Characterization of historical lime plasters by combined non-destructive and destructive tests: The case of the sgraffito in Bożnów (SW Poland)

Abstract

Mortars used in the two-color sgraffito, dated back to 17th century, have been investigated in order to fully characterize the material and to evaluate the state of its preservation. The non-destructive tests carried out “in situ”, comprising thermal imaging and ground penetrating radar, revealed the inner structure of the decoration, which was exfoliated from the wall in ca. 90%. The obtained thermograms and the GPR scans allowed to choose sampling areas for subsequent destructive-tests, performed by means of optical microscopy, chemical analysis, granulometric analysis, X-ray diffractometry, scanning electron microscopy, infrared spectroscopy and differential thermal/thermogravimetric analysis. The results reveal the decoration is composed of two separate coats of lime plasters: (1) the primer plaster and (2) the sgraffito plaster. Both the coats appeared to be a mixture of lime binder (aerial lime) and fine- to medium-grained sand, with addition of small amounts of brick chunks and charcoal. The latter is abundant in the sgraffito plaster, serving as a coloring agent. For elimination of plasters exfoliation, a calcareous PLM-A product of C.T.S. Italy was applied by injection. The non-destructive studies performed after this conservation demonstrates that the rehabilitation work has reestablished homogeneous structure of the sgraffito decoration.Our study indicates that full characterization and proper reconstruction of ancient mortars could be attained by complementary employment of destructive and non-destructive test methods. Moreover, repetition of the non-destructive tests after the rehabilitation works provide valuable insights into their effectiveness.