On the formation of hörnesite in a Fatimid manuscript folio

The chemistry of the arsenic sulfides and, in particular, the photodegradation of realgar (α-As₄S₄) as found in works of art and mineral collections have been extensively studied [1–8]. These studies have indicated the transformation of realgar to proceed stepwise through the intermediate χ-phase to the yellow polymorph of realgar, pararealgar (As₄S₄) and, ultimately, to arsenolite (As₂O₃). In the effort to explain the mechanism for this transformation, it has been proposed that incident light breaks As–As bonds in realgar, in preference to the stronger As–S bonds, thereby liberating free arsenic that is then intercalated in the rearranged cages of the pararealgar structure [7]. What has been shown to be the specific photosensitivity of realgar to wavelengths of visible light between 500 and 670 nm presents an intractable issue by itself where the display of works of art or mineral specimens is a concern. The physical alteration associated with the chemical degradation of arsenic-containing pigments, often seen as cracking and crumbling of paint layers and surfaces, poses a major conservation challenge [1, 9, 10].

However, beyond the structural breakdown of realgar and the consequent change in color, further effects are implied as a result of the dissolution of arsenolite in aqueous media, its oxidation to arsenic pentoxide and reaction with cations to form insoluble arsenates [1, 11]. As a possible example of one of these effects, we report here an occurrence of the magnesium arsenate mineral, hörnesite [Mg₃(AsO₄)₂·8H₂O], that illustrates yet another potential alteration product of arsenic sulfide pigments.

This occurrence was discovered on a rare, double-sided folio attributed to the Fatimid period in Egypt (969–1171 CE) and that is now held in the Department of Islamic Art at The Metropolitan Museum of Art (MMA acc. no. 54.108.3; Fig. 1a, b). Considered to be one of the earliest works in the Islamic collection, the folio was purchased by the museum in 1954 together with three other paintings on paper, all of which were reportedly found at the excavation site of Fustat in Old Cairo [12]. These works belong to a small group of surviving folios and their fragments that may once have been pages in bound manuscripts, of which no intact examples are believed to remain [12, 13].

On the page considered here, one side, labeled as the recto on the Museum’s website [14], depicts a lion in broad, brush-worked passages of what appears to be a muted and darkened yellow and orange paint, with outlines, details and verses of text rendered in black ink (Fig. 1a). The other side, labeled as the verso, depicts a hare with elongated ears and turned head, poised as if ready to jump. The features on this side appear to have been executed by the same hand, in the same style, and with the same palette as those on the side depicting the lion (Fig. 1b). Interpretations of the authorship and original context of the folio vary according to how the fragmentary Arabic inscriptions and literary associations are to be understood. One opinion emphasizes the text on the side bearing the lion, with a translation that reads, “In the name of God, the Merciful, the Compassionate/Ka’b al-Ahbar narrated and… through the speech of wild animals…/…the lion for the purpose of his teaching”, suggesting an association with a didactic tradition of fable literature [15]. Other views point to the text on the side with the hare and its partial translation as “Book on the speech of wild animals”, suggesting it to belong to Arab literary practices related to the classical Greek tradition of scientific handbooks and zoological treatises [12]; still other researchers have doubted whether the folio belonged to a book at all, citing the possibility that the inscriptions may well have been added at a later date [16]. Whatever its original context, the folio appears to be derived from or otherwise related to a manuscript titled, The Speech of Wild Animals, by the early author Ka’b al-Ahbar (d. 652/3) that was copied and expanded by Muslim scholars for centuries [17, 18]. As such, the folio belongs to the traditions established by Fatimid rulers, whose patronage of decorative arts and scientific inquiry led to novel figural forms that relied heavily on naturalistic animal motifs, featuring winged birds, spritely hares, and powerful lions [19]. Leaving aside the interesting issue of original purpose, our concern here is the present condition of the folio and, specifically, the discovery of an apparent alteration product affecting the orange and yellow pigments on both its surfaces.

The folio fragment was examined and analyzed by non-invasive techniques first to identify the pigments and the white crystals. The need to firmly identify these crystals and further characterize them prompted the removal of microsamples. To test whether paper deacidification treatments with Mg-based reagents may be responsible for the formation of the white crystals observed, experiments were carried out on paper samples painted with a mixture of As-based pigments in an aqueous gum arabic medium, followed by treatment with a MgCO₃ suspension.

