Raman spectroscopic analysis of an important Visigothic historiated manuscript

Abstract

Raman spectroscopy has been used to study fragments of early Visigothic historiated manuscripts from the important mediaeval library at Santo Domingo de Silos which were a part of a Beato dating from the tenth to the mid-eleventh centuries. These fragments are from some of the oldest manuscripts in the scriptorium of the monastery. In this study, a comparison is made between the pigments and inks used on these manuscripts and those used in a previous study of the unique Visigothic Beato de Valcavado in Santa Cruz, Valladolid, completed in the year 970, which is noted for its quality of execution as well as its content and is remarkable eschatologically in being identifiable as the complete work of only a single scribe. For comparative purposes, the pigments and inks used in the Silos Monastery Beato and a series of historiated early manuscripts from mediaeval times through to the Renaissance also held in the monastic library were analysed. Raman spectroscopy identified a range of mineral and organic pigments such as cinnabar, orpiment, minium, azurite and indigo. In addition, a number of admixtures were found, for example, indigo and orpiment to produce vergaut (green) and a mixture of cinnabar with iron-gall ink and cerussite to produce darker and lighter shades of red. Some interesting conclusions were drawn about the use of iron-gall and carbon-based inks.

This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’.

1. Introduction

The Visigoths, a conglomeration of tribes, flourished and spread during the late Roman Empire following their invasion of Italy and the sack of Rome in 410; they became Christianized and occupied the south of France, with headquarters in Toulouse and the Iberian Peninsula until the Mozzarabic occupation of Spain in 711 forced them north into the holding provinces of Cantabria and Asturias.

Santo Domingo de Silos, a Visigothic religious foundation, dates back to the seventh century and is located in the province of Burgos, in northern Spain. Up until the eleventh century, it was called San Sebastian de Silos after the abbot who first set up the library in 1041 but was later renamed after Saint Dominic of Silos, who died in 1073. In the early mediaeval period, Santo Domingo de Silos became the main repository of manuscripts along with Toledo Cathedral, but in the nineteenth century most of the manuscripts were dispersed, many being housed after purchase in the British Library, London, the Biblioteque Nationale in Paris and in Krakow. A particular feature of the scriptorium at Silos was the creation of written catalogues recording the manuscripts held between the eleventh and the eighteenth centuries [1]. This has facilitated the rehousing of several of the original manuscripts in Silos, including those manuscripts that are the focus of this research paper.

Towards the end of the eighth century, around 776, a monk called Liebana in the monastery of Santo Martin de Tuerieno prepared an illuminated manuscript commentary on the Apocalypse of St John. The original is now lost; however, it generated a proliferation of commentary texts over the next three centuries addressing heretical disputes and the terrors that awaited mankind at the end of the first millennium and the start of the eleventh century. Some 32 of these Beato manuscripts are known, 17 of which were written between the ninth and eleventh centuries, and these are now highly prized and treasured possessions in some of the most important libraries around the world including the Morgan Library in the USA, the National Library of Spain, the Bibliotheque Nationale of France and the British Library. The Beato de Valcavado in the Biblioteca del Colegio de Santa Cruz, Valladolid, is a Visigoth copy in two folio volumes which was completed in the year 970 and comprising 230 sheets of vellum and 87 historiated miniatures for which it is further admired and is considered to be the most complete and ancient survivor of these copies. A further five sheets reside in the National Library of Spain, detailing the genealogy of Christ. The Beato de Valcavado, also known as the Codex de Valladolid, was discovered at the Valcavado monastery in San Isidro de Leon and was then housed in the Society of Jesus College of San Ambrosio de Valladolid. It was then rehoused in the Santa Cruz Biblioteca after the expulsion of the Jesuits from Spain by King Charles VI in the mid-eighteenth century. It is unique among the existing Beato copies in that it contains the signature of its author and the dates of its commencement and completion [2]; the monk Obeco created this work in the monastery scriptorium of Valcavado, Palencia, Northern Spain, between 8 June and 9 September 970. The Beato de Valcavado is most interesting for its quality of execution as well as its content and is remarkable eschatologically in being identifiable as the complete work of only a single scribe. Because of the early dates of completion of the Visigothic manuscripts at Silos and that in Santa Cruz, which has been analysed previously, we are afforded an excellent opportunity to compare the pigments and inks that were in use at that time in different scriptoria.

