Previous studies of the mural paintings

ALEIX BARBERÀ-GINÉ, SÍLVIA LLOBET, SÍLVIA MARÍN, JAIME SALGUERO and PERE ROVIRA

Previous studies for the conservation of the decorative mural paintings in the Roman villa of Els Munts. The conservation problem of mural paintings in situ in outdoor environments

THE ROMAN VILLA OF ELS MUNTS IS ONE OF THE MOST IMPORTANT ROMAN SITES

IN CATALONIA, and presents occupation structures ranging from the 1st century BC until the 7th century AD.

This is a rich villa, profusely decorated with mosaics and mural paintings, intended for the leisure of the elites from the nearby capital of Tarraco. It consists of several sectors: the residential area itself (with rooms, triclinium, peristyle, etc.), the baths area, a garden area or hortus, a Mithraeum, an ambulacrum that connects the various sectors and an industrial area with several water tanks.

The residential area, from the high imperial period, consisted of a large peristyle and a two-storey building, of which only the semi-underground or cryptoportico area has been partially preserved, formed by a corridor with annex rooms. This space houses the group of most important Roman mural paintings currently preserved in situ in Catalonia.

Background

Excavations of the villa’s residential area began in the 1990s, and this is when this group of mural paintings that decorate a wide long corridor and five of the cryptoportico annex rooms were discovered. During the excavation, and to protect the paintings, they were bevelled with various types of mortars, and were protected by placing cloths adhered with resin, following common practice.

Furthermore, and aware of the importance of the uncovered ensemble, a temporary cover was placed to protect it from rainfall. However, after a short time, important signs of deterioration began to appear, associated with the presence of soluble salts which, due to their disruptive action, were disintegrating both the mosaic and the mural paintings. On occasion, several restoration interventions have been carried out to stabilise the ensemble, but to date they have not been able to guarantee the conservation of the mural paintings or to stop their deterioration.

At the end of 2018, in view of the pronounced and constant deterioration of the ensemble, the need arose to develop a project and prior studies that allow us to know the exact origin of the deterioration that affects the paintings, and suggest evidence-based solutions. This study has included a 3D survey, using photogrammetry of the whole ensemble; obtaining orthophotographs; making alteration maps; sample extraction and analysis (both of the paints and alteration indicators); cleaning tests; desalination tests and documentary research and cataloguing of previous interventions. The team of professionals who took part, under the guidance of the CRBMC, included: Aleix Barberà-Giné, M. José Gracia,

Aleix Barberà-Giné, conservator-restorer of cultural artefacts Sílvia Llobet, conservator-restorer of cultural artefacts (Àbac, SL) Sílvia Marín, conservator-restorer of cultural artefacts Jaime Salguero, conservator-restorer of cultural artefacts Pere Rovira, conservator-restorer, coordinator of the Area of Mural Painting, Stone Sculpture and Archaeology

General overview of the cryptoportico of the residential area of the Roman villa of Els Munts, where the mural paintings and mosaic have been preserved. Photo: Silvia Marín

Detail of the deteriorated surface of the mural paintings. The presence of a resin on the surface exposed to the weather causes small tears and losses in the pictorial layer. Photo: Aleix Barberà-Giné

Sílvia Llobet, Silvia Marín, Antònia Navarro, Pere Rovira, Jaime Salguero and Ricardo Suárez.

3D survey with photogrammetry and alteration maps

Photogrammetry allowed us to fully document the state of conservation of the ensemble at that time, and to carry out a study and analysis of the damage, both to the paintings in the rooms and to the mosaic.

For the 3D survey, a total of 4,953 photos were taken with the Sony a7RII camera, with a full frame size CMOS sensor with 42.4 mega-pixels, Sony Zeiss Planar T fe 50mm lens, indirect natural light, metric scales for sizing damage, and SpyderCheckr colour chart for colour and exposure management.

Independent 3D photo-models of the six polychrome rooms, the cistern, the annex rooms, the painted walls in the cryptoportico and the mosaic were obtained. An original of each photo-model is saved in OBJ format, approximately 30 million faces, to have a detailed reference at volumetric level and thus be able to control the appearance of future alterations, such as separation of preparation layers, superficial deformations or collapsing walls.

