Poultices as a Way to Eliminate the Yellowing Effect Linked to Limestone Laser Cleaning

One of the main drawbacks raised against stone cleaning by Nd:YAG Q-switched lasers (1,064 nm, 6–20 ns) is the yellow aspect left after cleaning. It is well known among stone conservators that one may attenuate this colour using waterbased poultices.We tested four kinds of poultices in two areas on the northern portal of the Saint Denis cathedral, which had been laser cleaned in 1997. The yellow aspect visibly decreased immediately after poultice removal, and the effect remained visible after 12 months. Colour measurements performed before, just after, 3 and 9 months after poulticing confirmed and quantified observations. Scotch tape tests, optical microscope and SEM-EDS observations and analyses show that this reduction is due at least partly to poultice remnants when poultice contains attapulgite or carboxymethyl- cellulose. Fume silica poulticing also seems to leave relics on the treated surface. The use of cellulose alone leads to a slight de-yellowing without any poultice relics being present. In that case, the de-yellowing could be due to solubilization of yellow products or physical detachment of particles.