A preliminary examination of both sides of the folio under low-power magnification revealed tufts of white acicular crystals superimposed on and exclusively associated with striated laths of orange and yellow pigment particles suggestive of arsenic sulfides (Fig. 2). To characterize the furnish of the paper, a microscopic fiber sample was removed from an edge of the folio and examined by polarized light microscopy (PLM), first as wetted with water and subsequently with Graff “C” stain, at 100×, 200×, and 400× magnifications.

Raman spectra of the orange and yellow pigments were acquired in situ, while samples of the white crystals were removed for analysis. A Renishaw System 1000 coupled to a Leica DM LM microscope was employed. A 785 nm laser excitation was chosen to avoid degradation of arsenic sulfide pigments that is known to occur when these pigments are exposed to light between 500 and 670 nm [7]. A 1200 lines/mm grating and a thermoelectrically cooled CCD detector were used. The laser beam was focused on the sample areas using a 50× objective lens with the power at the sample surface set to 0.2–1 mW using neutral density filters. Spectra were recorded with integration times between 60 and 240 s.

Using a tungsten needle under high magnification, samples of the white crystals found on the folio were also removed for analysis by energy-dispersive X-ray spectrometry (EDS), Raman spectroscopy and X-ray microdiffraction (XRD). No usable sample of the orange pigment could be removed for XRD analysis without risking damage to the surface of the folio. For this reason, only a few particles were sampled, and analyzed by SEM–EDS.

Scanning electron microscopy-energy dispersive X-ray spectrometry analyses (SEM–EDS) were performed with a Zeiss Sigma field emission-scanning electron microscope, equipped with an Oxford Instruments X-MaxN 80 SDD detector. Backscattered electron (BSE) images were acquired and EDS analyses were performed on uncoated samples mounted on carbon stubs under low vacuum (ca. 60 Pa) with an accelerating voltage of 20 kV. X-ray microdiffraction analyses were performed using a Rigaku Dmax/Rapid instrument. The analyses were conducted with Cu Kα radiation, an 800 μm collimator, and a graphite monochromator on the incident beam. Specimens were mounted on glass fibers with Ambroid^® nitrocellulose binder and oscillated about two axes of the goniometer during 15-min exposures. Patterns were interpreted with Jade 9.0 software from Materials Data, Inc. and the ICDD PDF-4+ database.

Strips of Whatman^® filter paper were painted with a ground mineral specimen containing a mixture of orpiment, realgar, and pararealgar (Ward’s Scientific) dispersed in an aqueous gum arabic medium. Subsequently, a MgCO₃ suspension was applied drop wise to the painted surfaces. The paper strips were then placed in sealed vials with sufficient water to insure a high relative humidity and allowed to age at room temperature with exposure to ambient laboratory lighting. These paper strips were examined monthly using a binocular microscope.

The mixture of yellow-orange arsenic-based pigments used to prepare the laboratory samples was determined by XRD to be predominantly a mixture of orpiment (As₂S₃) with realgar (α-As₄S₄) and pararealgar (As₄S₄). The magnesium carbonate suspension used was prepared by adding 5 g of MgCO₃ (Fisher Scientific) to 100 mL of distilled water and allowed to stand overnight.

The white crystalline material found associated with, and seemingly derived from, the yellow-orange arsenic sulfide pigments on the Fatimid folio fragment was determined to be the magnesium arsenate hörnesite, [Mg₃(AsO₄)₂·8H₂O]. While we have no record of treatment or other direct evidence to support an assignment of cause for this occurrence, the common usage of Mg-based reagents for the deacidification of art and archival works on paper–especially in archaeological fieldwork—suggests the possibility that in this instance the hörnesite may be a reaction product of such treatment. This conjecture may be supported by the apparent ease with which we have been able to synthesize hörnesite by aging, at high relative humidity and ambient temperature, a mixture of orpiment, realgar, and pararealgar applied on filter paper and treated with a MgCO₃ suspension, as well as by the well-documented sequestering of As by mobile Mg ions in As-bearing soils to form the arsenate. This work highlights the potential damage that can be sustained by works bearing arsenic sulfide-based pigments through exposure to aqueous deacidifying suspensions often used to treat works of art on paper and historic documents in museums and other institutions. Ultimately, in the case of the Fatimid folio, no conservation intervention was undertaken. The hörnesite presently exists on a scale that does not interfere with the aesthetic reading of the work of art, nor does it appear to present cause for concern with respect to long-term physical or chemical stability.