An opportunity was presented for the Raman spectroscopic analysis of the pigments used in the historiation of this very important work of art during recent operations to produce a limited number of high-quality Beato reproductions for specialist purposes. This also enabled a comparison to be made between the Beato de Valcavado and other selected early manuscripts from mediaeval times through to the Renaissance, which are also held in the Santa Cruz Biblioteca.

Raman spectroscopy has been demonstrated to provide a powerful analytical means of interrogation of pigment composition in art works, with special relevance here to oil paintings, frescoes, wall paintings and manuscripts; some notable results have been obtained and reported in the literature, such as the Lindisfarne Gospels, Gutenberg Bibles, mediaeval cantoral song books and early lectionaries [3–7]. From these studies, it has been possible to provide novel information for art historians, eschatologists, conservators and artefact restorers, such as:

• — the identification of mineral pigments, their composition and the mixing technologies adopted; examples include the hierarchical adoption of pigment usage, the operation of economic factors in support of particular pigment usage and the mixing of pigment particles to effect colour changes;

• — the presence of adulterants, binders and evidence for later unrecorded restorative measures; this is closely monitored by contemporary restorers to isolate areas that have been over decorated using synthetic pigments from later periods such as Prussian blue, chrome yellow, Scheele's green and Comper green [8–12];

• — the presence of organic dyes in admixture with mineral pigments, often unexpected but revealing for a particular painter's preference, such as the admixture of cinnabar with carmine red [13];

• — the possible geographical origins and sourcing of the pigments, especially cinnabar and lapis lazuli [14,15];

• — evidence for degradation of the pigments through exposure to contaminants and/or biological invasion, especially lichen or bacterial growths for the latter and the degradation of lead white to galena through contact with airborne or neighbouring sulfurous deposits [16–18]; and

• — the interaction between the vellum substrate and the pigment and detection of contemporary or later repairs to the substrate—vellum is a reactive keratotic, proteinaceous material with active organic functionality in the form of CONH, CO groups and aromatic rings. In a Raman spectroscopic study of a mediaeval cantoral, for example, the discovery was made that an ancient repair had been effected which involved the lining of the vellum with an early cellulosic paper and this had been hitherto unsuspected as the original script had been superimposed upon this new material [19]; it is significant to note also that in their preparation for scriptoria vellums were treated chemically with reagents such as slaked lime, clays and alums as depilatory and degreasing agents and residues of these have been detected in mediaeval manuscripts [19,20].

In the studies reported here, we seek to especially establish the composition of the pigments, be it mineral or otherwise, and to monitor the presence of biological degradation by extremophilic organisms which have evolved protective strategies for survival in the presence of toxic elements and ions such as arsenic, lead, copper and mercury, which are commonly found as components in mineral pigments [18]. Raman spectroscopy has already been shown to provide an early warning of the presence of biological colonization of artworks which has not been detected visually, hence facilitating the arrest of the biodeteriorative processes and the conservation of the artefact before irreparable damage has been sustained [21].

2. Experimental procedure

(a). Specimens

All samples were obtained from several manuscripts located within the Monastery of Santo Domingo de Silos in the southern part of the Burgos Province in northern Spain. With permission, the pigments were sampled using a scalpel to take a small amount for Raman microspectroscopic analysis. Samples (AMS-N5, AMS-N6, AMS-N9 and AMS-N12) were obtained from sheets from a partially destroyed unknown Beato in the Silos Monastery dated from the tenth to eleventh centuries. Sample AMS-FN24 is from a fragment from another manuscript dated around the eleventh century. AMS-N1 is a fragment of a manuscript from the Santa María la Real Monastery in Nájera; it was an important pilgrimage stop on the Camino de Santiago. This manuscript includes commentaries of the Apocalypse and is dated to be around the tenth century. A range of samples dating from 1598 to 1824 (AMS1–AMS13) were collected from the coaching inn books deposited at the monastery. The samples were collected from the letters to investigate the ink composition (black with some red in few cases). Samples AMS-M1, AMS-M2 and AMS-M3 were obtained from the cover of cantoral books dating from the eighteenth century, providing a total of 52 samples for Raman spectroscopy analysis.

