Repair Mortar for a Coloured Layer of Sgraffito Render – a Technological Copy

Abstract

Sgraffito technique was used to decorate renders by scratching the top layer of lime wash in the Renaissance time. In order to contribute to the preservation of surviving sgraffiti in the town of Slavonice in the Czech Republic a study was carried out assessing the possibility to replicate the original materials and the application techniques. Historical sgraffito layers were sampled in situ and studied in a laboratory by commonly used analytical methods - OM, TA, XRD, SEM-EDS. The raw materials, lime binder and sand, were characterised and the mixing proportion app. 1 to 0.7 (vol.) of lime putty to sand was determined. Based on the character of the local limestone, a similar raw material was obtained and burnt in an experimental lime kiln. The sand was obtained locally from a disused pit quarry. Obtaining the raw materials from similar sources as the historic ones allowed studying possible production technologies and application techniques. Appropriate and probable techniques were verified by a series of practical experiments. These considered: lime putty v. dry slaked hydrate; thickness of a layer; trowelling and final finishing; timing of drawing, scratching application of lime wash. The performance of the produced mortar mix was assessed by compressive and flexural strengths, capillary absorption, drying index, open porosity and water vapour diffusion coefficient. The mortar, designed as a material replica of the original, was used in a conservation project on a façade of a house, where missing parts of a sgraffito render were reconstructed.