black crusted materi al on the surface consisted of massive gypsum crystals and industrial deposi tions. Cross sections of eroded marble, below the surface of the gypsum region, examined by a polarizing microscope showed wide, open, i n t e r g r a n u l a r spaces that had been penetrated by sulfuric acid (Figure 2). This indicates that sulfates converted the marble into gypsum. Further analysis of deteriorat ed marble samples with ion chromatog
104 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 15, 1987
raphy was carried out to detect the ra tio of sulfates and nitrates embedded in the marble surface. Although both sulfates and nitrates exist in significant quantities in acid rain, our findings re vealed that the concentration of sul fates on the marble surface was more than 20 times greater than that of ni trates. This increases the likelihood that the sulfates in acid rain are the material that destroys marble. In exploring the destructive effects of sulfates on the marble surface, the following experiments were carried out. We placed two pieces of marble, an Ital ian type with a surface area of 42.1 cm2 and a U.S. type with a surface area of 23.2 cm2, side by side in a container circulating aqueous sulfuric acid solu tion for 24 h. Two concentrations of sulfuric acid, 30 μΜ and 100 μΜ, were 0003-2700/87/0359-104A/$01.50/0 © 1987 American Chemical Society
Table 1. Deterioration of marble by sulfuric acid and nitric acid
Figure 2. Cross section (500X) of deteriorated marble examined by a polarizing microscope revealed wide, open, intergranular spaces that had been penetrated by sulfuric acid
u s e d . T h e m a r b l e s a m p l e s were weighed before and after each trial to measure the weight loss or, in this case, the deterioration of the marble sample. The solution was circulated to mimic the effects of rain splashing on the mar ble surface. In addition, it was replaced by fresh solution every four hours to keep the concentration of sulfuric acid within 10% of the original concentra tion. The experiment with the 30 μΜ sulfuric acid was intended to simulate the environmental concentration of rainwater.
The experiment was repeated under the same conditions and concentra tions using aqueous nitric acid solution instead of sulfuric acid. We found that the loss of marble in the 30 μΜ sulfuric acid experiment was almost three times as much as that lost in the nitric acid experiment. In the case of the 100 μΜ acid solutions, the marble loss in the sulfuric acid experiment was 13-17 times as much as that lost in the nitric acid experiment (see Table I). Using Xray dispersive techniques, we analyzed the precipitates on the marble surface
Acid
Concentration (μΜ)
Sulfuric acid Nitric acid Sulfuric acid Nitric acid Sulfuric acid Nitric acid Sulfuric acid Nitric acid
100 100 100 100 30 30 30 30
Type of marble
Italian Italian U.S. U.S. Italian Italian U.S. U.S.
Weight loss
