Siegfried Siegesmund - Monument Future

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Claire Noël, Beatriz Menéndez

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.

Laboratoire Géosciences et Environnement Cergy, Université de Cergy Pontoise, 95000 Neuville-sur-Oise, France

In the Paris area there are important gypsum resources which results in an extended use of gypsum in traditional architecture as façade renders. Traditional renders are still visible today in urban areas and in the surrounding countryside. This study concerns renders of the vernacular architecture of the Vexin Français, few tens of kilometres North-West of Paris and is based on the work carried out by Tiffanie Le Dantec in her PhD thesis. We focus our research on the analysis of ancient and new renders in order to propose suitable conservation or restoration solutions. This preliminary study compares several physical, chemical and mineralogical properties of ancient render samples and modern renders formulations used for façade restoration, in order to find possible relationships between the characteristics of studied formulations and their physical properties. The final goal is to estimate physical properties of ancient renders from their mineralogical composition.

Keywords: Traditional gypsum renders, renders conservation, renders restoration, mechanical properties, chemical analysis, hydric properties

In the Vexin Français area, 40 to 70 km NW from Paris, two principal construction techniques are found: important buildings (churches, chateaux or big houses) are built in ashlar masonry, whereas smaller ones (farms, houses and other vernacular buildings) are made on random rubble masonry covered by renders. Nowadays many new owners of traditional buildings want to remove renders in order to show the stone masonry. This new tendency has two major disadvantages: it alters region landscape and it exposes masonry to weathering and decay induced by environmental conditions.

The long-term goal of the study is to provide proofs that traditional renders are much more adequate and durable when repairing stone façades than new cement base renders. Most of the façades in the Vexin area are made of rubble stones with a lime mortar and covered by a plaster of Paris, lime or plaster of Paris-lime render. The use of this kind of render has aesthetic purposes, helps to evacuate the humidity and has a protection role.

Our study focused on comparisons between ancient renders sampled in façades of the Vexin area and new formulations of renders supplied by Plâtrière Vieujot. This society has been producing plaster of Paris renders since 1880. One of its renders, “plâtre Briard” is used in rural areas of Paris region as external render 66because of its non-uniform aspect due to the presence of impurities like charcoal and small pieces of gypsum stone.

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.

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.