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Study of the degradation of 18th century alabaster sculptures through accelerated aging on test samples

Marta Oliveira

Abstracts

Two alabaster sculptures dating from the late 18th century show an unknown superficial degradation characterised by striated erosion, whose cause seemed to be water condensation or, alternatively, rain water. To test these two hypotheses, accelerated aging tests were performed on alabaster samples, and the morphology of the degradation layer and its composition were characterised. The results suggest that the condensation conditions were the main cause of the alteration.

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The author would like to thank MSc Fernando Costa, lecturer at the Polytechnic Institute of Tomar, for the mentorship of this study as well as for helpful suggestions; MSc Elsa Murta, technician at the Conservation and Restoration Department in the Institute of Museums and Conservation, for the mentoring provided during the internship; MSc Inês Gomes, Dr. Maria José Oliveira and BSc Ana Mesquita-Carmo for her contribution with the analytical techniques. A special thanks goes to Dr. António Candeias for the use of the SEM-EDS at the Hercules Center (Évora University), and also to Dr. Luis Nunes and Dr. Silvia Pereira of Stone & Ceramics Division from National Laboratory for Civil Engineering (Lisbon) for the use of the 3D optical profilometer. The author is thankful to Dr. António João Cruz, director of the Conservation and Restoration Master at Polytechnic Institute of Tomar for the proofreading of the paper. Finally, a last thanks to all the other people that have supported the author during her master thesis.

Introduction

1During the internship in the Sculpture Department of Conservation and Restoration of the Institute of Museums and Conservation (IMC) in Lisbon, Portugal, a set of two sculptures in gypsum alabaster was studied and restored (Fig. 1). The sculptures, dating from the eighteenth century and belonging to the collection of Grão Vasco Museum, in Viseu (inventory number 993/118 and 994/119, respectively) represent two mythological figures (Poseidon, the god of the sea, and Athena, the goddess of war). They are about 50 cm high and are carved on the front and on the sides with elegance and grace which points to influences from the Italian Baroque and Rococo.

2

Fig. 1 – The alabaster set: Poseidon and Athena

Fig. 1 – The alabaster set: Poseidon and Athena

The set of gypsum alabaster, representing two mythological figures, Poseidon (left) and Athena (right).

Credits: Luis Piorro, Conservation and Restoration Department in the Direcção-Geral do Património Cultural.

3In general, the sculptures were in reasonable condition, presenting some fractures, adhesives from old collages and a few missing parts. However, during the preliminary observation, in the front areas, an unusual kind of degradation characterised by a roughening erosion (Fig. 2) has been detected which was thought might be related with the nature of the substrate – the alabaster.

4This material consists mainly of gypsum, i.e., calcium sulphate dihydrate (CaSO4·2H2O), a sedimentary rock of the group of evaporites, precipitated from ancient seas and lakes due to water evaporation in arid climates (Dimes 2001). Alabaster is a fine-grained and compact material, extremely soft (hardness 2 in the Mohs scale), and easily carved. As a rule, a pure alabaster is translucent, and has a whitish tone, although alabasters with beige or red tone or coloured veins, caused by traces of minerals such as iron oxides and other impurities, are also common. Another feature of alabaster, with great relevance for conservation, is its solubility in water which is why the works are usually found inside buildings (Cheetham 1984:11).

5Based on the preliminary observation and taking into account the characteristics of a typical alabaster such as the solubility in water, it became clear that the roughening erosion should result from gypsum dissolution. However, for that dissolution at least two possible causes can be considered. The first is the dissolution caused by condensation of the atmospheric moisture, which, according to Larson, is one of the main causes of degradation of alabaster (Larson 2001). According to him, “the recurring movement of water droplets over a polished surface causes erosion similar to that of a river in a valley, although a chemical reaction between water and stone is also involved” (Larson 2001). The second possibility is the direct exposure of the sculptures to weather conditions such as sun, wind, fog and especially rain, what could have happened if the sculptures had been placed in outdoor, for instance, in the cloisters of the Grão Vasco Museum or, before the works have entered that collection, in any open space.

6To test these two hypotheses, accelerated aging tests were performed in samples of gypsum alabaster and the altered surfaces resulting from the exposure to condensation conditions and to tap water simulating the rain’s action were characterised with several analytical techniques and compared with the sculptures surface. This study, undertaken as part of the internship that took place at the Direcção-Geral do Património Cultural (Lisbon, Portugal) and was included in the Conservation and Restoration Master Course of the Polytechnic Institute of Tomar, Portugal, is described here.

