Patologías y estudio analítico de materiales procedentes de mosaicos de Carmona e Itálica-Pathologies and analytical study of mosaic materials from Carmona and Italica

Este trabajo presenta los resultados del estudio realizado en muestras de mosaicos romanos procedentes de Carmona e Itálica (Sevilla). El objetivo principal del estudio consistió en la caracterización químico-física de teselas de vidrio y fragmentos de mortero para determinar su composición, evaluar su estado de conservación y estudiar su tecnología de producción. Las muestras se caracterizaron mediante lupa binocular, MEB-EDS, espectrofotometría visible, DRX y análisis petrográfico. Los resultados indicaron que las teselas se realizaron con vidrio de silicato sódico cálcico, aunque una de ellas mostró elevados contenidos de óxidos de plomo y de cobre. Las muestras procedentes de Carmona presentaron patologías de deterioro asociadas a la humedad retenida por el mosaico durante su enterramiento, mientras que las muestras de Itálica presentaron huellas de biodeterioro. Asimismo, los resultados indicaron que en las teselas se utilizó un vidrio de inferior calidad técnica que el empleado en piezas de vajilla de cronología semejante.


SUMMARY
This work reports on the characterization of Roman mosaic samples from Carmona and Italica (Spain). A physico-chemical study of both glass tesserae and mortar fragments to determine their composition, to evaluate their state of conservation and to study their production technology was carried out. Samples were characterized by means of a binocular microscope, SEM-EDS, visible spectrophotometry, XRD and petrographic analysis. The tesserae were made from soda lime silicate glass, even though one of them showed high contents of lead and copper oxides. The samples from Carmona showed deterioration pathologies associated to the humidity affecting the mosaic during burial, while those from Italica presented traces of biodeterioration. The data also indicated that the glass used in the tesserae had a lower technical quality than that used in tableware pieces of similar chronology.

INTRODUCTION
The mosaic has been used throughout the History as a decorative item, even though it also plays a structural function since it is part of a floor or wall. They are composed of different layers named statumen, rudus, nucleus, mortar and the tesserae final layer (Figure 1a). The preparation of a Roman mosaic was documented by Vitrubio in De Architectura and by Plinio in Naturalis Historia libri. The first step of the process consisted on setting the statumen (formed by coarse stones), followed by the rudus (a lime and sand mortar mixed with stone fragments). Then the nucleus was added, which was composed of a lime mortar and finer elements such as grounded bricks or marble powder. On the nucleus the sinopia (a coloured design) was accomplished by means of ceramic or lead patterns, rules, strings, sectors, and other tools. Finally the tesserae were inserted (1, 2). These pieces, in general, were cubic in shape and could be produced on stone, marble, ceramic or glass. The different colouring of the tesserae allowed the elaboration from simple geometric motifs to complex figurative representations.
The most ancient mosaics were used as wall decoration. In the ancient city of Uruk (Fourth millennium BC), very simple geometrical motifs made of fired clay with black, red and white colours were found. During the seventh to the third centuries BC, the mosaics made with river or sea pebble stones were expanded throughout the Mediterranean basin. They were mainly used for decorating temples and sanctuary floors. This kind of mosaics was usually produced for private houses up to the first century BC. Nevertheless, the higher In the Iberian Peninsula the most ancient mosaics are dated between the seventh and the fourth centuries BC. They were produced with small stones which formed simple geometrical motifs. After the Roman conquest (third to second century BC), mosaic craftsmen coming from the Lazio were settled in the main cities of the Iberian Peninsula. They founded local workshops in which they made mosaics following Italic techniques and motifs. Polychrome tesserae started to be used in the second century AD, contemporary to the white and black pavement that was still used up to the fourth century AD (2-4).
The ancient city of Italica (Santiponce, Seville) is an example of the wide variety of mosaics. In this site more than one hundred mosaic pavements have been documented. Among them there are big rich polychrome decorated mosaics, as well as other simpler and less luxury. Unfortunately, a great part of these mosaics has been lost as a consequence of spoliation or poor conservation. In addition, those conserved are subjected to weathering conditions such as sun, rain and frost, as well as to local fauna and flora which attacked and disintegrated them slowly (5). In previous works carried out on several mosaics from Italica, the advanced biodeterioration experienced by both tesserae (6, 7) and mortars (8,9) has been shown.
The main objective of the present research was the characterization of two sets of building materials formed by glass tesserae and mortar fragments from a mosaic of Carmona and three mosaics of Italica. The characterization was undertaken with the purpose to determine their chemical composition and to evaluate their state of conservation in order to diagnose deterioration mechanisms developed on such materials. Finally, this work was also focussed on determining some technological aspects concerning production of these materials.

