Description
REVERSIBLE LOW-TEMPERATURE HYDRATION STATE TRANSITION AND WHITENING OF GYPSUM PLASTER INDUCED BY ATMOSPHERIC ATOMIC OXYGEN IN NON-EQUILIBRIUM HELIUM PLASMA:Plaster artworks, such as casts, sculptures and architectural elements, typically display subtle variations in their characteristic whiteness, and preserving their authentic appearance remains a key challenge for conservators. While the innovative atomic oxygen (AO) cleaning has emerged as a green and highly effective non-contact method for fire-born soot removal [1], recent experimentation revealed that AO may induce unexpected whitening effects on plaster. The effect varies depending on AO parameters and plaster compositions, and may be perceived as excessive cleaning. This study aimed to investigate the causes of AO-induced whitening, its implications and ways to mitigate the risks of unwanted changes by adjusting AO parameters, highlighting non-thermal AO in helium (He) plasma-driven dehydration as the dominant and entirely novel mechanism over conventional thermal dehydration. Initial tests showed that AO treatment at relatively low temperatures (30–50 °C) may increase brightness due to rapid conversion of calcium sulfate dihydrate (gypsum) to its hemihydrate form (bassanite), which may self-reverse within weeks at ambient humidity. To investigate the whitening mechanism, several plaster typologies were treated using AO generated in a non-equilibrium He plasma at radiofrequency power between 1–8 W, with He and Argon as shield gasses. To distinguish AO effects from heat-induced changes, plaster test samples were subjected to controlled heating (40–300 °C) for varying durations (30–120 minutes). Spectrocolorimetric analysis showed significant brightening occurring between 100–250 °C, corresponding to gypsum-to-bassanite transformation and the onset of irreversible dehydration, while AO and He plasma caused brightening already at 35–50 °C. Unlike the temporary AO effects, heat-induced brightening continued over time, indicating a propagating alteration of the plaster matrix, which could only be reversed under high relative humidity (90% or higher). These findings indicate that while heat causes brightening, the mechanisms differ from the AO effects, supporting the hypothesis of rapid non-thermal AO and He plasma-induced dehydration of the crystalline lattice. Structural and compositional changes in AO and thermally treated plaster were compared using SEM, XRD, FTIR and GC-MS. Profilometry was used to assess the micro-roughness of pristine and bleached plaster as well as texture alterations of the casting skin, i.e., the mould-contact surface capturing fine details and influencing the appearance of plaster casts. This research emphasises the need to refine AO cleaning protocols to prevent unintended changes, ensuring the long-term preservation of plaster artefacts and their historical and artistic value.
| Period | 7 May 2025 |
|---|---|
| Event title | Technart 2025: International Conference on Analytical Techniques for Heritage Studies and Conservation. |
| Event type | Conference |
| Location | Perugia, ItalyShow on map |
| Degree of Recognition | International |