In the case of the mosaic, a digital elevation model (DEM) and virtual flush light treatments were also carried out to study the relief distortions and the wear of the mosaic tiles in very specific areas.

At the same time, a 3D photo-model of the entire archaeological site was also generated, to obtain general and up-to-date documentation, without the current cover that protects it.

From the different photogrammetries, orthoimages were obtained at scale and in high quality (0.3 pixels per millimetre) of each of the paintings in the rooms, as well as the mosaic. This allowed us to have accurate and quantifiable documentation of the losses and wear to the different layers of the mural painting, to determine endangered

Photogrammetric model obtained individually for room 4600. Author: Jaime Salguero

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One of the alteration maps made from the orthophotographs obtained through photogrammetry, on one of the walls of room 4600. Author: Jaime Salguero

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Key

Erosion of pictorial layer Loss of pictorial layer Loss of preparation layer Efflorescences of soluble salts Biological attack

areas, the appearance of efflorescences and soluble salts or biological attacks.

This valuable graphic documentation obtained allowed us to draw up very precise alteration maps, which made it possible to see these pathologies globally and to locate the most affected areas of the ensemble. Thus, it was possible to study the potential causes of the appearance of soluble salts and the consequent deterioration of the mural paintings and the mosaic tiles.

Analytical characterisation of materials

To characterise the mortars used to make the mural paintings, the mosaic of the cryptoportico and the paintings of a room in the domus, as well as discover the mineral composition of the numerous efflorescences that are documented in this area, an initial study was commissioned to the Materials Laboratory at the Escola Politècnica Superior d’Edificació de Barcelona ( EPSEB-UPC).

Samples were taken from three points with efflorescences: from a mosaic ceramic tile, from the middle of the east wall of the cryptoportico and from the mural painting of one of the rooms in the domus. Since the site is located next to the sea, initially it could seem that they were sodium

FTIR spectrum of saline efflorescence analysis compared to the sodium sulphate pattern. X-ray diffraction corroborated that the crystalline form corresponds to thenardite

M5T sample Sodium sulphate pattern

chlorides. The study of efflorescences was done with X-ray diffraction, and the results point to the presence of sodium carbonates on the mosaic and mural painting of the cryptoportico, in particular thermonatrite (Na2CO3H2O), gaylussite (Na2Ca(CO3)25H2O) and trona (Na3H(CO3)2(H2O)2). However, the analyses determined that almost all of the soluble salts present were thenardite (Na2SO4), a very hygroscopic and especially aggressive type of sodium sulphate, because it can increase up to 400 times its volume during its crystallization, with changes due to environmental humidity. This data introduced us to a much more aggressive type of salt than sodium chloride. Also, the study of the preparation mortars of the pictorial layers, both in the cryptoportico and the domus, identified chlorides, halite NaCl in particular.

The mortar granularity study was performed through dry disintegration and sieving, and the conglomerate composition was determined by X-ray diffraction and gravitational thermal analysis. It has been confirmed that the conglomerate is aerial lime and that the aggregates are of mixed siliceous and calcareous composition, with angular shapes and a maximum grain size of 8 mm. The mortar weight of the cryptoportico paintings is 1:4 and that of the domus room paint is 1:2. Furthermore, the mosaic preparation mortar has similar characteristics, although the mortar weight is 1:3.

Detail of the disruptive effect on the pictorial layer that generates the presence of soluble salts in the support. The constant crystallization cycles of the salts disintegrate the mural paintings. Photo: Aleix Barberà-Giné

Given the serious salt problems that were documented in all the structures preserved in the villa, the CRBMC physiochemical analysis laboratory extended the sampling and study. 38 samples of domus, peristyle, Castel and Tartana and southern bath deposits were analysed, with the presence of sodium sulphate being very widespread, although sodium carbonate and chlorides were also identified throughout. At the same time, 11 soil samples were taken from the site surroundings, which were identified as calcium carbonate, aluminosilicates and chlorides.

