In November 1967, water flooded into the Calouste Gulbenkian Foundation in Lisbon, Portugal, coating a collection of 16th century Islamic manuscripts with mud. Conservators spent years trying to clean the valuable papers and parchments, scraping them and applying chemical solvents. The problem with these methods, of course, is that they not only attack the mud, but they also affect the inks on the documents, and the solvents can sink into the paper or parchment and destroy it.
Fortunately, a better method may exist. Excimer lasers have been used for several years to clean paintings, and recently conservators have tried applying similar methods to documents. Researchers at the Centro de Ciencias e Tecnologias Opticas (Porto, Portugal) and the Instituto Soldadura e Qualidade (Oeiras, Portugal) are trying to determine just what long-term effects laser ablation has on these irreplaceable artifacts.
The documents were cleaned with a krypton fluoride laser emitting at 248 nm. The beam was put through a mask with a rectangular slit to cut out the lower-energy portions, then focused through a lens with a 200-mm focal distance, creating a spot on the document of 1.3 x 4 mm. The average energy density delivered was 30 mJ/mm2, and the 170-mJ pulses, at a repetition rate of 9 Hz, lasted on the order of 10 ns each. Approximately 94% of the ultraviolet radiation delivered to the document was absorbed by the mud, exceeding the binding energy threshold of the contaminants and causing the material to break into particles that were blasted away. Each pulse penetrated to a depth on the order of tenths of a micron.
