Monument Future

Traditional renders in the Vexin Français

Traditional houses in “Vexin Français” can be classified into three groups: big farms for growing of cereals with closed court-yards; workers’ houses and wine growers’ houses. Before the arrival of railway in the area, the price of raw materials forced people to build with local quarry materials: mainly limestones, sandstones in some areas or “meulière” (millstone). Ashlars and “meulière”, prestigious materials, were apparent but rubble stones were covered by renders.

Nowadays, many people think that plaster of Paris is a weak material, soluble in water (~2 g/l at 20 °C) and very sensitive to environmental conditions. Gypsum has been used in façades since antiquity and it has been usually employed in the last century in very different climate areas (Spain, France, Germany, Czech Republic, etc.). Some of these renders are well preserved. Blottas (1839) showed that gypsum at Montmartre (Paris, France) is a sulphurous limestone (“chaux sulfatée calcarifère”) composed of gypsum with more than 12 % of calcite. This mineralogical composition, (Blottas, 1839; Debauve 1884; Flavien 1887), does not correspond to the amount of calcite actually found in Parisian gypsum, varying from 2 % to 5 %. In traditional Plaster of Paris we find overcooked, undercooked and uncooked gypsum due to the inhomogeneity of the temperature distribution inside the kiln. It can be thought that the high durability of gypsum plaster with respect to outdoor environmental conditions comes from the presence of other minerals as clays (Sanz Arrauz and Domínguez, 2009).

Different kinds of renders have been used in the Paris area. Some of them are mixtures of plaster of Paris and aerial lime, around 12 % (Thénard, 1834; Le Dantec, 2016). In some cases the amount of lime in gypsum renders can be explained by a batching of plaster with lime water. Some gypsum-free mortars have been employed in the region, as mortars or as renders (Toussaint, 1841). Lime mortars are more difficult to apply, they harden slower than gypsum and it is less easy to achieve a flat surface. Rugosity of lime mortars is higher than that of gypsum renders due to the present aggregates and for this reason they are less used as finish renders in this region.

Materials and Methods

In this work we studied renders sampled in old façades and new renders formulations in order to compare their physical properties, composition and microstructure. New plasters are used in the rehabilitation of vernacular buildings but also in the restoration of buildings and monuments in historical cities.

Twelve fragments of façades from the Vexin Français area and about ten render formulations supplied by Plâtrière Vieujot were studied. Some façade samples come from the Tiffanie Le Dantec collection (TLD) (Le Dantec, 2019), while others were collected from the sites during this study (CN). They were taken before rehabilitation campaigns or from walls where the renders were about to be taken off. Some of the samples presented two different layers that were characterized separately. It is very difficult to date these renders by documents as most of the studied buildings are private houses or farms without any record on ancient restoration.

TLD collection consists in samples from:

— Théméricourt (THM, XVII century): A façade with two different states of weathering and different colour layers.

— Condécourt (CON, Château de Villette, 1663–1669): Façade with two layers of render, a grey layer and a white one.

— Hédouville (HED, XIX century): Two samples, one from the front façade with a grey upper layer and a white inner layer, and one from the rear façade with only one white layer.

— Fontenay Saint Père (FStP, XVIII century): White façade.

— Arthies (ART, probably XX and XIX centuries respectively): Two samples from one white farm façade, called “Moellons” et “Charbons”.

67CN collection

— Fontenay Saint Père (FStP): White façade, quite degraded

— Saint Clair sur Epte (StC): Two façades with lime render with large amount of aggregates.

— Genainville (GEN): Small part of a white render, almost completely disappeared.

— samples from non-well-maintained enclosure walls at Saint Gervais and Ambleville (W1, W2, W3) We observed façade samples under polarized light and digital incident light microscopy.

Formulations of new render samples of “Plâtrerie Viejot” have also been fabricated according to the experience of this company specialized in outdoor and indoor plaster renders. They reproduce in industrial kilns the traditional cooking of gypsum in “four culée” kiln type (XVIII and XIX centuries).