Study of the contents of soluble salts

One of the causes of deterioration that most seriously compromises the conservation of mural paintings is the presence of soluble salts in the substrate and their cyclical disruptive effect. Before starting a new conservation intervention, we considered it necessary to carry out a study to identify the types of soluble salts and evaluate their distribution in the facings, to check the level of impact that their presence had on the integrity of the paintings.

To study the soluble salt contents in the facings it is worth bearing in mind that this is a complex approach for several reasons. First of all,

because the soluble salt contents varies depending on the environmental conditions and the source of the salts. Secondly, because the salt contents also changes depending on the intrinsic characteristics of the facing, such as porosity, composition or possible materials added in prior interventions, so that it does not provide a homogeneous and regular response on its entire surface. And, ultimately, because any method to obtain a soluble salt content value will have technical limitations regarding the equipment used, the degree of dissolution of the salts extracted and other technical and methodological conditions.

Aware of these limitations, the intention of the study was to obtain systematic and comparable data, which could provide information on salt distribution patterns in the wall and be repeated over time. The possibility of extracting quantitative values, although relative to the previously mentioned factors, would allow us to evaluate the relationship between the paintings’ state of conservation and the temporary concentration of salts located at each point of analysis.

The methodology used to obtain the soluble salt value is based on conductivity tests with agarose. The analyses were carried out with the application of 5% Panreac® agarose discs in deionised water, which were applied for 20 minutes on the surface of the paintings, previously moistened with a drop of deionised water. Quantitative extraction of soluble salts was analysed with the Horiba® LAQUAtwin EC11 conductivity meter. The analyses were carried out systematically every 30 cm on the facing, but the distance could have been modified if deemed necessary. The values obtained were transferred to tables, and to make interpreting the results

Sample of the working method for the content analysis of soluble salts. The whole facing was divided with string into a 30-centimetre grid, and agarose discs were applied to the intersections, which after 20 minutes would be analysed with a conductivity meter. Photo: Aleix Barberà-Giné Statistical colour map with the conductivity results of the facings of room 4600, in which the uneven distribution of the salt concentration in the facings is observed. Below are the measurement points for reference that allow it to be repeated. Author: Jaime Salguero

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easier, colour statistical maps were also made to enable tracking and comparison with alteration maps.

The use of colour statistical maps is a graphic identification system that, apart from the quantitative value of salts (which is what changes depending on the different variables mentioned), gives us the areas of greatest risk, that is, with higher salt concentration. Unfortunately, the usefulness of this work methodology was corroborated only after three months, when new paint losses were identified within the systematic ensemble control and evaluation system. All alterations that occurred in the following months coincided with areas that had previously been identified as being of particular risk. Thus, we can anticipate the most sensitive areas within the same wall and define intervention priorities based on the relative values obtained. Further-

Table with conductivity results for room 4600 of the cryptoportico. The values have the corrected conductivity value (rest of the initial conductivity value of the agarose). Author: Aleix Barberà-Giné

LECTURES CORREGIDES DE CONDUCTIVITAT - HABITACIÓ CORRECTED CONDUCTIVITY READINGS. ROOM

A B C D E F

1 2 3 4

763 663 433 783 1073 463 303 563 6873 773 783 793 433 743 883 1043

5 6 7 8 9 10 11 12 13 14

3073 4573 1593 903 923 5773 1493 833 573 1863 1413 503 953 933 953 8953 763 733 533 523 3173 383 553 683 993 933 923 693 343 813 2173 393 313 643 433 673 643 443 833 763 443 443 1223 963 673 343 423 523 343 283 303 1083 533 443 523

15 16

833 543 613 563 453 603 973 1013 933 423 353 413

17

513 373 1073 433 613 433 18 6573 723 783 363 393 523

19

273 1773 1343 863 393 243 20 1133 1573 4073 933 1473 833

21

363 1743 1973 2373 733 1633

22

633 2273 763 2073 1173 653

23 24 25 26 27

543 3473 1223 1043 643 633 393 463 503 543 403 193 433 1433 453 853 553 393 873 823 1493 313 323 493 393 503 503 433 513 323

