Why a plasma jet for Cultural Heritage?
In the last years as atmospheric plasma was gaining more space in industrial applications, some groups in the scientific commented started to experiment the atmospheric plasma also in cultural heritage applications. The plasma technology can offer an alternative way to clean relative to mechanical methods or solvents and laser, in particular has the following features:
- acts chemically
- can be room temperature
- is not in contact
- is solvent free
- acts only at the top surface
- on organic removal does not affect inorganic material
- can also reduce corroded layers
- can work on multi material substrates
At the same time actually plasma torches available on the market can have one of these defects:
- Metal particles are deposited on the surface due to electrodes erosion;
- Temperature at the surface can be higher than 80 °C;
- Removal speed is too low to consider sustainable for restoration aims.
Stylus Plasma Noble is the innovative soft plasma jet specifically designed for Cultural Heritage applications that solved these drawbacks and allows a safe use of the technology.
The atmospheric-pressure plasma technology developed by Nadir makes possible to have efficient restoration processes such as:
- Full removal of organic protective coatings also aged from Paraloid® to epoxies, parylene, graffiti and protein coatings and glues;
- Fast surface activation and depolymerisation of cross-linked aged polymers for a faster and easier removal with no toxic and less hard solvents;
- Controlled and compliant cleaning at the interface of organic coatings;
- Reduction of corrosion product on silver, silver alloys, brass, iron also on archaeological artefacts;
- Reduction of corrosion products on metal wires in textiles;
- Plastics yellowing removal;
- Removal of biological patinas even endolithic organisms;
- Removal of stains from paper.
* SMP Photoconservation and University of Venice
Nadir Stylus Plasma demonstrated to successfully clean and reduce silver oxide and silver sulfide parts avoiding any damage of the delicate nanoparticles structure that compose the daguerreotype image.
The graffiti on stone are usually removed by solvents or by laser. Solvents have the drawback that can dissolve the coating inside the stone (as a function of the porosity) even if applied with gels or poultices. Lasers have to be tuned in order to reach the best compromise between a good visual removal and avoid stone damaging (change in surface roughness or in color).
Atmospheric plasma can be an alternative for graffiti removal. Arc plasma torches are very fast in cleaning but depending on the stone can induce surface color changes due to surface heating and by the deposition of metal particles coming from the electrodes to the surface.
Stylus Plasma Noble allows the graffiti removal avoiding the drawbacks of arc plasma torches in the full respect of the artifacts. The soft jet at the same time doesn't reach the removal speed of arc plasma torches as of solvents or lasers. Therefore the best results have been obtained combining the Stylus Plasma Noble with solvent removal. Solvent can be used as a first step in order to remove the thicker part of the graffiti. As second step, when arrived at the interface, plasma can be used to remove the last graffiti layer and clean the pores. The difference between solvent and solvent+plasma cleaning is shown in the figure below.
However, when the surface of the stone is not smooth and presents imperfections, for example from the cutting, the removal of the graffiti paint is with both method (solvent and plasma) impossible.
Acrylic polymers removal
Acrylic-based coatings as Paraloid® B72 or B67 have been used since longtime for surface consolidation and protection. With aging the coatings properties deteriorate and often has to be removed. The atmospheric plasma technology by Plasma Stylus Noble allows their removal just on the surface, leaving the polymer inside the material untouched. This feature allows to recover the original surface with its colors and materials, allows to treat the surface again and maintain the consolidation polymer inside the material, so without any loss of compactness.
The removal is mainly a chemical etching without solvents, not in contact a room temperature and with the ability to stop the cleaning process when desired. The removal is gradual and the chemical etching acts only at the nanometer scale surface.
Due to the simple linear structure of the acrylic polymer its removal doesn't have preferential paths, as can be seen by the FT-IR spectra all the bonds of the polymer are uniformly affected by the oxidation process.
The removal rate of the polymer has been measured by the FT-IR measurement and can reach 20μm/min. The measurements have been performed also by quartz balance obtaining a removal rate of 1μg/s.
acrylic removal from fresco or secco…
Soot removal from frescos
Plastic surface yellowing removal
Plastics materials are always more present in contemporary art collections or in science and technology museums. Their restoration is often quite complex due to the wide range of compositions, the presence of different materials and their unknown aging evolution.
Several plastic materials such as ABS or HDPE during their aging mainly by UV radiation show a yellowing of the surface due to polymer cross-linking. This yellowing layer is usually several microns thick. Atmospheric plasma technology offers the possibility to erode gradually the layer leaving the possibility to stop at the desired cleaning stage.
Etching of a aged polymer