The experimental result of primary study on the micro-contaminants from underwater Kraak Porcelain, which indicated the disease types, disease sources and potential dangers of the contaminants of JDZ-QH and ZZ-QH were differ in degree rather than in kind.
Disease types
Different types of contaminants with elemental components of Fe, S, Ca, Na, Mg, Cl were found in the pores or cracks of the porcelain items by SEM-EDS. Among several contaminants, FeOOH and CaSO4·2H2O are confirmed by Raman spectroscopy and FT-IR analysis. Moreover, based on the research on harmful salts in underwater porcelain, it can be inferred that there are two kinds of harmful salts in the inner part of the porcelain. One is soluble salt, such as NaCl [8, 11]. The other is insoluble salts, such as FeOOH, FeS2, and CaSO4·2H2O.
Disease sources
These two types of diseases mentioned above (stains and cracks) are a result of a long term burial in the marine environment. Sources of the contaminants probably from seawater and/or other associated items in the shipwreck.
It was showed that, sea water contains a large number of ions, such as Na+, K+, Mg2+, Ca2+ and Cl− [20], and its pH value is stable in the range of 7.8–8.2. Slightly alkaline surface waters with a large number of ions make an undesirable environment for porcelain preservation [21]. These ions have small ionic radii (Na+: 102 pm, K+: 138 pm, Ca2+: 100 pm, Fe3+(hs): 65 pm, Cl−: 167 pm). Therefore, they can quickly enter to the porcelain through pores (about 50–100 µm), and finally cause corrosion and precipitation of sediments inside the porcelain. According to the detrital mineral grains statistics of Northern South China Sea, the average content of magnetite (Fe3O4) is 3.9%, and its kurtosis is 9.3. The average content of pyrite (FeS2) is 2.4%, and its kurtosis is 51.6 [22]. These data indicate that Fe and S are common elements in seawater. Moreover, it has been reported that FeS2 generally exists in artifacts excavated from the marine environment [23,24,25].
The redox in seawater influences the contaminant formation and transformation, and the process could be described by the following reactions:
$${\text{SO}}_{ 4}^{ 2- } + 9 {\text{H}}^{ + } + 8 {\text{e}} \rightleftharpoons {\text{HS}}^{ - } + 4 {\text{H}}_{ 2} {\text{O}}$$
(1)
$${\text{ClO}}_{ 3}^{ - } + 6 {\text{H}}^{ + } + 6 {\text{e }} \rightleftharpoons {\text{ Cl}}^{ - } + 3 {\text{H}}_{ 2} {\text{O}}$$
(2)
$${\text{Fe}}\left( {\text{OH}} \right)_{ 3} + {\text{H}}^{ + } + {\text{e}}^{ - } \rightleftharpoons {\text{Fe}}\left( {\text{OH}} \right)_{ 2} + {\text{H}}_{ 2} {\text{O}}$$
(3)
$${\text{Fe}}\left( {\text{OH}} \right)_{ 2} + {\text{H}}^{ + } + {\text{e}}^{ - } \rightleftharpoons {\text{FeOH}}^{ + } + 2 {\text{H}}_{ 2} {\text{O}}$$
(4)
$${\text{Fe}}^{ 3+ } + {\text{e}}^{ - } \rightleftharpoons {\text{Fe}}^{ 2+ }$$
(5)
The above chemical equations are reduction reactions in the marine environment from left to right, and oxidizing reactions from right to left. Equation (1) describes variation of the content of pyrite and its transformation, Eq. (2) describes the modification of the content of sodium chloride, and Eqs. (3), (4) and (5) describe variations of the content and conversion of iron hydroxides, from ferric hydroxide to ferrous hydroxide. Thus, as the environment changes, some of the crystals of soluble salts and contaminants would deform and cause damage in the vicinity.
The “Nan’ao I” shipwreck contains artifacts made of diverse materials such as ironware, bronze ware and woodenware. After the metal material was eroded by the seawater, the corrosion products might interact with each other or were transported by seawater from one object to another. Generally, high salinity environment can turn Fe into FeOOH easily and FeS2 is widespread in the wooden material in the sea. Other kinds of ferrous contaminants could come from the ironware corrosion [26, 27]. To evaluate whether cross-contamination occurs in the marine environment, sediments in different kinds of artifacts were analyzed by XRD. It was found that there was FeS2 in woodenware, FeFe2O4 and Fe8(O, OH)16Cl1.3 in bronze ware and FeOOH in ironware. From the XRD results (Fig. 6), it can be seen that different kinds of iron chemical compounds exist in different kinds of artifacts in the shipwreck. Therefore, it can be confirmed that cross-contamination occurred in the marine environment among all of the remains.
Marine microorganisms can cause biodegradation and biodeterioration, and cause several kinds of inorganic products including ammonia, nitrate, phosphate, carbon dioxide and hydrogen sulfide. These products might also affect the contaminants and salts formation in the porcelain body. These processes should be further explored in the future.
Potential dangers
An previous study suggested that the dissolving and recrystallization process of NaCl can lead to a fracture pressure of 221.9 mPa (2190 atmospheres equals to 221.9 mPa). High-pressure expansion or constriction in a small pore or crack can cause a detrimental impact in porcelain on a micro level [28]. Previous studies also reported that the repeated crystallization process of NaCl is the main cause of porcelain glaze cracking and peeling off [29]. However, as it was shown above, NaCl, CaSO4·2H2O, FeOOH and FeS2 are all found in the pores and cracks of the two kinds of porcelain items. The phase transition pressure of Fe2O3, FeOOH, and FeS2 can reach up to 60–120 GPa [30,31,32], which is at least 270 times higher than the dissolving or crystallization pressure of NaCl. Theoretically, it can be inferred that ferrous contaminants can cause severe damage to the underwater porcelain.
Potential dangers to JDZ-QH
Deionized water desalination method was used for JDZ-QH to preliminarily investigate the effect of ferrous contaminants. After 1 day of soaking, it was found that the NaCl crystals in the crack disappeared, but the ferrous contaminants expanded and the crack enlarged. The changes in size are shown in Fig. 7, and it can be seen that the amount of maximum deformation was 147%. This data indicates that, the ferrous contaminants can increase the crack size or make the pores irregular.
During drying, iron salts may expand to fill voids caused by unrelated forces. They may also grow to provoke fractures. Any reduction in the concentration of the ionic matrix (e.g., desalination bath) may cause a change in the separation of micro-cracks through alteration of Van der Waals forces, which may cause an expansion in the cracks filled with ferrous salts.
Potential dangers to ZZ-QH
As showed in Figs. 1 and 2c, the cracks at the porcelain are filled with orange contaminants, after the above experiments we can speculate that this orange contaminants are probably ferric or ferrous compounds. And deterioration of ZZ-QH caused by these contaminants is megascopic, the cracks are already been broadened and a whole porcelain item could rupture during the deionized water immersion.