28 29

853 573 523 473 513 603 833 903 203 673 303 263

30

713 363 253 593 553 353 31 1343 753 203 313 318 213 32 5073 623 323 353 193 303 33 6373 613 663 493 563 713 34 3473 993 413 403 633 35 1823 423 203 403

36

683 433 163 243 37 1973 523 393 313 38 6573 473 39 3273 733

40

643 493 693 41 1593 663

* Valors on s’han restat els 127 µS/cm 2 del gel d’agarosa abans d’haver-lo aplicat.* Values where the 127 µS/cm2 of agarose gel have been subtracted before applying it

TOTAL : 209 MESURES TOTAL: 209 MEASUREMENTS

Surface deposits

Concretions Paraloid B72 layer

Pictorial layer: fresco or dry polychrome

Preparation layer: lime mortar

Stratigraphic scheme of the surface layers on the mural paintings. Author: Aleix Barberà-Giné

FTIR spectrum sample calcium carbonate pattern Paraloid® B72 pattern Analysis of a sample of the pictorial layer that identifies the varnish layer as Paraloid® B72

more, it also allows us to monitor the long-term progression of the distribution of soluble salt contents, if the operation is repeated at different times or before and after a desalination intervention.

Then again, to assess the real-time effect on the paintings we placed a camera that videorecorded the paintings for 20 days in a timelapse, with data captured every 5 minutes. In the video obtained we can see how the salts crystallize in efflorescences on the surface of the paintings and re-dissolve almost immediately in daily periodic cycles. Efflorescences form during the warmest and driest times of the day, while they solubilise during the wettest and coldest periods. This visual tracking is complemented with the placement of a weather station to measure both the environmental and surface temperature and relative humidity. Thus, we will know exactly during which environmental ranges salts begin to crystallise and, therefore, the paintings begin to deteriorate.

In relation to the source of these salts, which is essential for their subsequent treatment, we ruled out that they could come from the sea, as thenardite is not a common salt in seawater. We also ruled out its origin in the subsoil through the extraction and analysis of multiple soil samples. No sodium sulphates were identified in any of the subsoil points analysed on-site. Mortars applied in previous interventions were also analysed. Sodium sulphates were also not identified in the vast majority, but gypsum, i.e. calcium sulphate, was observed in some of them. It is possible that several mortars added in previous interventions, either cement or lime, may have contained gypsum in some proportion.

As found in the specialised bibliography, the formation of sodium sulphate in calcium sulphate and sodium chloride solutions is possible, due to the formation and stability constants of the various products in aqueous solution. Calcium sulphate is partially soluble in water and in the presence of marine sodium chloride can form sodium sulphate. Therefore, today, the main hypothesis is that the salts that affect the paintings are directly related to the application of materials added in prior interventions, in the presence of sea spray. Due to its solubility and morphology, the salt identified (thenardite) is much more aggressive and deteriorating than sodium chloride, which is why both accelerated and sustained damage has been observed over time.

In order to be effective, any treatment intended to resolve the impact of salts should include the replacement of all previously added mortars which may be susceptible to including gypsum in their composition.

Cleaning study

The cleaning tests are carried out on room 4600 and aim to remove several surface layers currently covering the mural paintings, but with

special emphasis on removing the remains of improper materials applied during previous interventions.

The presence of a thick layer of resin that covers the mural paintings obstructs the mortar’s porosity and affects the proper circulation of steam. It is logical to think that it may be a factor that aggravates the damage caused by salts. Then again, under the layer of resin there is a layer of deposits that irregularly covers the paintings. Therefore, completely cleaning the surface requires a double approach, which includes removing both the resin and the deposits and other layers of dirt.

A) Surface water-based cleaning tests

To work in a stratified and controlled way, we first chose to assess possible water-based cleaning of the most superficial layer of dirt, before removing the acrylic resin. It should be noted that prior water-based cleaning usually facilitates the subsequent action of the solvents, in the case of having to remove a layer sensitive to organic solvents (the absence of surface dirt on the resin enables better direct contact between it and the solvents).

