Laser Cleaning of Undyed Silk: Indications of Chemical Change

Abstract

Three different undyed, unweighed silk fabrics (new clean, new soiled, and naturally aged) were cleaned with a computer-controlled Q-switched Nd:YAG laser at 532 nm in 30 combinations of fluence and pulse numbers. They were studied for chemical change by viscometry, X-ray diffraction, and FIB-SIMS in combina-tion with temperature calculations. While physical changes only occurred above the tested parameters, chemical changes could be detected as low as 0.2 J cm −2 with four pulses. Yellowing was observed at lower and bleaching at higher fluence/pulse number combinations. Melting was observed in naturally aged silk cleaned with 64 pulses at 4.2 J cm −2 . The temperature reached at 0.1 J cm −2 is sufficient to evap-orate carbon. Excess energy is transferred into the silk substrate causing thermal degradation. Different chemical processes leading to chain scission and to crosslink-ing seem to occur simultaneously, even at low fluence and pulse number. An increase in pulse numbers also leads to increasing damage.