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Laser technologies in the analysis, conservation and restoration of historic stained glass windows

Final Report Summary - LASERGLASS (Laser technologies in the analysis, conservation and restoration of historic stained glass windows)

The art of stained glass windows making belongs to the field of monumental arts. Its existence is directly connected with Europe (where probably the first stained windows were made) and Catholicism, since stained glass windows have been and are an inseparable part of church decorations. It is totally understandable that historic stained glass windows are a valuable part of European cultural heritage. Consequently, the conservation and preservation of those outstanding works of art is very important issue. However, their significant role is not only limited to the decorative or religious functions. Stained glass windows play also the protective function against outer climate and through many years have been exposed to various atmospheric conditions: water, sunlight, wind, temperature changes, biological activity, and from the beginning of 20th century a high air pollution levels. Those damaging factors caused irreversible changes in bulk glass structure such as glass surface soiling and corrosion, deterioration of grisailles, and also structural damages to panels' construction part: lead cames and ferramenta.

Many efforts are internationally undertaken for the proper conservation of historical stained glass windows. However, still the common approach to the problem treats conservation process as a typical craftsman work, omitting the importance of scientific investigation of the objects. Scientific and technical development of novel analytical and documentation methods as well as new tools for restoration of glass such as lasers, together with advanced computer science permit the application for glass conservation safe and highly efficient methods allowing to restore them on modern, sustainable and preventive way.

The project LASERGLASS aimed to realise the complex study on the interaction between laser irradiation and a corroded glass and as a continuation to construct the laser cleaning station, dedicated to stained glass windows, equipped with the microscopy and spectroscopic control of the ablation process.

In particular, the effect of ns Nd:YAG laser irradiation with the corroded layers of historical stained glass windows was investigated. The special emphasis was put onto the selection of optimal laser parameters as laser wavelength, fluence and frequency in order to obtain satisfactory level of cleaning without making any damage to the glass. The online control of the laser cleaning was carried out by laser induced breakdown spectroscopy (LIBS), while the post cleaning effects were examined by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDX) and optical coherence tomography (OCT).

The research was carried out on the model samples and on the original, historic glasses, as:

1) Medieval glass from St. Mary's church, Krakow, Poland, having very thick layer of corrosion products as gypsum, bassanite, syngenite and calcite;
2) the seventieth century glasses from stained glass window panel from Grodziec, Poland, presenting different corrosion problems, manifested in corrosion crust, dirt and superficial polymer layer and
3) the ninetieth century colourless glasses, dismounted from the stained glass windows originated from St. Catherine church, Krakow, Poland, covered by the superficial layer of the dark soiling.

In all cases, the best cleaning results were obtained irradiating with one pulse at the fundamental wavelength of Nd:YAG laser (1064 nm, pulse duration of 6 ns) with fluences of 100 - 600 mJ*cm-2. Moreover, when the cleaning were proceeded on the colourless glass the process was self limiting due to the glass transparency to 1064 nm. Using wavelength of 355 nm significantly damaged the model glass, while cleaning at 532 nm required the application of many laser pulses and higher energies of laser beam. Until now, no long lasting effects of the areas treated with laser were revealed.

Results obtained during this research project demonstrated the successful removal of the contamination and corrosion layers from the historic glass surface by the pulse ns laser. It shows that the laser could be used as a powerful conservatory tool in the field of glass restoration. However, it must be stressed the importance of proper determination of irradiation conditions for each, individual case as well as the online control of the cleaning process.