After taking pH and conductivity measurements in the area where the surface dirt cleaning tests are to be performed, we observed that the area has a surface pH of 7.1 and a conductivity of 650 µS/cm2. A damping solution capable of removing surface dirt (basically formed by excavation soil remnants and saline efflorescences) is required, but which does not damage the pictorial substrate. Because it is a carbonate support, the choice of a chelating agent such as sodium citrate (pKfCa-Citrate 4.68), in addition to a neutral or slightly alkaline pH (within the safety range of the wall painting and in harmony with the pH of that area of the wall) should be a safe cleaning system.

Also, other mild damping solutions, with no chelating action, are tested, such as sodium acetate. In this case, the pH of the solution is lowered to 5.5 in case removing carbonate crust or other insoluble salts is required, on the points with less presence of acrylic resin (areas that seem to have a greater presence of surface carbonates other than the pictorial layer).

Furthermore, we decided to test an effective chelating agent to seize calcium ions from a carbonate salt, such as CaCO3, thanks to its pKfCa-DTPA 10.9. Two damping solutions with chelating agent DTPA applied: one with a pH capable of enabling the removal of carbonates, both by chelation (pK3) and by dissolution (pH 4.5), and another with a safe pH, for the carbonates constituting the work (pH 8.5), but with chelating capacity (pK4). Both solutions are undoubtedly systems that could be effective in cleaning the typically archaeological carbonate and carbonate soil layer, which is under the Paraloid® B-72 layer.

Area where the first solubility tests of the resin layer have been performed with different solvent mixtures: 1, acetone (Fd 47); 2, 75% acetone and 25% ethanol (Fd 44); 3, 30% isoctate and 70% acetone (Fd 63); 4, benzyl alcohol (Fd 48); 5, 50% acetone and 50% benzyl alcohol (Fd 48); 6, Xylene (Fd 83). We clearly observed how among the various mixtures, acetone leaves the surface saturated, while with the rest of the combinations the Paraloid® is removed in more depth, leaving the layer of surface deposits exposed. Photo: Aleix Barberà-Giné

FTIR spectrum sample Paraloid® B72 pattern

The following tests were carried out: A. 0.5% pH 5.5 sodium acetate buffer (pH buffer solution): it barely removes surface dirt. B. 1% pH 7 sodium citrate buffer (pH buffer solution): it removes surface dirt very effectively. C. 1% sodium citrate + 4% pH 4.5 DTPA buffer (pH buffer solution): it effectively removes surface dirt and appears to remove carbonate sediment residues in places where there is little or no resin present. Still, it does not seem to affect the pictorial layer since we do not observe colour removal. D. 1% sodium borate + 4% pH 8.5 DTPA buffer (pH buffer solution): it effectively removes surface dirt and appears to remove carbonate sediment residues in places where there is little or no resin present. Still, it does not seem to affect the pictorial layer, since we do not observe colour removal. E. Repetition of B (1% sodium citrate at pH 7) on an area with little or no resin layer: it removes surface dirt very effectively while not removing colour or attacking the original surface. F. Repetition of D (1% sodium borate + 4% pH 8.5

DTPA) on an area with little or no resin layer present: it very effectively removes surface dirt and also carbonate crusts, while not removing colour or attacking the original surface.

As a conclusion for this first surface dirt cleaning, the 1% sodium citrate buffer solution at pH 7 is effective and harmless for mural paintings. Even for the final cleaning proposal, the buffer concentration could be reduced to 0.2 - 0.5% until reaching isotonic conductivity with the surface of around 800 µS/cm2 .

B) Solubility tests with solvents

The resin was identified with FTIR as Paraloid® B-72 and the approach to its removal, according to usual logic, is based on the use of solvents. According to common practice for resin removal,

FTIR spectroscopy analysis of a sample of the pictorial layer that had been cleaned with acetone to remove the resin layer. Its peaks are clearly identifiable with the analysis, so we concluded that there is a large amount of residue with this work method

the first tests were based on the use of solvents (see table) to determine their solubility range based on the combination of various mixtures of isoctane, acetone and ethanol, and testing with other common solvents.

