Alteration and protection methods for glass in Cultural Heritage. Multi-analytical investigation into the composition effects for glass alteration under atmospheric and immersion conditions, and a study of two possible protection methods based on zinc salts and silica nanoparticles

Abstract

The composition effect on glass alteration was studied through a multi-analytical approach focused on plate and powder replicas. These were synthesised in a way to simplistically represent different glass formulations typically found in Cultural Heritage objects. Mixed alkali glass, characteristic of Medieval central Europe, was reproduced and studied in comparison with other compositions for which the alkali/alkaline earth ratio was systematically varied. Artificial ageing tests under both atmospheric and immersion conditions were carried out to investigate the different behaviours of the studied glass. The difference between stable and unstable compositions resulted straightforward as the latter easily presented flakes and salts formation upon ageing. The analytical results revealed they had undergone a severe hydration and, in some cases, also a change in the silica structure. Carbonate salts formed on samples surfaces as a secondary phenomenon, which was assessed to be temperature dependent. The ageing under immersion conditions resulted less aggressive, causing an incongruent dissolution that affected only a very thin layer of the material surface. Two possible methods for glass protection were also studied: one based on zinc salts and the other on a hydrophobic sol-gel coating. While the former was tested on plate samples, for the latter silica nanoparticles of different dimensions were synthesised and characterised as a preliminary step of the product formulation.