Experimental

Alabaster samples

7Alabaster samples for the aging tests were obtained from a block of a decorative column made of gypsum alabaster (identified by X-ray diffraction), cut in cube shapes approximately 4.0 cm high, 3.5 cm long and 4.0 cm wide. The upper side of the samples was polished in a polishing device (DP-U3, Struers). The polishing was finished with a 3 µm and 1 µm abrasive pads combining with paraffin wax. The mass of the samples was determined after dried at 70 ºC during 24 h.

Accelerating aging tests

8Two series of tests have been carried out. The first, intended to test the condensation hypothesis, was done in a salt mist chamber (Ascott S120T model) that allows to control the fog and drying periods in addition to temperature and relative humidity. The samples were subject to six condensation cycles, each one composed by the following phases: 144 h of nebulisation at 35 ºC and 24 h at 50 ºC. Deionised water was used and the alabaster samples were labelled CT1 to CT4 (CT=condensation test). The second series was done to test the exposure to rainwater hypothesis. The alabaster samples were placed at the bottom of a washbasin and for the rain simulation a perforated bottle attached to the faucet with holes from which drops of water fell equally in all the surface of the samples has been used. In each cycle the samples were exposed to water with a slow flux during 8 h and after that were dried at 70º C for 24 h. The samples, labelled RS1 to RS6(RS=rain simulation), were exposed to three cycles.

Analytical techniques

9For the characterisation of the test samples, several analytical techniques were used. After each cycle, the test samples were weighed and the changes in the surfaces were observed by means of a binocular magnifier Leica IC 3DM2-16A.

10At the end of each aging test, the colour surface was measured in the CIELAB system, that uses the coordinate L* related to the lightness, the coordinate a* to the red/green value and the coordinate b* to the yellow/blue value. The samples were measured in three areas with a 3 mm diameter with a spectrophotometer Datacolor Check II Plus®, with a D65 illuminant and a 10º observer. The average of the three measurements was recorded.

11For each series, the samples were directly analysed, at the end of tests, by scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) (HITACHI S-3700 N microscope connected to a Bruker X Flash 5010 SDD, accelerating voltage of 20 kV, beam current of 0.1 mA, high vacuum). For each sample, the characterisation was done at three different areas.

12For the identification of the minerals, X-ray diffraction analysis has been carried (Bruker, D8 Discover AXS, using a Cu Kα radiation (40 kV, 40 mA and graphite monochromator), with a general area detector diffraction system (GADDS)). The diffractograms were obtained with a scanning angle of 8.0º to 70.0º (2θ angle), a step size of 0.02º and a scan step time of 300 s.

13The roughness increase due to alteration of the surface sample was measured in an area of approximately 6 mm2 in a selected location of each sample. For that purpose a 3D optical profilometer, Talysurf CLI 1000, was used. The analysis has been performed with a laser of a diameter of 8 nm, 2 mm/s of measuring speed, 5 mm/s return speed and a 5 µm spaced between lasers. Two roughness parameters were determined, Sa and Sdr. The Sa, expressed in μm, is the mean surface roughness and Sdr, expressed in percentage, is the increment of the interfacial surface area related to the area of the projected xy plane (for a completely flat surface Sdr = 0 % and for a non flat surface Sdr > 0 %) (Queiroz 2012).

14In all cases, the results obtained on the test samples after aging were always compared with those obtained on the same samples before aging. Moreover, the degradation surface of the two sculptures was characterised with the binocular magnifier and with SEM-EDS and that degradation morphology were compared with the alterations of the test samples after aging.

Results

Sculptures surface

15The alteration only occurs in the front surface of the both sculptures, and more extensively on Athena sculpture. With the binocular magnifier, it was observed that the surface alteration occurs in a form of translucent tiny furrows randomly distributed, which gives a striated erosion surface, similar to that described by Larson (2001) (Fig. 2).

Fig. 2 - Striated erosion  

Fig. 2 - Striated erosion  

Degradation observed with binocular magnifier (left) and a photograph of the same place (right), located on the left leg of Athena's sculpture.

Credits: Inês Gomes and Marta Oliveira.

16Due to its small size, the Athena's head, which was detached during the treatment, was placed in the SEM-EDS chamber and directly analysed on the right cheek. In the striated areas, the furrows show a loss of polishing and the surface is roughened (Fig. 3).

