Читать книгу Monument Future - Siegfried Siegesmund - Страница 84

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.