Atomic oxygen plasma for controlled removal of carbonaceous deposits from delicate heritage substrates: soot removal from undyed silk objects.

Plasma-generated atomic oxygen (AO) is a highly reactive and chemically selective species with strong potential for the precise removal of carbonaceous contaminants from heritage materials. However, its application to organic substrates is limited by competing oxidation processes that may induce chain scission, mechanical degradation, and mass loss. In this study, the parametric dependence of AO-induced soot removal from undyed silk model substrates was systematically investigated using a prototype developed within the MOXY project. The effects of power and total cleaning time were evaluated to quantify their influence on cleaning efficiency and thermal load. Cleaning performance was assessed using optical and morphological techniques, including 3D optical microscopy, profilometry, drop tests, and hyperspectral imaging, while infrared thermography enabled in situ temperature monitoring. Results show that increasing power or treatment time enhances soot removal, but excessive conditions lead to alterations in fiber morphology and mechanical properties. By correlating cleaning efficiency with thermo-mechanical responses, a safe operating window was identified. Tensile testing, thermogravimetric analysis, dynamic mechanical analysis, and dynamic vapor sorption revealed sub-critical damage thresholds. These findings demonstrate that AO plasma treatment can be optimized for fragile proteinaceous substrates, providing conservators with reliable and controlled cleaning parameters for textile heritage objects.