Detection of the optimal laser fluence ranges to clean graffiti on silicates

- The polymineral characteristic of the granite influences on the laser cleaning effectiveness.
- Nd:YVO4 ns laser at 355 nm cleaned granite of red, blue and silver graffiti.
- The satisfactory cleaning shows a complete extraction of the graffiti without damage in the substrate.
- Quartz and K-feldspar grains were successfully cleaned with 0.1-0.2 J.cm−2.
- Plagioclase cleaning depended on the colour of graffiti to be cleaned.
- Biotite at 0.06 J.cm−1 started to be cleaned but already damaged.

The polymineralic characteristic of granite implies an additional challenge during the laser cleaning of cultural heritage built of this rock, because the forming-minerals experience different behaviours under the laser radiation, leading to damages of variable nature and intensity.The aim of this work was to identify the ranges of fluence to satisfactorily and safely clean the four main constituent minerals of granite (quartz, K-feldspar, plagioclase and biotite) painted with three different graffiti colours. The cleaning was performed using a nanosecond Nd:YVO4 laser which works at high pulse rate (kilohertz). The evaluation of cleaning effectiveness was performed in terms of graffiti extraction and mineral damage evaluated by optical, electronic and confocal microscopies and hyperspectral imaging technique.The results showed that, regardless of the graffiti colour to be cleaned, quartz and K-feldspar grains were satisfactorily cleaned at the same fluence range. However, in case of plagioclase, it depends on the colour of graffiti to be removed. In case of biotite the analytical techniques allowed to confirm that it was already cleaned with the lowest fluence tested and it was the mineral with the highest sensitivity to laser radiation.Therefore, in order to obtain a satisfactory and safe laser cleaning it would be required the prior identification of the granitic forming minerals to adjust the laser fluence.