Paraloid®B-72 still showed high reversibility in solvents in which it is usually soluble.

Based on initial tests, we observed that while acetone seemed to achieve more satisfactory aesthetic results compared to other solvent mixtures, it was probably because it did not completely remove Paraloid®B-72. In fact, analysis with FTIR spectroscopy of samples cleaned with acetone still showed very significant signs of resin. All other solvent mixtures manage to remove the resin more effectively, exposing a white layer of deposits, probably carbonate, and a less satisfactory visual result.

It should be noted that all the solvent mixtures tested take on a greater or lesser degree of colour in the range of dark reds, probably because the nature of the pigment makes them more sensitive. The possibility of treating these areas

Results of the removal of resin with solvents

Solvent Fd Saturation Removal Evaporation Color Acetone 47 Yes Superficial Very fast It removes red AE1 44 No More effective Fast It removes red

IA7 63 No More effective Moderate It removes red

Benzyl alcohol 48 No More effective Very slow It removes red Acetone: BeOH 48 No More effective Slow It removes red

Observation with Dinolite® digital microscope (x10) before and after removing the resin layer with free solvents. We observed bright points of residues that could not be removed with the work method. Photo: Aleix Barberà-Giné

individually should be assessed. This colour removal is probably not due to the fact that the solvent can be aggressive to the paint layer, but because the original application of Paraloid® B-72 fixed dusty particles that, when removed, disappear with the solvents.

At a macroscopic level, removal of the resin using solvents seemed an effective and relatively fast method. However, observing samples with a digital microscope verified that, despite having removed most of the surface resin, there was still residue in pores and cracks. The risk of finding residues in hard-to-reach areas or even the danger of the resin re-entering the porous system of the mural painting was verified with FTIR analysis. We thus found that it was appropriate to choose another working methodology.

The next consideration could be the use of gel solvents, but traditional gel surfactant solvents are not stable on ionic surfaces. Therefore, given the large amount of salts present in the paintings, they were ruled out. In addition, they also involve

Working process for a cleaning test with polymeric emulsions with xanthan gum. Photo: Aleix Barberà-Giné the use of free solvents for removal (i.e. solvents that are not applied as gels or emulsions, such as those applied with a cotton swab or brush).

C) Cleaning with gel emulsions

Finally, we decided to remove the resin using polymeric emulsions. The use of traditional emulsions was ruled out due to the high content of surfactants they include in their formulation, which would become an added residue on the surface of the work, which we wanted to avoid. Using an emulsion could offer many advantages to solve the specific cleaning needs of the mural paintings at Els Munts:

–The resin could be removed without it re-entering the porous system of the mural painting. - The removal of both the resin and the layer of deposits could be achieved in a single process, if the aqueous phase is formulated for this purpose. - The use of solvent could be drastically reduced, if the work is performed in a much safer manner, both for the environment and for professionals. - It allows us to rinse the cleaning product with water and therefore basically use water-based cleaning to remove the resin. - Since the cleaning product is an emulsion without surfactants, the presence of possible residues derived from the intervention would be avoided.

For the water phase of the emulsion, we designed a solution of sodium borate at pH 8.5, with the addition of DTPA, with a total conductivity around 3,000 µS/cm2. This solution is in a safe pH range for the preservation of fresco mural paintings, but with the addition of chelating agent it is slightly able to remove thin carbonate deposits present on the pictorial layer. This

Low-light view of a cleaning test with polymeric emulsions, where we can observe the satisfactory removal of the resin layer and surface deposits, with the use, mainly, of water-based methods with the addition of only 10% of solvent. Photo: Aleix Barberà-Giné

water-based phase is then gelled with 2% Vanzan® xanthan gum.