Multiple sampling was effected to facilitate the study of pigment mixtures and to address the question as to the way that particular colours had been achieved—for example, green can be produced by the use of naturally occurring minerals and earths such as malachite, verdigris and terre verte, or alternatively by the mixing of blue and yellow pigments. Other questions could also then be answered such as how were the tonal qualities of colours such as orange, purple, dark red and lights red achieved and were there any differences in the ways in which these colours were produced.

(b). Spectroscopic instrumentation

Detailed spectral analysis was undertaken using a Renishaw Raman inVia Reflex Microscope (Renishaw plc, Wotton-under-Edge, UK), equipped with an air-cooled charge-coupled device camera. The spectrometer was fitted with holographic notch filters and two gratings (1200 mm per line (visible), 2400 mm per line (near infrared)). The attached microscope was a Leica DM LM and was equipped with three objectives (×50/0.75NA, ×20/0.40NA, ×5/0.12NA) and a trinocular viewer that accommodated a video camera, allowing direct viewing of the sample.

Sample excitation was achieved using an NIR laser (TOPTICA Photonics AG, Graefelfing, Germany) emitting at 785 nm. Daily calibration of the wavenumber axis was required and achieved by recording the Raman spectrum of silicon (one accumulation, 10 s). If necessary, an offset correction was performed to ensure that the position of the silicon band was maintained at 520.50 ± 0.10 cm⁻¹. The spectrometer was controlled using a PC with instrument control software (Renishaw WiRE 3.0). Spectra were recorded using the ×50 objective over the spectral range of 3200–100 cm^–1 or 1800–100 cm^–1 with the accumulation of scans, exposure and a laser power modified to suit each sample. Spectra were not corrected for instrument response. All experiments involved the acquisition of spectra in triplicate. Spectra were obtained using a different sub-sample for the acquisition of each replicate spectrum of the 52 samples analysed a total of more than 200 spectra were collected.

3. Results and discussion

Preliminary studies of the Beato de Valcavado using a Raman portable spectrometer and probe head were undertaken in situ using both 633 and 785 nm excitation within the Historical Library of Valladolid [22]. Following these studies, it was decided to undertake a comparative investigation of the Beato de Valcavado with fragments of sheets from two early Visigoth historiated manuscripts located at the Silos Monastery from the tenth and eleventh centuries and also assumed to be a part of another Beato. In addition to this analysis, a further comparison was undertaken to include manuscripts dating from the late sixteenth to the early nineteenth centuries which comprised three samples taken from the covers of eighteenth century cantoral books, and 13 specimens from coaching inn books that were dated from 1598 to 1824.

(a). Silos Monastery Beato samples

Figure 1 presents a range of spectra measured from reference samples collected from the Silos Monastery Beato. These spectra are of pigments and materials whose characteristic Raman bands are shown in table 1. The identified pigments are orpiment, minium, cinnabar and the organic material indigo, which are not unexpected for the tenth century. These pigments were also identified in the Beato de Valcavado using in situ Raman microspectroscopic analysis.

Figure 1.

Raman spectra of typical pigments and materials found on the Silos Monastery Beato. (a) Orpiment, (b) minium, (c) cinnabar and (d) indigo. (Online version in colour.)

Table 1.

Position and assignment of bands of pigments and other materials identified in this study. Here vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad; sh, shoulder.

pigment/material identified	characteristic Raman bands	references
cinnabar	351 (sh), 343 (m), 286 (vw), 253 (vs), 142 (vw), 121 (vw)	[23]
orpiment	136 (w) 154 (s) 181 (vw) 202 (w) 220 (vw) 230 (vw) 292 (m) 309 (s) 353 (vs) 381 (w)	[23]
minium	550 (m), 390 (m), 313 (w), 225 (vw), 151 (m), 122 (s)	[23]
indigo	1704, 1617, 1584 (sh,s), 1573 (s), 1488 (w), 1462(w), 1365 (m), 1310 (w), 1248 (w), 1226 (w), 1086 (w, sharp), 758(w), 674 (w), 599 (m), 545 (s), 312(w), 278 (m), 265 (m), 252 (m-s), 236 (w), 173 (br), 137 (br)	[24–26]
azurite	1575 (w), 1456 (w), 1426 (m), 1095 (m), 841 (w), 399 (s), 245 (s)	[23]
iron-gall ink	1577 (br), 1480 (s, br) 1424 (sh), 1341 (m, br), 937 (w), 774 (w), 705 (w), 614/550 (br) 402 (w)	[27]
carbon black	1591 (br), 1316 (br)—ivory black—AMS2 1591 (br), 1320 (br)—ivory black—AMS42618 (br), 1608 (sh), 1578 (s), 1308 (br)—unidentifed—AMS61601 (br), 1316 (br) —black chalk—AMS10	[28]
vellum	2921 (m, br), 2869 (sh), 1664 (br, s), 1449 (s), 1298 (m), 1024 (w), 1002 (m)	[20]
cerrusite	1054 (s)	[23]
gypsum	1008 (w to m)	[23]
baryte	980 (m)
ilmenite	682 (vs), 370 (m), 230 (w), 166 (w)	[29]