Water has been added manually to the powder until desired workability was achieved. This procedure corresponds to what is done during building works depending on the experience of the plasterer. For these renders only general results are presented as manufacturer consider the data confidential. Formulations consist in gypsum with different amount of lime but also with marble powder and charcoal.

Mineralogical compositions were determined by thermo-gravimetric analysis (TGA) and X-Ray powder diffraction (XRD). Porosity and apparent density were measured by the triple weight method (NF EN 1936); water vapour sorption/desorption tests were determined following the procedure presented in standard NF EN ISO 12571 with a Vsorp Plus apparatus. Water vapour permeability was measured by the standard NF EN 15803, method of wetcup. Compressive and flexural strength were measured following NF EN 13892-2 standard.

Most of new plaster samples have been measured about one week after their preparation, and some of them have been re-measured 30 days later. All samples were dried at 60 °C during 48 hours. Not all properties could be measured for all of the samples due to the limited size of historical samples and to the preparation procedure of new ones.

Results
Historic renders

The results cannot be considered representative of all the renders from the Vexin area because mainly gypsum renders were considered. The percentage of gypsum vs. lime renders in the area is unknown. Having survived weathering, sampled renders represent the most durable renders of XIX century. Experimental results show that samples from historic façades have a relatively uniform mineralogical composition (Table 1), close to formulations proposed nowadays. However, their physical properties are quite different, probably due to changes produced with time.

According to TGA analyses, in most of the historic samples, except StC, the amount of gypsum ranges from 80 to 93 % and the amount of calcite from 1 to 8 %. The latter may either be an impurity of the natural gypsum, indicate high temperature calcination, or correspond to the addition of lime or calcite aggregate to the plaster. The amount of non-detected minerals by TGA varies from 2 to 16 %. In order to identify this unspecified portion, XRD analyses were done. Quartz but also anhydrite, 68halite and aragonite were found. In StC samples, only calcite was found with over 80 % of unidentified minerals by TGA, which indicates a lime render with a large amount of aggregates.

Table 1: Thermo gravimetric analysis results for ancient samples. Gyp: Gypsum, Cal: Calcite, ND: Non-determined.

The grey colour of samples from CON and HED may come from the addition of charcoal. The change of colour from grey to white and a peak at 430 °C seem to indicate the presence of charcoal (Shi et al, 2012).

Thin sections and polished surfaces (Fig. 1) of samples from THM, CON, HED, FStP, ART and StC were observed. Figure 1. THM, CON and ART samples present two different layers. We can observe the differences in grain size between the two layers but also between different samples taken at the same façade. THM sample has a red-colour superficial layer which is partially detached. CON sample is the only one that contains agglomerations of small gypsum crystals as well as bigger crystals. In the THM sample the two layers appear detached by a fissure. In ART there is also a fissure parallel to the surface but in the inner layer. HED sample shows a thin layer in the surface with recrystallization zones. Quartz sand grains in the Saint Claire sample are a little bit bigger than gypsum crystals in the other samples. In most of the samples charcoal grains can be observed, only ART and FStP do not show any charcoal. In TGA results this last sample has only 2.5 % of “undetermined” components, which is the lowest value of all the samples. Porosity and density results for historic samples are shown in Figure 2. A general direct relationship is observed for samples FStP, ART, HED and THM, while the CON sample has a higher porosity with respect to its density which can be explained by its higher content of calcite compared to the other samples. Density of calcite and gypsum are 2,700 kg/m³ and 2,300 kg/m³, respectively. FStP has an extraordinarily high porosity and a low density that can be explained by the small grain size and the high gypsum content. Figure 2 In general, good correlation was found between density and porosity with the exception of the lime-rich sample mentioned above, a fact that supports the similarity in composition between the respective samples.

Figure 1: Digital optical incident light microscopy. All images are at the same magnification.

Water vapour absorption/desorption results for samples from historic renders are presented in Figure 3. A first group of samples absorbs much less water vapour than the others at 90 % relative humidity, THM (0.15 %), FStP (0.5 %) and ART (0.77 %). Two other samples present intermediate values, StC (1.66 %) and Heudouville (2.5 %). The 69CON sample has the highest value, 4 %. Samples with low water vapour absorption have higher contents of gypsum. CON samples, with the highest absorption, have particles with very fine crystals which can explain this result due to a high specific surface. HED has the highest percentage of undetermined minerals in TGA which may correspond to the charcoal content (Fig. 2). The StC sample is quite different from the others, with lime binder containing almost 85 % of quartz with low water vapour absorption properties.

