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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.