Deterioration of stony cultural relics and their geochemical characteristics

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  • Vot. 5 No. 3 GEOCHEMISTRY 1986

    Deterioration of Stony cultural Relics and Their Geochemical Characteristics

    WANG MINGKANG (~EIt~), Mo TIANLIN (~) and CHEN TUaUA (~ l l t~)

    (Nanjing University)

    Chemical reactions between cultural relics explosed to the atmosphere and aerial substances will lead to the deterioation of the relics tll. The aim of this paper is to relate the deterioration rate otF cultural relics to environmental conditions.

    Samples and Measurements

    Samples were colleted from the stone carving and stony city wall built up in the Ming Dynasty about 600 years ago in the eastern part a scenic spot and the southern part - - an industrial district of Nanjing, respectively. The samples, light grey in color and Carboni ferous in age, consist mainly of calcite with a minor amount of quartz. Thin sections prepared from altered layers of the samples were studied by using ihe optical microscope and X-ray energy dispersion analyzer.

    Results

    The typical thin sections prepared from altered layers of the samples collected from the eastern and southern parts of Nanjing City are shown in photos 1 and 2, respectively. Obvious differences are observed in alteration thickness for these samples. On the right side of the altered layer ( I in photo 1) are seen abundant air bubbles with diameters ranging from 50 to 1000 ~m, which amount to 60--70 percent by volume. These air bubbles look like a honeycomb on the whole. On the left side of the altered layer (II in photo 1 ) are observed some, but much less, air bubbles, which are smaller in size and amount to 20--30 percent by volume. In the unaltered layer (III in photo 1)consisting of calcite (comprising 70--80 percent by volume) and quartz there are some perfect clo~-spaced and regularly arranged crystals.

    Chemical compositions of these samples were determined by using X-ray energy dispersion analyzer and the typical results are given in Tables 1 and 2. It can be seen from Table I that the altered layer on the right side contains more sulfur and chlorine than that on the left side, i.e., the contents of sulfur and chlorine are 0.54~0.79% and 0.11-- 0.29% on the right side against 0.25% and 0.08% on the left side. Table 2 shows that the contents of sulfur and chlorine in the altered layer are 0.10--0.15% and 0.08--0.09%, respectively; they decrease to 0.03% and 0.03~ towards the unaltered layer. It is clear that the samples collected from the industrial district contain much higher sulfur and chlorine than those from the scenic spot.

  • No. 3 GEOCHEMISTRY ~7

    Table 1. Chemical eompositlon of the sample eolleeted from the industrial distriet

    No. Part measured

    1 Altered layer I

    2 Altered layer I

    3 Altered layer I

    4 Altered layer II

    5 Unaltered layer

    Composition(%)

    SiOt AltO s CaO Na20 MgO KzO C1 S 39.95 2.42 15.64 0.10 0.22 0.14 0.14 0.54

    11.57 1 .15 38.32 0.15 0.22 0.22 0.11 0.63

    1.73 0.38 49.17 0.25 0.15 0.14 0.27 0.79

    5.14 0.94 46.29 0.13 0.54 0.14 0.08 0.25

    1.38 0.68 49.36 0.09 0.29 0.20 0.07 0.O6 ] I

    FeO CuO

    0.41 0

    0.72 0

    0.13 0

    0.38 0.03

    0.42 0

    Table 2. Chemical composition of the sample collected from the seenir spot

    No. Part measured

    1 Altered layer

    2 i Altered layer

    3 : Unaltered

    Composition(%)

    "SiO~ AllO 8 CaO NatO MgO KtO CI S FeO Cu() 0.07 0.11 52.37 0.02 0.29 0.03 0.09 0.10 0.17 0

    0.28 0.34 50.03 0.07 0.31 0.05 0.03 0.15 0.37 0.04

    O.O6 O.O8 52.43 0 0.i7 0.01 0.03 0.03 0.12 0

    Significant differences in thickness, structure and composition of altered layers of the samples collected from a middle-sized city like Nanjing can be attributed to different environmental conditions. In the scenic spot the deterioration rate of cultural relics is much lower than that in the industrial district because in the former environment the population is sparse and there are no factories but more trees, while in the latter environ- ment the air has been polluted.