Figure 2: Porosity and density values of ancient samples.

Figure 3: Water vapour absorption/desorption curves for some old renders.

The CON sample absorbs the maximum of vapour and has the highest hysteresis of desorption. 20 % of the absorbed water remains in the sample after the test. THM sample absorption is the lowest one but it keeps 18 % of the water after desorption. StC sample keeps 10 % and the three others around 4 %. The differences in the sorption/desorption hysteresis cannot be explained.

Laboratory samples

TGA analysis of laboratory made plasters was performed shortly after their preparation, in which lime had not time to carbonate. The results obtained are not comparable to samples from historic renders.

Porosity of new plaster samples goes from 58 % in samples of rough render for exterior walls with lime or charcoal grains, to 44 % in fat or pure gypsum renders.

In new render samples, the amount of absorbed water vapour by unit of mass goes from 0.18 to 0.50 i. e. values similar to THM, FStP and StC samples. The highest values correspond to samples with the highest content of lime or charcoal. Lowest values correspond to pure lime samples.

Water vapour permeability was measured for new mortars. Permeance in kg/(m² × s × Pa) goes from 1E-13 to 2E-13. The highest values correspond to samples with 20 % of lime and the lowest ones to pure plaster renders. Tests have been done at three different ages, 10, 25 and 90 days, and the permeability values slightly decrease with time. Compression and flexural strength were measured for several samples of new plaster. Compressive strength varies from less than 1 MPa in some samples of pure gypsum with charcoal to more than 6 MPa in samples of pure gypsum with or without lime. At the tested ages the admixture of lime does not produce any increase in strength, as more time is necessary to complete carbonation of lime.

The amount of mixing water controls mechanical and hydric properties. With increasing amounts of mixing water, compression strength decreases, porosity and water vapour absorption increase and density decreases. The most important variation has been observed in water vapour permeability. In several studied renders, a diminution of 15 % in mixing water induces a decrease of water vapour permeability between 30 and 40 %.

Conclusions

In this paper we present preliminary results in order to understand the properties of gypsum renders as a function of their composition, binder and aggregates, but also of the samples preparation, especially the amount of mixing water, It is difficult to draw general conclusions, as the sampling process is not easy and obtained samples cannot be considered as representative of the whole population of renders in the Vexin Français area.

Another difficulty of the study is the age of the samples. This study has been done during a Master degree research project of 6 months. Samples from buildings are at least 100 years old but laboratory samples have less than 3 months. This difference is especially important for samples with lime. In new 70samples we find some portlandite, not found in old samples, not even in Saint Clair sur Epte sample, fabricated entirely with lime binder.

Nevertheless, some conclusions can be inferred from this work:

a) Historical studied renders are very different from one to the other, even if most of them are composed of more than 80 % of gypsum. Aggregates granulometry and composition, amount of charcoal, number of layers, colour, etc. vary from one sample to the other.

b) The amount of calcite or quartz aggregates is more important in renders of farms or enclosure walls than in façades. Decorative elements are composed almost exclusively of gypsum. The composition also depends on the location of a building in respect to the areas of plaster of Paris production.

c) Mixing water and the amount of lime control the properties of renders. In this way, the plasterer’s skills determine the quality of the products.

d) Porosity cannot be directly related to the water absorption/desorption of plasters. Sorption/ desorption test seems more adequate to characterise hydric behaviour of plasters than porosity measurements.

e) Permeability of plaster is directly related to the capacity of samples to desorb the absorbed water upon drying.

f) It is difficult to compare new and old plasters due to the alteration of their properties with time.

Acknowledgements

For their help during this work we thank Guillaume SODEZZA (PNR Vexin Français), Marc POTIN and Keltoum BALBZIOUI (Plâtrière Vieujot), Mae PRADAL (UCP student) and Tiffanie LE DANTEC and Jean DUCASSE-LAPEYRUSSE (Cercle des Partenaires du Patrimoine – LRMH).