Fig. 3 – Furrow surface

Fig. 3 – Furrow surface

Electron secondary image (SEM-EDS) of the furrow surface, surrounded by polished areas, in the right cheek of Athena’s sculpture.

Credits: António Candeias, Hercules Center.

17The gypsum crystals shapes resemble fine slivers, due to their tabular cleavage (Fig. 4).

Fig. 4 – Gypsum grains

Fig. 4 – Gypsum grains

Backscattering electron image (SEM-EDS) of the gypsum grains of the furrow surface in the right cheek of Athena's sculpture.

Credits: António Candeias, Hercules Center.

18The elemental data show differences in composition related to the alteration. In the non altered surface aluminium, carbon, sulphur, calcium, strontium and silicon were detected. The origin of the elements besides gypsum was inconclusive. Although it’s possible that the elements can be derived from the abrasive pastes used during polishing by the sculptor or from the protective layer, which is a kind of wax. In the altered surface occurred a loss of carbon, aluminium, silicon and strontium, probably removed during the gypsum dissolution (Fig. 5).

Fig. 5 – Elemental analysis map

Fig. 5 – Elemental analysis map

Elemental map obtained through SED-EDS for carbon, aluminium, silicon and strontium, respectively, on the same sample and area as Fig. 3.

Credit: António Candeias, Hercules Center.

Aging tests

Microscopy

19In the accelerating aging test in condensation conditions, all samples loss the polished surface and became roughened and less translucent after the first cycle. At the end of the second cycle the CT3 and CT4samples revealed a degradation similar to that found in the alabaster set with the same striated form (Fig. 6, Fig. 7), while this was not the case for the other samples of the CT-type.

Fig. 6 – Superficial degradation

Fig. 6 – Superficial degradation

Striated erosion, observed with the binocular magnifier, on Athena’ssculpture in abdomen area.

Credits: Inês Gomes and Marta Oliveira.

Fig. 7 – Alteration of CT4 sample

Fig. 7 – Alteration of CT4 sample

Alteration observed with the binocular magnifier of CT4 sample after the test.

Credits: Inês Gomes and Marta Oliveira.

20After the following cycles there was an increasing development of the alteration in all samples, most pronounced in the CT2, CT3 and CT4samples. The striated morphology first appeared at the edges of the upper side of the samples and at the end of the cycles it spreads to the centre. In the CT1sample the surface alteration was much less obvious and restricted to the formation of a few tiny furrows. This difference might be explained by the heterogeneity of the block from which the samples were cut. The samples lost some weight at the end of the test, 3.84 % in average.

21Different results were obtained on the test samples exposed to the action of rain water. At the end of the test, the surface of the samples became concave or tilted with a roughened texture (Fig. 8), and a significant loss of weight was observed (23.8 %, in average).

Fig. 8 – Alteration of RS3 sample

Fig. 8 – Alteration of RS3 sample

Alteration observed with the binocular magnifier of RS3 sample after the test.

Credits: Inês Gomes and Marta Oliveira.

Colorimetry

22According to the colorimetric measurements, the aging processes caused a significant increase in opacity (revealed by the increase in the lightness parameter, L*) and enhanced green and the blue tones (shown by the decrease in a* and b* parameters, respectively) (Tables 1-2). The changes were significantly more pronounced in the samples subject to the condensation conditions.

Table 1 – Results of the colorimetric analysis, before and after the accelerated aging test under condensation conditions

Table 1 – Results of the colorimetric analysis, before and after the accelerated aging test under condensation conditions

Table 2 - Results of the colorimetric analysis before and after the accelerating aging test with rain simulation

Table 2 - Results of the colorimetric analysis before and after the accelerating aging test with rain simulation

SEM-EDS

23The SEM-EDS images showed different changes in the two test series. In the samples aged by condensation a loss of polishing and the formation of gypsum crystals with flat and angular shapes were observed (Fig. 9) similar to those observed in Athena’s sculpture (Fig. 4).

Fig. 9 – Gypsum grains of the CT3 sample

Fig. 9 – Gypsum grains of the CT3 sample

Backscattering electron image (SEM-EDS) of the gypsum grains of the CT3 sample after the test.

Credits: António Candeias, Hercules Center.

24In the samples exposed to the rain simulation, the gypsum grains are round shaped due to the dissolution by the water flow (Fig. 10).