(i). Inorganic pigments

Orpiment (As₂S₃) is a naturally occurring yellow mineral used as a pigment from ancient times until the end of the nineteenth century. The description of the colour ranges from canary yellow or golden to a brownish yellow [30]. The pigment was thought to have gone out of fashion around the ninth century, as it was not often found on manuscripts until later in the twelfth to fifteenth centuries. Orpiment was mixed with other pigments to produce other colours. For instance, mixing indigo and orpiment produced a deep green pigment known as vergaut [31]; this mixture was less deleterious to use on a manuscript compared with verdigris which could react with pigments such as orpiment and lead white and could result in parchment decay [32].

Cinnabar (HgS) has been used in Europe since Neolithic times. The main source of cinnabar in the ancient world was the Almadén district in Spain, which has been mined for approximately 2000 years [33]. During and after the Middle Ages, cinnabar was significantly more expensive than minium (Pb₃O₄), another red pigment commonly used on manuscripts [34]. Both minium and cinnabar were found on the Beato samples as well as on the other selected early manuscripts from mediaeval times confirming the importance of this manuscript. As cinnabar was found in addition with other red pigments, this would suggest that it was sparingly used.

The Raman spectrum of cinnabar, figure 1a, is dominated by a very intense band at 253 cm⁻¹ attributed to a ν(HgS) mode of A₁ symmetry together with two ν(HgS) modes observed at 343 and 286 cm⁻¹ assigned to the degenerate E’ modes [35,36]. Interestingly, previous studies have established that cinnabar from the Almadén mines contains α-quartz because of the volcanic origin of this cinnabar [37]. However, in the 29 spectra collected from the seven different cinnabar samples in this study, the characteristic Raman signature of α-quartz at approximately 463 cm⁻¹ was not observed. This finding raises the following two questions (i) what is the likely source of the cinnabar if not the Almadén mines? (ii) Or could the cinnabar source be the Almadén mines but was the pigment purified? A similar question was posed by Frost et al. [35], when using a combination of Raman spectroscopy and scanning electron microscopy (SEM) to investigate the likely origin of cinnabar from King Herod's palace in Jerusalem. It was proposed that two techniques could have been used for purification (i) sublimation or (ii) crushing and washing the ore [35] producing what is referred to as vermilion.

Typically, the term vermilion is used to describe the synthetic pigment manufactured by either a dry-process or a wet-process [38]. The process of making vermilion was invented in China around 300 AD but it was not until the eighth or ninth century that a manuscript Compositiones ad tingenda (Recipes for colouring) first described how to make this pigment [39]. It would indeed appear that in the thirteenth century it was novel to manufacture vermilion but was commonplace in the fourteenth century [38]. Cinnabar was an expensive pigment used sparingly in early medieval manuscripts until at least the eleventh century.

Another red pigment commonly used throughout history and found on the Beato is minium. Minium is a naturally occurring form of lead tetroxide (Pb₃O₄), also known as red lead, believed to be one of the earliest artificially produced pigments still in use today. Controversially minium has been historically used to describe variously a range of pigments including cinnabar and red lead. Fortunately, the Raman spectrum of minium which has a very intense band at 121 cm⁻¹ attributed to a ν(PbO) mode (figure 1b) is quite distinctive from the ν(HgS) mode cinnabar (figure 1c) at 253 cm⁻¹. In medieval times, the term minium was almost exclusively used to describe red lead [37], and it was produced by heating cerussite (lead carbonate) in air until the required colour was achieved [40,41].