    Discussion

    In addition to the differences in thickness, structure and composition, the deterior- ation of samples collected from different parts of the city proceeds in quite different ways. Shown in photo 3 is a sample (thin section) collected from the industrial district, in which there is a narrow alteration belt in the unaltered part along some cracks. X-ray energy dispersion analysis shows that the contents of sulfur and chlorine, as shown in Table 3, are much higher in the inside of the belt than in the outside. Tile high contents of sulfur and chlorine in the belt seems due to air pollution. No sign of such deterioration on the samples collected from the scenic spot can account for the differences in deterioration rates of samples collected from different parts of Nanjing City. Due to the expansion of deterioration, the city wall surface in Nanjing exhibits an honeycomb structure (Photo 4).

    In order to clarify the effect of acid rain on the deterioration of cultural relics, pH values were determined for rain water which had been collected at our station located in the centre of the city where the air quality is above the average. Acid rain is defined as rain water with a pH value below 5.6. The observed pH values for rain water from June, 1980 to August, 1981 are given in Table 4. Similar observations in the scenic spot show that the precipitation with pH lower than 5.6 only accounts for 10.64% t21. The data suggest that different frequencies of occurrence of acid rain might be responsible for different de- terioration rates.

  • 288 GEOCHEMISTRY Vol. 5

    Table 3. Chemical composition of the secondary altered belt within the unaltered part of the sample

    No. Part measured

    1 Left side of the belt

    2 Left side of the belt 3 In side of the belt

    4 Right side of the belt

    5 Right side of the belt

    Composition(%)

    SiO 2 AlzO3'i CaO NazO MgO KzO Ct S FeO 0.41 0.27 48.43 0.91 0.18 0.11 1.02 0.78 0,15

    0.86 0.53 46.98 0.33 0.60 0.14 0.07 0.54 0.27

    5.48 2.38 7.98 3.24 0.67 3.53 1.34 8.79 0.85

    13.47 30.15 12.15 0.78 0.76 0.06 0 0.36 0.21

    8.88 39.70 10.30 0.91 0.56 0.66 0.11 0.31 0

    Table 4. The observed pH values for rain water from June,1980 to August, 1981

    Total pre-

    cipitation

    (mm)

    1420.7

    Precipita-

    tion with

    pH higher

    than 5.6

    (ram) 778.9

    Percent

    frequen-

    cy

    54.83

    Precipita-

    tion with

    pH between

    5.O---5.6

    (ram) 281.2

    Percent

    frequen-

    cy

    19.79

    m

    Precipita-

    tion with

    pH between 4.0---5.0

    (mm)

    360.6

    Percent

    frequen-

    cy

    25.38

    Photos 1--4: 1. Thin section prepared from the altered layer of the sample collected from the city wall (

    ~0),I the outer part of the altered layer, II the inner part of the altered layer,

    IIl the unaltered layer. 2. Thin section prepared from the sample collected from the

    scenic spot ( 160).I the altered layer, II the unaltered layer. 3. The secondary

    alteration belt (III) in the unaltered part (II) of the sample (I the ahered layer exposed

    to the atmosphere). 4. Honeycomb structure observed on the city wall surface in Ntmjing.

    Conclus ion

    Optical microscopic and X-ray energy dispersion analyses show that there are signifi-

    cant differences in thickness, structure, composit ion, and way of deterioration for the

    samples collected from various parts of different environmental qualities. Observations

    on acid rain lend great support to the conclusior, that different deterioration rates of

    cultural relics are closely related to atmospheric pollution,

    References

    [1] Del Monte M. et al., Atmos. Environ. 15,5(1980), 645---652.

    [2] Li Zhengfang and Gao Xuping, Environmental pollution and protection, 4(1982), 40.

  • No. 3 GEOCHEMISTRY 289

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