Fig. 10 – Gypsum grain of the RS2 sample

Fig. 10 – Gypsum grain of the RS2 sample

Backscattering electron image (SEM-EDS) of the gypsum grains of the RS2 sample after the test.

Credits: António Candeias, Hercules Center.

X-Ray diffraction

25With the aim of identifying the alteration products, X-ray diffractograms were obtained for the CT1, CT2, CT3, RS2 and RS6samples. Each exposed surface were analysed in 5 or 6 locations, but all diffractograms showed only the presence of gypsum (CaSO4·2H2O) and no differences were observed in relation to the diffractograms of the gypsum before the tests (Fig. 11). Consequently, no alterations products were detected.

Fig. 11 – X-ray diffraction analysis

Fig. 11 – X-ray diffraction analysis

X-ray diffraction pattern of the CT2 sample after the test.

Credits: Maria José Oliveira, Conservation and Restoration Laboratory José de Figueiredo in the Direcção-Geral do Património Cultural.

Profilometry

26As an example, Fig. 12 presents the 3D maps obtained for the sample CT3 before and after aging, which show a significant increase of the roughness and the development of a striated erosion morphology.

Fig. 12 - 3D maps

Fig. 12 - 3D maps

3D profile of the surface of the CT3 sample before (left) and after (right) the test.

Credits: Silvia Pereira and Luis Nunes of Stone & Ceramics Division from National Laboratory for Civil Engineering.

27In general, the same was observed in the other test samples. The intensity of the alteration is also evident through the parameters reported in Table 3: both the roughness (Sa) and the superficial area (Sdr) parameters significantly increase during the aging tests for the samples of both series.

Table 3 - Sa and Sdr parameters of the samples analysed by 3D optical profilometry, before and after the aging tests

Table 3 - Sa and Sdr parameters of the samples analysed by 3D optical profilometry, before and after the aging tests

28The mass loss of the samples made the earlier smooth and polished surface became quite rough, which is reflected in the results in these two parameters.

Conclusion

29The experiments were helpful to understand which conditions caused the striated erosion on the alabaster set, by the alterations on the surface samples after the tests. The results of the two series of aging tests were quite different and only the series that employed condensation conditions lead to an alteration pattern, characterised by a striated morphology, similar to that observed in the sculptures. This lead to the conclusion that the condensation exposure is the main cause of alteration. According to what Larson argues, this morphology results from the action of the water drops that condense on the polished surface through a chemical reaction of the gypsum that leads to its dissolution and to the formation of a net of tiny furrows (Larson 2001). Although the mechanism of this process remains to be determined, the present study documents a case that adds to the scant literature on the alteration of works in alabaster.

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Bibliography

CHEETHAM, F. (1984). English mediaeval alabasters, with a catalogue of the collection in the Victoria and Albert Museum, Oxford, Phaidon.

DIMES, F. G. (2001). Sedimentary rocks, in J. ASHURST, F. DIMES (eds.), Conservation of Building & Decorative Stone, Oxford, Butterworth Heinemann, pp. 61-134.

LARSON, J. (2001). The Conservation of Stone Monuments in Churches, in J. ASHURST, F. DIMES (eds.), Conservation of Building & Decorative Stone, Oxford, Butterworth Heinemann, pp. 185-196.

QUEIROZ, J. R. C. (2012). Effect of airborne particle abrasion protocols on surface topography of Y-TZP ceramic, in Cerâmica, vol. 58, no. 346, pp. 253-261.

