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Restoring medieval stained-glass transparency: use of new task specific luminescent ionic liquids for corrosion crusts removal
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Transparency is a fundamental feature to preserve on a stained-glass panel. The passage of light is the essence of this art form and, as so, a crucial piece of its original intention, both through the colored glass as through the paintings. Medieval stained-glass is characterized by a relatively high content of alkali and alkali-earth ion oxides, mainly potassium, calcium and sodium, and low contents of silica compared to contemporary glass. These ions are leached, and when in contact with the atmosphere a gel-layer (hydrated silica-rich surface) is formed, and consequently there is the formation of a corrosion crust. This is mainly composed by insoluble salts such as calcium carbonate (CaCO3), sulfate (CaSO4) and oxalate (CaC2O4), which are very difficult to remove from the glass surface. The methods that present higher efficiency for the removal of those crusts – like chelate agents (e.g. EDTA solutions), weak acids and ionic resins – are the same that may induce damage or long-term risks to the glass surface.
The aim of this research was to develop a new product – an ionic liquid (IL) – for the removal of medieval stained-glass corrosion that is effective and harmless both for the stained-glass and for the user. To assure the complete removal of both corrosion and cleaning material (the ionic liquid) after the procedure, the IL was functionalized using a light emitting marker – intrinsically luminescent ionic liquid. It was our objective to have an IL with a dual function: having a binding site for metal cations (in this case, Ca2+), while exhibiting bright fluorescence.
The effects on the surface of model glass of three different ILs, an EDTA solution and the effect of a high relative humidity (RH) environment are described and compared. A comparison between the efficiency of two ILs and an EDTA solution for the corrosion crusts removal was made using corroded archaeological stained glass samples. The tests performed confirmed the effectiveness of the cleaning material in removing the corrosion crusts, and also demonstrated that there were no detected alterations to the glass surface even when in direct contact with the ionic liquids for a long period of time.
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Stained-glass Medieval glass Ionic liquids Glass corrosion Chemical composition Cleaning methods