doi:10.14311/AP.2017.57.0139 Acta Polytechnica 57(2):139–148, 2017 © Czech Technical University in Prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/ap SURFACE CONSOLIDATION OF WALL PAINTINGS USING LIME NANO-SUSPENSIONS Jan Vojtěchovský Studio of Restoration and Conservation of Wall painting and Sgraffito, Faculty of Restoration, University of Pardubice, Jiráskova 3, Czech Republic correspondence: Jan.Vojtechovsky@upce.cz Abstract. Within the field of the conservation of historical and cultural monuments, lime nano- suspensions are still a relatively new and unexplored material. This study examines their effect on the consolidation of architectural surfaces and, consequently, on wall paintings. Previous experiments showed that considerably deteriorated materials may not be adequately strengthened using only lime nano-suspensions. Therefore, the effects of their admixtures and gradual applications with silicic acid esters were examined. For verification, a simulation of a deteriorated lime-based paint layer was created on panels of plaster. The results of the consolidation were subsequently studied using objective (peeling test, water absorption capacity test, measuring colour changes using a mobile spectrophotometer) and subjective methods (comparison of visual changes to a set standard and by testing cohesion using a cotton swab). The microstructure of a consolidated paint layer was studied with a scanning electron microscopy. Tests proved that with either individual lime-alcoholic suspensions or with successive applications and mixtures of silicic acid esters it is feasible to achieve good consolidation results, whilst the alkoxysilane content of the agent indisputably increases the consolidating effect of these materials. Keywords: lime nano-suspensions; silicic acid esters; surface consolidation; conservation; wall painting. 1. Introduction In recent years, the consolidation of paint layers of wall paintings or architectural surfaces is, as in other areas of the care of monuments, associated with the notion of material compatibility. On the basis of this notion, materials using the same type of binder initially used in the original artefact are being increas- ingly applied in the consolidation and conservation of historical and culturally significant buildings and artefacts. A result of the above-mentioned efforts is that a group of consolidating agents was developed at the beginning of the new millennium – so-called “lime nano-suspensions” [1]. As they are materials whose only consolidation agent is the calcium hydrox- ide Ca(OH) 2 , they are designated for strengthening other porous calcareous materials such as lime plas- ters and their surface layers (washes, paintings). As opposed to a traditional lime-based consolidant, which is a saturated solution of the Ca(OH) 2 in water (lime- water), these consolidants have a higher concentration of the active component and, moreover, the consol- idant is dispersed in an alcoholic media, which, in many cases, as in the case of risks connected with moisture, can be more advantageous (for instance, in case of activation of water soluble salts). The aim of this study was to investigate the effects and risks of strengthening lime based paintings and surface layers of historical plasters using this type of consolidant as well as its modifications. In spite of the fact that, as opposed to limewater, the concentration of the active matter in lime-alcoholic suspensions is higher, in the cases where the strength- ened substrate had disintegrated excessively, the con- solidating effect was still inadequate. Therefore, the tests included modifications of these suspensions with silica based consolidants, where the resulting content of the solid component is approx. 10 times higher. On the basis of previous experiments described in literary sources [2], the decision was made to test mix- tures and combinations of silicic acid esters, which, in the conservation field, are the most commonly used consolidants for porous inorganic materials, including plaster, renderings and their surfaces. 2. Basic characteristics of lime alcoholic suspensions The term lime-alcoholic nano-suspensions (or nano- sols) is used for suspensions of calcium hydroxide in aliphatic alcohols with particles of calcium hydroxide, which size is about 50 to 300 nm. Theoretically, they are not nanomaterials in the true sense as their size would have to be 100 nm at the most [3]. The individual suspensions, available commercially or as an experimental developmental material, differ from each other in their particle morphology, their con- centration or type of alcohol. The most common dis- persing agents are ethanol, 1-propanol and 2-propanol. The suspensions are produced in various concentra- tions, commercially available in concentrations from 5 to 50 g of Ca(OH) 2 to 1 l alcohol. The viscosity and “whiteness” of the suspensions differ partly accord- ing to their concentration. Suspensions with higher concentrations have slightly higher viscosity and they 139