(ii). Plant material

Indigo is a pigment that has been used since antiquity by the ancient Egyptians, Greeks and Romans. Indigo was extracted from Indigofera tinctoira plants native to India and was traded to Europe, in particular from Persia to Muslim Spain, from where it was distributed to other European countries [42]. A European source of indigo was produced from the woad plant Isatis tinctoria. It was common for indigo to be mixed with other pigments to produce other colours most commonly with yellow to make green [43]. Indigo has been well characterized using Raman spectroscopy and its identification is based on the presence of a band at 1571 cm⁻¹ which is attributed to the ν(C=C), ν(C=O) and ν(N–H) vibrations of the cross-conjugated system [24–26]. A typical Raman spectrum collected from indigo samples in this study is presented in figure 1d.

(iii). Admixtures

Pigments were often mixed together to achieve different colours or different shades of a colour. A range of admixtures found on the manuscript (table 2) included a green pigment found to be vergaut (figure 2a) a mixture of orpiment and indigo, a mixture of orpiment and red lead produced orange. Yellow was red lead and cerussite, impossible to say if this was a deliberate mixture of the two pigments or if the colour was produced by heating cerussite until yellow was formed. Calcite was added to indigo to produce a light blue, and in one instance both calcite and gypsum were found together. The Raman spectrum of a light blue pigment collected from sample AMS-NS also revealed the presence of a broad feature at approximately 1000–980 cm⁻¹ attributed to alum, a product used in the vellum manufacture. The presence of calcite in the tenth century manuscript samples could also reflect reaction of excess lime with atmospheric carbon dioxide in a moist atmosphere rather than its presence arising from a whitening agent.

Table 2.

Samples analysed and the pigments and other materials identified in this study.

sample identifier	year	pigments and other materials identified
Silos Monastery Beato
AMS-N5 (n = 6)	tenth to eleventh	orpiment, red lead, indigo, iron-gall ink (IGI)admixutres: orpiment + red lead (orange), orpiment + indigo (green), indigo + calcite (light blue) + alum
AMS-N6 (n = 2)	tenth to eleventh	red lead, admixture: red lead + cerussite (yellow), animal hair fibre, vellum
AMS-N9 (n = 5)	tenth to eleventh	orpiment, red lead, indigo, IGI,admixture: orpiment + indigo (green), orpiment + gypsum + baryte
AMS-N12 (n = 1)	tenth to eleventh	admixture: indigo + calcite + gypsum, orpiment + indigo
AMS-FN24 (n = 4)	eleventh	orpiment, cinnabar, iron-gall inkadmixture: orpiment + indigo, indigo + anhydrite + cerussite
Najera Monastery
AMS-N1 (n = 2)	tenth	cinnabar, IGI
samples from the covers of cantoral books
AMS-M1 (n = 8)	eighteenth	cinnabar, azurite, IGI, vellum,admixtures: cinnabar + IGI, IGI + baryte + gypsum, azurite + unidentified pigment
AMS-M2 (n = 3)	eighteenth	cinnabar, IGI, unidentified material
AMS-M3 (n = 4)	eighteenth	cinnabar, azurite, IGI
samples from coaching inn books—dating from approximately 1600 to 1800
AMS1 (n = 1)	1598–1602	admixture: IGI + barytes, vellum
AMS2 (n = 1)	1631–1635	IGI, carbon black
AMS3 (n = 1)	1640–1646	IGI, vellum
AMS4 (n = 1)	1697–1711	carbon black, IGI, IGI + gypsum
AMS5 (n = 1)	1680–1696	IGI
AMS6 (n = 1)	1668–1680	vellum, carbon black
AMS7 (n = 1)	1649–1662	ilmenite, IGI, admixture: ilmenite + IGI, ilmenite + IGI + baryte
AMS8 (n = 3)	1756–1758	cinnabar, IGI, admixture: IGI + lead white
AMS9 (n = 1)	1748–1756	vellum
AMS10 (n = 1)	1726–1747	carbon black, cerussite
AMS11 (n = 3)	1824	cinnabar, admixture: cinnabar + IGI + baryte + gypsum, IGI + gypsum
AMS12 (n = 1)	1777–1795	unidentified
AMS13 (n = 1)	1779	unidentified

Figure 2.

Raman spectra of (a) vergaut, an admixture of indigo and orpiment, and (b) iron-gall ink (IGI) found on the Silos Monastery Beato. (Online version in colour.)