Description of samples
Two sample sets from Carmona and Italica (Seville) have been studied. Samples from Carmona belonged to a mosaic fragment recently discovered during public works. The fragment corresponds to the right bottom part of a big mosaic that represent the four seasons. The first set is composed of one complete tessera, seven tessera fragments and four mortar fragments ( Table 1). The tessera fragments were directly taken from the inner side of the mosaic (Figures 1b and 1c).  (2), b) Image from sample C-7, c) Image from sample C-12, d) Image from sample I-3, e) Image from sample C-3. f) Image from sample C-5, g) Image from sample I-2 (scales in cm).

Tesserae
Mortar Nucleus

Rudus Statumen
The mortar fragments showed remains of silica gel layers from the tesserae to which they were adhered ( Figures  1e and 1f). The colour indicated in Table 1 for mortar samples refers to the colour of this silica gel layer.
The second set of samples belonged to three different mosaics dated on the second century AD from the city of Italica (Santiponce, Seville): the Neptune mosaic from the Neptune House, the Planetary mosaic from the Planetary House and the Tellus mosaic from the Birds House. The set is composed of 5 samples: one complete tessera, three tessera fragments and one grip mortar to which a glassy tessera and a calcareous tessera were adhered ( Table 1). None of the samples is higher than 2 cm in length (Figures 1d and 1g). The Tellus samples presented, when they were taken before 1991, some parts of the original mortar and evidences of the biodeterioration caused by lichens (7).

Characterization techniques
The characterization techniques used to study the set of samples have been the following: binocular microscope, scanning electron microscopy and energy dispersive X-ray spectrometry (SEM-EDS), reflection visible spectrophotometry, X-ray diffraction (XRD) and petrographic analysis.

Surface characterization of glasses Samples from Carmona
Samples from the mosaic of Carmona showed breaks, cracks and wearing. Moreover, all the tesserae presented  (Figure 2a). Such iridescent layers are due to surface dealkalinization. The soil humidity directly in contact with the mosaic pieces causes the ion-exchange of water protons H + with the alkaline ions from the alkaline and earth-alkaline oxides of the glass (Na 2 O, K 2 O, MgO) following the reaction [1].
Humidity is propagated through the cracks and breaks of the mosaic pieces. That is why iridescent layers are also formed in the inner part of the tesserae, which could affect the mechanical integrity of the piece (Figures 2a and 2d).
Silica gel layers showed white or yellowish colouring.
In between iridescent layers of the glass samples, dark dendritic deposits were detected (Figures 2b and 2e). Sometimes such deposits coated the entire sample surface, as can be observed in sample C-10. EDS microanalyses carried out on the dark deposits of the most attacked samples (samples C-2, C-10 and C-11) showed a high content of lead oxide (PbO), iron oxide (Fe 2 O 3 ) and manganese oxide (MnO) ( Table 2).
On the surface of samples C-10, C-11 and C-12 some filamentous structures were observed containing a high calcium percentage (Figures 2c and 2f), which could be the result of biological activity in the past.

Samples from Italica
The tessera fragments from Italica showed the usual pathologies of a chemical attack developed after biological interaction (Figure 3a), as previous works conducted with similar pieces demonstrated (7), even though some tessera surfaces did not show evidences of the biological attack (Figure 3b).

Characterization of the glass bulk
The glass bulk characterization of tesserae gathers valuable information on the original glass since, in contrast to the surface, it has not experienced deterioration processes. From a total of 17 samples, only 4 (C-2, C-10, C-11 and I-2) presented a sufficient size to undertake a confident study of the glass bulk of the piece.

Samples from Carmona
The chemical composition of samples C-2, C-10 and C-11 corresponds to a soda lime silicate glass ( Table 2). This is the common composition of most Roman glasses of this chronology. The macroscopic observation from the bulk of pieces revealed a glass with many bubbles and not melted inclusions with brown and black colours ( Figure 4). This indicated a low quality and poor refined glass.
The sample C-2 is an opal glass, turquoise blue coloured. EDS microanalyses carried out on its polished surface, and consequently on the bulk glass, showed a high content of antimony oxide (Sb 2 O 3 = 3.9 wt. %) ( Table 2) that could be attributed to the presence of calcium antimonite. This compound was used in the Iberian Peninsula up to the fifth century AD as an opacifying agent (10)(11)(12). On the contrary, samples C-10 and C-11 are translucent glasses with a Sb 2 O 3 content lower than 2.5 wt. % (Table 2).    Table 2). This composition is similar to that used in frits and tile enamels of later chronology (fourteenth century AD) (13), and suggests a possible reusing of wastes from other glasses. However, the restoration and consolidation processes undertaken in this mosaic during the twentieth century, in which some polyester tesserae and glass from contemporary bottles were even used, could explain the different composition of this tessera.