For the solvent phase of the emulsion, several options can be considered, as long as they are immiscible in water. The first option that was considered was a 1:1 mixture of acetone and benzyl alcohol, since they are solvents that have low toxicity in small amounts. The proportion of solvent in the water-based phase was about 10% and the resulting emulsion is gently applied with a brush to the mural paintings. The application time is lengthened if working with this method, since small areas of about 10 square centimetres could require around 5-10 minutes. Finally, the cleaning product is rinsed with another buffer solution.

Another solvent option that was tested was the use of Rhodiasolv Iris®, a product that is considered a “green solvent” by the manufacturer, and which has a high solvent capacity for various types of resins. It is a dibasic ester, based on short branched alkyl chain dicarboxylic acids. Specifically, it is a 2-methyl dimethyl glutarate (INCI name: Dimethyl 2-methylglutarate, CAS number: 14035-94-0), presented by Solvay as a green alternative to many chlorinated organic compounds. According to Hansen’s solubility parameters (δH = 5.0; δP = 8.7) it is very close to the solubility of acetone, N-methyl-2-pyrrolidone, ethyl acetate and methyl ethyl ketone, among others.

Emulsions that incorporated only between 5 and 10% of this solvent had the same cleaning efficiency, but with a much shorter working time, less than 5 minutes. The resin removal capacity is evident at macroscopic and microscopic level, as corroborated by laboratory analyses.

Conclusions

Prior studies with the participation of multiple professional profiles have been decisive in understanding the state of conservation of the paintings and the source of the causes and factors of deterioration they present. The 3D survey of the ensemble and the use of alteration plans allows us to determine an initial state of conservation of the mural paintings and mark the guidelines for their conservation-restoration. This overview of the site’s state of conservation, which has been in place for many years, is an essential guideline for monitoring the deterioration of the ensemble over time.

Observation with Dinolite® digital microscope (x13) before and after removing the resin layer with polymeric emulsions. We observed a more regular surface at cleaning level and without perceptible Paraloid® residue. Photo: Aleix Barberà-Giné

FTIR spectrum sample cleaned with BeOH emulsion Paraloid® B72 pattern

FTIR spectroscopy analysis of a sample of the pictorial layer that had been cleaned with a polymer emulsion with a 10% solvent phase (50% acetone - 50% benzyl alcohol). No resin residue is observed Sample of an area cleaned with a polymeric emulsion with only 10% solvent phase and rinsed with water. With a single process, both the resin layer and the layer of carbonate deposits and surface dirt can be removed. Photo: Aleix Barberà-Giné

When it comes to cleaning, the use of emulsions is a more efficient system for removing layers of resin on murals, because it minimises the use of solvents and prevents the re-penetration of the resin inside the pores. It also allows us to remove the resin and the layers of deposits or surface dirt. In fact, today, the choice of waterbased systems is the most sustainable, safe and environmentally-friendly solution. The removal of this resin placed many years ago is very important to maintain the walls’ proper transpiration and prevent further alterations due to the interference of soluble salts present throughout the site.

Concerning the salt content studies, the methodology for obtaining conductivity readings with agarose discs is effective, and the combination with colour statistical maps allows us to determine the areas of greatest risk in a facing. Apparently, the presence of sodium sulphate, the main cause of deterioration, is due to the formation of a new salt, through the combination of gypsum present in some mortars and sodium chloride in sea spray. Therefore, any desalination treatment must provide for the extraction of all intervention mortars that may contain gypsum. At a strategic level, the widespread presence of these salts throughout the site is a reality, and is considered a priority given its high deteriorating effect on mural paintings and mosaic, and is confirmed as one of the main challenges at a conservation level in the coming years.

DETAILS

GENERIC CLASSIFICATION: archaeological and palaeontological material | OBJECT: site | MATERIAL / TÈCNIQUE: mural painting | DATA / ERA: Roman, from the 1st century BC to the 7th century AD | LOCATION: National Archaeology Museum of Tarragona. Roman villa of Els Munts, Altafulla (Tarragonès) | CRBMC REGISTRY NO.: diverse | CATALOGUING: Archaeological ensemble of Tàrraco declared a UNESCO Human Heritage Site. 2000. Cataloguing BCIN 2044-ZA