(iv). Iron-gall ink

The black colour in the Beato samples was attributed to iron-gall ink and no trace of carbon was found, which is unusual. From the twelfth to the late nineteenth century, the most commonly used black inks were iron-gall [27]. Iron-gall ink was used as a writing medium supplanting carbon-based inks as the writing medium of choice from approximately 1130, but in this instance it has been used as a pigment [44]. The Raman spectra collected from a range of iron-gall inks sampled from the various vellum pages were very consistent and a typical example is presented in figure 2b. The identification of iron-gall inks was based on bands observed at 1478, 1341, 598 and 400 cm⁻¹. Within the literature, it is noted that the position of the band at 1341 cm⁻¹ is variable and can be found within the range of 1350–1310 cm⁻¹ and the broad band observed in figure 2b at 598 cm⁻¹ can be found with the range of 640–490 cm⁻¹ [27].

(v). Parchment/vellum

Vellum having superseded papyrus was used from the fifth century until the mid-fifteenth century when paper became more commonly used, because it was cheaper and easier to prepare [45]. The terminology of parchment and vellum is an interesting one: parchment is a generic term for animal skin that has been treated for writing purposes, so this would include goat, calf and pigskin. The term vellum is reserved for the very best, that is, calf skin. The highest quality calf vellum was soft, the texture was smooth, and there were no holes or ‘lacunae’ or other damage. Pigskin was a little coarser and goatskin was about the same. In order to prepare vellum the animal skin washed with water and then lime (calcium hydroxide) and was then soaked in a lime solution for several days to soften the skin, which was then scraped to remove the hair. The skin was then desiccated using either sodium or potassium chloride and the pH adjusted by treating with ammonium chloride or sulfate with lime and application of a potash alum. The addition of flour and egg yolk was used to produce a more supple skin [20].

Figure 3 presents two representative Raman spectra of proteinaceous material found within the range of analysed samples. The characteristic features of a biopolymer is evident in figure 3a being dominated by a number of intense bands within the CH stretching region (3000–2800 cm⁻¹), attributable to the CH groups of proteins and lipids, and the amide I band arising from a combination of the ν(C=O) and δ(NH) modes of the peptide bond [46]. This spectrum was collected from a sample of the Beato (AMS-N6) and is consistent with Raman spectra in the literature of a hair fibre which is not an unexpected discovery [20,46].

Figure 3.

Raman spectra of proteinaceous material collected from the Silos Monastery Beato: (a) hair fibre and (b) vellum. (Online version in colour.)

Figure 3b presents a spectrum of a second sample collected from the same sheet (AMS-N6) and is confirmed to be that of vellum. The chemical composition of vellum is primarily collagen type I, a protein with a triple α-helical structure. Degradation of vellum results in cleavage of the amino acid chains and peptide backbone bonds due to chemical oxidative deterioration. This leads to a secondary structure conformational change from an α-helix to a β-sheet or β-turn structure. The position of the amide I band at 1664 cm⁻¹ is indicative the secondary structure being predominately that of β-pleated sheet and not that of an α-helix (1654 cm⁻¹). In addition to chemical changes, bacteria attack will target the disulfide bridges of cysteine residues which have a characteristic signatures at approximately 643, 621, 522 cm⁻¹, assigned to stretching modes of the C–S and S–S bonds. These bands are not present in figure 3b, which indicates breakage of the S–S bonds and deterioration of the manuscript [20]. The broad feature centred at approximately 781 cm⁻¹ can be ascribed to a limewash a composite of calcium oxalate and calcium hydroxide used in the production of the vellum. The shoulder at 1711 cm⁻¹ and the band at 1375 cm⁻¹ can be attributed to ν(C=O) and ν(CH) modes of lipids, respectively. These bands may well be the result of lipid oxidation as recent studies hypothesized that oxidation of parchment lipids could lead to collagen degradation [47].

(b). Cantoral and coaching inn manuscript samples

Extension of this work to associated manuscripts held in the same location was also accomplished; and it was interesting to see the changing trend of pigment usage, as illustrated in table 2. Analysis of the later manuscripts indicated the continued use of iron-gall as a black pigment as well as an admixture with a range of pigments to achieve different colours. In samples from the Cantoral books covers from the fifteenth century iron-gall ink was mixed with (i) cinnabar to produce dark red and with (ii) baryte and gypsum to produce a grey colour. Iron-gall ink was found throughout the coaching inn samples dated approximately 1600–1800 again as a pure component or as admixtures. Of particular interest was sample AMS7, dated 1649–1662, where iron-gall ink was found mixed with another black pigment ilmenite (FeTiO₃) [17]. The Raman spectrum collected from ilmenite is presented in figure 4c and is characterized by bands at 682, 370, 230 and 166 cm⁻¹ [29].