Characterisation of the glass colours
The samples studied did not present a sufficient size to prepare a slab (1 mm in thickness) to be analyzed by absorption spectrophotometry in the visible range. For this reason the study of chromophores was undertaken by visible reflection spectrophotometry, even though the signal obtained by this method is weaker and less resolved than that obtained by the absorption method. In most cases the habit of the spectra did not allow the complete assignment of chromophores.

Samples from Carmona
The study of samples C-2, C-10 and C-11 was carried out on a clean and polished surface of the pieces. The reflection spectrum of sample C-2 showed the characteristic band of Fe 2+ -ions (1007 nm). The turquoise colouring and the EDS analysis of this piece (Table 2) indicate the overlapping of the Fe 2+ -ion band with that of the Cu 2+ -ion (not resolved in the spectrum). The spectrum of the dark blue sample C-10 presented the characteristic bands of Co 2+ -ions (535, 591 and 648 nm, respectively) and Fe 2+ -ions (1011 nm). In the spectrum of sample C-11, the reflection bands of Fe 3+ -ions (377, 420 and 460 nm, respectively) were assigned ( Figure 5).

Samples from Italica
Chromophores characterization was carried out on the surface of the glass samples I-1, I-3, I-4 and I-5, even though the surface irregularity diminished notably the spectra resolution. The habit of the spectra from samples I-1 and I-5 did not allow a conclusive assignment of chromophores. The spectrum of sample I-3 (dark blue) showed the absorption bands of Co 2+ -ions at 536, 592 and 649 nm, respectively. In the reflection spectrum of sample I-4, the bands of such ions were assigned at 540, 596 and 651 nm, respectively, as well as the bands of Fe 3+ -ions at 380, 425 and 457 nm ( Figure 5). The blue greyish colour of the sample is attributed to the chromatic combination of both chromophores: cobalt and iron.

Characterization of the mortar
The mortar is the layer, together with the nucleus, the rudus and the statumen, on which the tesserae are placed (Figure 1a). Mortar samples C-3, C-4 and C-6 presented remains of iridescent layers from the corresponding extracted tesserae, whereas sample I-2 exhibited two tesserae adhered, one of them glassy and the other calcareous.

Samples from Carmona
Crystalline phases detected by XRD (calcite and quartz) (Figure 6a), petrographic results ( Figure 6b)  mortar chemical composition (87.8 CaO, 6.9 SiO 2 wt. %) indicated the use of a lime mortar with a small proportion of silica sand. This kind of mortar is commonly used in mosaics of this chronology. Sample C-4 is the only one with an additional red layer between the mortar and the iridescent layer, showing variable thickness and porous appearance (Figure 7a and 7b). Its EDS analysis (  Sample I-2 is a mortar fragment adhered to a glassy tessera and a calcareous tessera. On its surface big sand grains (250 µm) have been detected, as well as lichen remains described in previous works (9) (Figure 6c). EDS surface microanalysis of the mortar showed a lower concentration of CaO and higher content of SiO 2 and MgO ( Table 2). This result could indicate that this lime mortar contains more silica sand than that used in the mosaic from Carmona. The presence of other crystalline phases, as dolomite, albite or aragonite, identified in mortars of the same chronology (14) must not be discarded. This mosaic was consolidated and restored with reposition materials and filling mortars in the twentieth century. Therefore, it is not possible to assign a Roman origin for this mortar.

CONCLUSIONS
The characterization of a set of mosaic samples from Carmona and Italica determined the chemical composition of the glass tesserae and their state of conservation. The majority of tesserae were produced with soda lime silicate glass, which is the common composition of Roman glasses used for tableware pieces and windows. Nevertheless, one of the samples from Italica showed a different chemical composition characterized by high contents of lead and copper oxides. Such composition could suggest the reusing or recycling of contemporary glass wastes. Moreover, the presence of bubbles, strings and not melted particles in the tesserae indicates the use of a low quality glass in comparison with that used in tableware glass.
Samples from the mosaic of Carmona showed a severe surface deterioration. The deep dealkalinization process experienced by the pieces caused the formation of thick iridescent layers which, together with dark deposits, have completely transformed the surface. In addition, deterioration has also damaged the integrity of the pieces since they were irreversibly fissured, fractured and fragmented. On the contrary, samples from Italica showed evidences of a chemical attack from biological origin. Such samples have been subjected to the attack of lichens, due to their outdoor exposition for many decades in which no cleaning or maintenance treatments were accomplished.