CONSOLIDATION OF A DECOHESIVE PLASTER The case of Robert Nilsson's reliefs at the Gothenburg University Humanities Library

Abstract

This thesis explores possible consolidants to stabilise the decohesive, lime-based plaster of the two 20th century plaster reliefs by Robert Nilsson, located in the Humanities Library at the University of Gothenburg, Sweden. The reliefs are suffering from cracks and loss of material, and in these areas the plaster is decohesive. The methodology included an on-site condition assessment and mapping; environmental monitoring and testing of original material for salts to identify possible causes of deterioration; laboratory testing of consolidants on mock-ups, and application of consolidants on site. The environmental monitoring showed RH fluctuations between 21.7% and 52.4%; presence of sulfates in the plaster was detected, possibly the cause for plaster decohesion. Three consolidants - nanolime, nanosilica, and di-ammonium hydrogen phosphate (DAP) - were chosen for laboratory testing based on information given by the literature reviewed. Laboratory results showed that nanosilica provided the best consolidation at the surface, while DAP was assessed to penetrate deeper and improve cohesion in depth. On site, combinations of nanolime, nanosilica, and DAP were applied. Three combinations were tested on site: a two-step DAP treatment, DAP followed by nanolime, and DAP followed by nanosilica, each applied with 24 hours between the first and second application. The combination of DAP and nanosilica showed the most promising results. However, all consolidants caused yellowish tidelines around the treated areas, possibly caused by the solubilisation of an organic material present in the stratigraphy. This suggests the need for further investigation into the materials of the relief as well as different application techniques of the consolidants.