Effectiveness and environmental performance of dry ice blasting for graffiti removal from outdoor metallic sculptures

CO₂-based cryogenic cleaning by dry ice blasting is increasingly regarded as a low-impact alternative for the removal of unwanted deposits from sensitive surfaces. The present study investigates the application of this method to the removal of unauthorised graffiti from contemporary outdoor metallic sculptures, focusing on steel and aluminium substrates protected with acrylic-urethane and acrylic-styrene clear coats. These protective coatings represent the original surfaces to be preserved, whereas the aerosol paint layers constitute the removable material. The experimental assessment evaluates removal efficiency, the persistence and morphology of residual paint deposits, and the energy demand and CO₂ consumption associated with the process. The characterisation of cleaning performance and residue distribution was achieved through the utilisation of high-speed imaging and optical and electron microscopy. The findings indicate that dry ice blasting enabled effective removal of a wide range of synthetic aerosol paints. No residual paint was detected on the acrylic-urethane coated steel substrate M; residual areas on substrate N remained below 0.023%, and most values on the acrylic-styrene coated aluminium substrate R were below 0.24%, except for the A94 aerosol, which reached 2.07%. Removal times for a fixed area of 4.34 cm² ranged from 0.46 to 4.35 s, while energy demand ranged from 0.41 to 2.28 kWh/m² depending on aerosol formulation and coating response. Overall, CO₂-based cryogenic cleaning can be considered a selective and controlled method for graffiti removal on synthetic clear coats commonly used in outdoor contemporary artworks, provided that CO₂ use, energy demand and repeated-treatment effects are transparently assessed.