References

Blottas, M. (1839). Traité complet du toisé des ouvrages de maçonnerie. Carilian-Goeury et V. Dalmont, Paris.

Debauve, A. (1884). Procédés et matériaux de construction. Quatrième partie. Matériaux de construction. Vve Ch. Dunod, Paris. 680 p.

Flavien, F. (1887). Plâtre, in: Dictionnaire encyclopédique et biographique de l’industrie et des arts industriels. Lami,Tharel & Cie, Paris, pp. 393–404.

Le Dantec, T. (2016). Le plâtre en façade, une architecture francilienne historique. fabricA 10. hal-01611369.

Le Dantec, T. (2019) Les façades enduites au plâtre d’Île-de-France. Le déclin du plâtre extérieur, du XVIIe au XXe siècle. PdD. 555p. Université Paris-Saclay.

NF EN 1936 May 2007. Natural stone test method -Determination of real density and apparent density, and of total and open porosity.

NF EN ISO 12571 October 2013. Hygrothermal performance of building materials and products – Determination of hygroscopic sorption properties.

NF EN 15803 March 2010. Conservation of cultural property – Test methods – Determination of water vapour permeability.

NF EN 13892-2 September 2003. Methods of test for screed materials – Part 2: determination of flexural and compressive strength.

Sanz Arauz, D., Domínguez, L. V. (2009) Evolución de los morteros históricos de yeso al exterior en la España Central. Congreso Nacional de Historia de laConstrucción. Valencia V2:1329–1336, ISBN978-84-9728-316-8.

Shi, L., Liu, Q., Guo, X., Wu, W., Liu, Z. (2013) Pyrolysis behavior and bonding information of coal – A TGA study. Fuel Processing Technology 108, 125–132. doi.org/10.1016/j.fuproc.2012.06.023.

Thénard, L-J ; (1834) Traité de chimie élémentaire théorique et pratique, Paris, Crochard,p. 221.

Toussaint, C.-J. (1841). Nouveau manuel complet du maçon-plâtrier, du carreleur et du paveur, EncyclopédieRoret. Libr encyclop de Roret.

71

STUDY OF DECAY PATTERNS AND DAMAGE ASSESSMENT OF THE ACHAEMENIAN ROCK-RELIEFS OF NAQSH-E RUSTAM

Sahar Ahmadinezhad¹, Antonio Sansonetti², Andrea Pane³, Danilo Biondelli²

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Polytechnic University of Milan, Dep. of Architecture and Urban Studies (DASTU), Milan, Italy

2 Institute of Heritage Science (ISPC) Italian National Research Council (CNR), Milan, Italy

3 University of Naples Federico II, Architecture Dep.(DiARC), Naples, Italy

Abstract

Ancient rock reliefs in the necropolis of Naqsh-e Rustam (Iran) are important testimonies of the development of an outstanding monumental art over the centuries, in close relationship with their natural context. The rock reliefs underwent natural and anthropic decay processes in the course of time leading to the loss of fragments and in some cases to severe structural instability. This paper focuses on the oldest group of rock reliefs, dating back to the Achaemenian period; due to their location on the top of a sloping cliff, they are less accessible and more challenging by a conservation point of view. The reliefs have been studied in field and by means of a multi-analytical laboratory procedure, in order to identify the decay patterns, along with an assessment of the state of conservation. In order to frame the problem in a proper historical context, archival material including photographs, drawings and descriptions – created by western scholars between the 17th and the 20th century – were also analyzed focusing on any indications as regards deterioration problems. Fragments were studied by means of optical microscopy, SEM-EDX and XRD analyses. Among the main decay causes and mechanisms, the chemical dissolution of the stone substrate and the heavy microbiological subsurface growth play a major role. The layered aluminosilicate encrustations imply a continuous exposure of the limestone monument to the moisture ingress from the outer environment. Also, indications of the recent impact of atmospheric pollutants were observed, which is noteworthy, considering the distance of the monument from the urban areas.

Keywords: damage characterization, decay pattern, Iran, rock relief