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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.