Figure 4.

Characteristic Raman spectra of pigments and materials found on the samples taken from the covers of cantoral books and coaching inn books. (a) Azurite (b) carbon black (i) AMS2* (ii) AMS4 (iii) AMS6 (iv) AMS10* (c) ilmenite (*Spectra have been baseline corrected for clarity). (Online version in colour.)

It was also interesting that in the eighteenth century samples collected from the coaching inn books azurite replaced indigo and the discovery of the addition of carbon black as a darkening agent and not iron-gall ink. The spectra of the carbon black material collected from samples AMS2, AMS4, AMS6 and AMS10 are presented in figure 4b. The position of the D and G bands indicates that there were three different sources of carbonaceous material [28]. The spectra collected from AMS2 and AMS4 are attributed to ivory black, as this pigment is prepared by charring either bone or ivory it would be expected that the Raman spectrum would contain a band approximately 960 cm⁻¹ attributed to the inorganic phosphate band of bone and/or ivory. However, based on the work by Coccato et al. [28], who investigated a range of carbon-based black pigments using both 532 and 785 nm laser excitations, the phosphate band is not observed in spectra collected using a 785 nm laser as was used in this study. In future work, it would be interesting to re-analyse these samples with different excitation lines. The position of the D and G bands in the Raman spectrum of sample AMS10 at 1601 and 1316 cm⁻¹, respectively, is attributed to black chalk [28]. The final spectrum of this series collected from AMS6 is characterized by a number of spectral features including a broad band at 2618 cm⁻¹ (not shown), a shoulder on the D band at 1608 cm⁻¹, and the position of the D and G bands at 1578 and 1308 cm⁻¹, respectively. The origin of this carbonaceous material is unidentified. The presence of quartz in some samples was ascribed to the use of fine river sand both for grinding pigments and preparation of the vellum substrate for manuscript usage. Signatures of gypsum and alum were also observed in spectra (data not shown) collected from series of historiated early manuscripts (AMS1, AMS3, AMS6 and AMS9) indicative of the processing that the vellum underwent.

4. Summary

The combination of in situ analysis together with microscopic laboratory sample interrogation allows a more comprehensive interpretation to be made for a holistic appreciation of the early Visigothic historiated manuscripts. Further to the in situ studies undertaken on the Beato de Valcavado which used a more limited sampling regime, we note the presence of minium and cinnabar as the red pigments, orpiment as yellow, indigo as blue, an orpiment and indigo mixture as green and iron-gall ink as the black pigment. We also note the presence of hydrocerussite and vellum substrate in many of the spectra recorded here; the vellum would have been treated with lime and alum in its preparation for the scriptorium. In the earlier Beato studies, the presence of carbon was noted which was not found in the contemporary Silos samples; instead it is clear that the black pigment used in the Visgothic Silos manuscripts comprised iron-gall ink. However, the later manuscripts were found to replace iron-gall ink with carbon black, which was also used within admixtures of lead white to produce grey and red to produce a darker red. The green pigments were found to be a mixture of indigo and orpiment and not verdigris as reported in an earlier study of the Beato de Valcavado manuscript [22]. The Raman spectra of the vellum sampled illustrated that samples were generally well preserved, however, the initial spectroscopic signs of degradation were evidenced by the presence of lipid oxidation and the loss of the S–S cysteine bonds.

Authors' contributions

All authors were involved in the initial concept of the experiments. F.R.P. and J.M.G. conducted the preliminary experiment with portable equipment and sampled the various manuscripts. E.A.C. collected all the data used in this paper. E.A.C. and H.G.M.E. equally contributed to data analysis, interpretation and manuscript writing. F.R.P. and J.M.G. were involved in revising it critically for important intellectual content and all authors gave final approval of the version to be published.

Competing interests

We declare we have no competing interests.

Funding

This research was supported by the Australian Research Council (ARC International Linkage: LX0776464 and ARC LIEF: LE0883036 grants).