Measuring crystallinity of laser-cleaned silk by X-ray diffraction

We present an effective and sensitive method of analyzing the condition of silk following laser cleaning. New silk samples were analysed; two sets were soiled with carbon black before laser cleaning and two sets were left unsoiled. Samples were exposed to laser cleaning at a wavelength of 532 nm and a fluence of 1.5 J/cm2 for 4, 16 and 64 pulses. Two sample sets were also treated at fluence levels of 0.5, 1.0 and 4.2 J/cm2 to assess the effects of fluence on the silk structure. Wide angle X-ray diffraction was carried out using the NanoSTAR facility at Cardiff University. Using the main equatorial reflections from silk the crystallinity of the samples was calculated. Upon laser cleaning at 1.5 J/cm2, the silk displayed a reduced level of crystallinity as the number of pulses increased, with soiled silks displaying a greater crystallinity loss than unsoiled silks. Coupled with this, the crystal size, as analysed using the Scherrer formula, was shown to increase as the crystallinity reduced. The effects of fluence on the sample crystallinity was less obvious: samples treated at 0.5 J/cm2 displayed an intrinsically higher crystallinity than all other samples, but there was no progressive loss of crystallinity with increasing fluence level as may have been anticipated.