The paper support and the orange-yellow pigments
The microscopic examination of the paper sample by PLM revealed the furnish to consist of 80% bast and 20% cotton fibers, with protein and carbohydrate constituents. The absence of associated cellular material in the sample also suggested that the paper was not made directly from plant material, but rather from secondary or tertiary forms of recycled cloth or other material. A photomicrograph of the stained fibers is included in Additional file 1: Figure S2.
EDS analysis of the orange-yellow crystals removed from the folio (Fig. 3, top) identified As and S as major elements in an approximate atomic ratio of 1:1. A few small particles that appear brighter in the BSE image (Fig. 3, top) were found to contain Hg and S. The corresponding EDS spectra are included in the Additional file 1: Figure S1A, B. Raman spectra acquired in situ from the orange-yellow and yellow pigment particles used to paint the bodies of the lion and hare gave bands at ca. 172, 187, 191, 201, 220, 234, 273, 318, 331, 344, and 361 cm−1 that are consistent with frequencies reported for mixtures of pararealgar and the orange χ-phase (Fig. 4) [5]. The shoulders at ca. 183 cm−1 and at 211 cm−1 in this spectrum may indicate the presence of realgar [5]. Other bands reported for realgar overlap those of pararealgar and/or the χ-phase, while additional bands observed at ca. 137 and 149 cm−1 cannot be firmly assigned. Strong and medium-strong Raman bands expected for orpiment at ca. 292 and 309 cm−1 are not observed in the spectra acquired in the orange-yellow and yellow paint areas. A strong feature is expected at ca. 353 cm−1 for orpiment, and a maximum at this frequency is visible in the spectrum shown in Fig. 4; however, the χ-phase and realgar also give strong bands at this frequency. In the spectra recorded from the yellow areas, a relatively weak band was also observed at ca. 1087 cm−1, likely due to calcium carbonate [20]. This band is above the range of the spectrum presented in Fig. 4. Arsenolite was not detected by Raman spectroscopy in the painted areas of the folio. Analysis of the red outlines seen on figures, such as those delineating the head and upper part of the lion’s back (Fig. 1a), showed characteristic Raman bands for vermilion (α-HgS) at ca. 252, 282 and 343 cm−1. These results suggest that the figures in the Fatimid folio were originally painted red–orange with realgar and possibly other arsenic sulfides, and that their present, less intense hue is due to photodegradation.
The white crystalline overgrowth
EDS analysis of samples of the white crystals removed from the folio (Fig. 3, bottom) found As and Mg as principal elemental components (Fig. 5a). Raman spectra acquired from these crystals showed main bands at ca. 810 and 880 cm−1 that are consistent with data reported for the AsO43− ion (Fig. 5b) [21]; the feature expected at ca. 365 cm−1 for this ion is within the signal to noise ratio in the spectra obtained. X-ray microdiffraction analysis of the white crystals yielded a well-defined pattern with numerous sharp lines. An unrestricted search of the entire ICDD PDF-4+ database matched the pattern of hörnesite [Mg3(AsO4)2·8H2O] with a very high level of confidence. The file pattern for this mineral effectively accounts for the entire pattern of the unknown with excellent matches for all d-values and intensities. No other phases, including arsenolite, were detected (Fig. 6). A table listing the XRD data is included in the Additional file 1: Table S1. The XRD results are consistent with the EDS and Raman results discussed above as well as with the observed texture and morphology of the material, which compare favorably with published images of the mineral as found in nature [22].
Hörnesite belongs to the so-called vivianite group that includes several other arsenate and phosphate members with the structural formula, M3(XO4)2·8H2O, where M represents a divalent cation and X is either As or P. In other contexts, hörnesite is reported as a common As-bearing species in arsenic-contaminated soils, where it is thought to be formed by the reaction of As with mobile Mg ions [23,24,25]. The mineral has also been found in underground cave settings in association with other arsenates [26], and mineral specimen images have been published showing hörnesite as an overgrowth on arsenic sulfides [27].
It has been proposed that when orpiment and realgar photodegrade, arsenic trioxide is formed, which, if water is present, may dissolve and further oxidize to arsenic pentoxide; the latter species may then react with ions present such as lead, calcium and others, and become deposited in the paint system as an arsenate [1, 11]. Whether such is the mechanism in the present case remains a topic for further study. However, the results recall the widespread use of Mg-based reagents for the deacidification of paper documents and works of art and prompt the question as to the origin of this occurrence. These treatments have been developed to address the acid hydrolysis of cellulose, the major component of paper, that can lead to severe degradation of its mechanical properties [28]. Sources of acid in papers include organic acids that may form from the decomposition of residual lignin during the natural ageing of the paper [29, 30], as well as environmental pollutants, such as sulfur dioxide, nitrous oxide and nitrogen dioxide. Housing and mounting materials and artistic media, such as inks, paints, and adhesives, also present a risk [31,32,33]. Various deacidification methods have been proposed to remove or reduce the acidity of paper (see, for example, [29, 31, 34,35,36]). Treatments that involve immersion of the paper in aqueous alkaline solutions were introduced at the end of the nineteenth century, and basically consist in the use of reagents such as calcium hydroxide, calcium bicarbonate and magnesium bicarbonate to neutralize the acidity and reportedly provide an alkaline reserve that may protect the paper against further acidic attack [29, 31, 36,37,38]. While it has been reported that the calcium-containing compounds give better results compared to magnesium-containing ones due to an increased bond strength of the cellulose, both types of reagents have been widely used [33, 36, 39].
Synthesis of the white crystals
To explore the possibility that paper deacidification treatments with Mg-based reagents might lead to reaction with arsenic sulfide pigments to produce hörnesite, strips of filter paper painted with a mixture of orpiment, realgar, and pararealgar, in an aqueous gum arabic medium, were treated with a MgCO3 suspension and allowed to age at a high relative humidity and room temperature as described in the Experimental section. After 3 years, tufts of a white material were observed to have formed on their painted surfaces (Fig. 7a). X-ray microdiffraction analysis of the white material yielded a well-defined pattern with more than forty sharp lines in the front-reflection region. An unrestricted search of the ICDD PDF-4+ database resolved the pattern with a high level of confidence as representing predominantly arsenolite with strong evidence of hörnesite (Fig. 7b). A table listing the XRD data is included in Additional file 1: Table S2. These results suggest both the relative ease with which the arsenate may be formed as well as the conditions of time and/or environment necessary for the transformation to hörnesite, which appear to have been achieved completely on the folio but only partially on our synthetic example.