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List of illustrations

Title Fig. 1 – The alabaster set: Poseidon and Athena
Caption The set of gypsum alabaster, representing two mythological figures, Poseidon (left) and Athena (right).
Credits Credits: Luis Piorro, Conservation and Restoration Department in the Direcção-Geral do Património Cultural.
URL http://ceroart.revues.org/docannexe/image/3187/img-1.jpg
File image/jpeg, 52k
Title Fig. 2 - Striated erosion  
Caption Degradation observed with binocular magnifier (left) and a photograph of the same place (right), located on the left leg of Athena's sculpture.
Credits Credits: Inês Gomes and Marta Oliveira.
URL http://ceroart.revues.org/docannexe/image/3187/img-2.jpg
File image/jpeg, 60k
Title Fig. 3 – Furrow surface
Caption Electron secondary image (SEM-EDS) of the furrow surface, surrounded by polished areas, in the right cheek of Athena’s sculpture.
Credits Credits: António Candeias, Hercules Center.
URL http://ceroart.revues.org/docannexe/image/3187/img-3.jpg
File image/jpeg, 96k
Title Fig. 4 – Gypsum grains
Caption Backscattering electron image (SEM-EDS) of the gypsum grains of the furrow surface in the right cheek of Athena's sculpture.
Credits Credits: António Candeias, Hercules Center.
URL http://ceroart.revues.org/docannexe/image/3187/img-4.jpg
File image/jpeg, 88k
Title Fig. 5 – Elemental analysis map
Caption Elemental map obtained through SED-EDS for carbon, aluminium, silicon and strontium, respectively, on the same sample and area as Fig. 3.
Credits Credit: António Candeias, Hercules Center.
URL http://ceroart.revues.org/docannexe/image/3187/img-5.jpg
File image/jpeg, 108k
Title Fig. 6 – Superficial degradation
Caption Striated erosion, observed with the binocular magnifier, on Athena’ssculpture in abdomen area.
Credits Credits: Inês Gomes and Marta Oliveira.
URL http://ceroart.revues.org/docannexe/image/3187/img-6.jpg
File image/jpeg, 52k
Title Fig. 7 – Alteration of CT4 sample
Caption Alteration observed with the binocular magnifier of CT4 sample after the test.
Credits Credits: Inês Gomes and Marta Oliveira.
URL http://ceroart.revues.org/docannexe/image/3187/img-7.jpg
File image/jpeg, 48k
Title Fig. 8 – Alteration of RS3 sample
Caption Alteration observed with the binocular magnifier of RS3 sample after the test.
Credits Credits: Inês Gomes and Marta Oliveira.
URL http://ceroart.revues.org/docannexe/image/3187/img-8.jpg
File image/jpeg, 68k
Title Table 1 – Results of the colorimetric analysis, before and after the accelerated aging test under condensation conditions
URL http://ceroart.revues.org/docannexe/image/3187/img-9.jpg
File image/jpeg, 36k
Title Table 2 - Results of the colorimetric analysis before and after the accelerating aging test with rain simulation
URL http://ceroart.revues.org/docannexe/image/3187/img-10.jpg
File image/jpeg, 36k
Title Fig. 9 – Gypsum grains of the CT3 sample
Caption Backscattering electron image (SEM-EDS) of the gypsum grains of the CT3 sample after the test.
Credits Credits: António Candeias, Hercules Center.
URL http://ceroart.revues.org/docannexe/image/3187/img-11.jpg
File image/jpeg, 56k
Title Fig. 10 – Gypsum grain of the RS2 sample
Caption Backscattering electron image (SEM-EDS) of the gypsum grains of the RS2 sample after the test.
Credits Credits: António Candeias, Hercules Center.
URL http://ceroart.revues.org/docannexe/image/3187/img-12.jpg
File image/jpeg, 76k
Title Fig. 11 – X-ray diffraction analysis
Caption X-ray diffraction pattern of the CT2 sample after the test.
Credits Credits: Maria José Oliveira, Conservation and Restoration Laboratory José de Figueiredo in the Direcção-Geral do Património Cultural.
URL http://ceroart.revues.org/docannexe/image/3187/img-13.jpg
File image/jpeg, 68k
Title Fig. 12 - 3D maps
Caption 3D profile of the surface of the CT3 sample before (left) and after (right) the test.
Credits Credits: Silvia Pereira and Luis Nunes of Stone & Ceramics Division from National Laboratory for Civil Engineering.
URL http://ceroart.revues.org/docannexe/image/3187/img-14.jpg
File image/jpeg, 72k
Title Table 3 - Sa and Sdr parameters of the samples analysed by 3D optical profilometry, before and after the aging tests
URL http://ceroart.revues.org/docannexe/image/3187/img-15.jpg
File image/jpeg, 35k
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References

Electronic reference

Marta Oliveira, « Study of the degradation of 18th century alabaster sculptures through accelerated aging on test samples », CeROArt [Online], EGG 3 | 2013, Online since 10 May 2013, connection on 17 October 2017. URL : http://ceroart.revues.org/3187

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About the author

Marta Oliveira

Marta Oliveira is a graduate of the Polytechnic Institute of Tomar, in Conservation and Restoration (2010) and has a Master’s degree in Conservation and Restoration - Stone specialization, by the same institute (2012). Contact: marta.oliveira88@gmail.com

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Copyright

Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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