THE OHIO CONSERVATION CENTER: The conservators at the Ohio Conservation Center are always looking for new technologies to treat the myriad of materials we encounter. In 2014 the center purchased an ICETECH KG20 dry ice solid carbon dioxide blasting machine from Continental Carbonic, a model in the lower medium size for a little more than $5,000.00. We use this with our large diesel-fueled blasting compressor. We felt this model size was most appropriate for our expected uses.

Dry ice blasting is a relatively new process but has been met with great success in industry. We tested the process out on differing materials and substrates and were impressed with the versatility and effectiveness of dry ice cleaning. We found it can be used in many application ranging from (but not limited to) removing cross-linked clear coatings from bronzes, cleaning painted sculpture, and the removal of graffiti on weathering steel.

Conservator Jim Gwinner CO2 cleaning Rodin's iconic sculpture The Thinker

But what is CO2 cleaning?

CO2 cleaning involves three primary factors: kinetic energy, thermal shock, and the kinetic-thermal effect.

Kinetic Energy: Incorporating specially designed high-velocity nozzles, pellets about the size of rice are propelled at the substrate surface using compressed air. These pellets are quite soft and do not have the density of harder more common blasting media like sand,  glass beads or even walnut shells. The pellets change from a solid to gas almost instantaneously upon impact that provides an almost abrasion free blasting process.

Thermal Shock: The thermal shock effect occurs once the dry ice impacts the coating. The pellets that are -109 F (-79 C) absorb a massive amount of heat from the coating.  The temperature of both the coatings layer and the substrate decreases. The different materials contract unequally and the adherence between them decreases causing the bond between them to fail.

Thermal kinetic effect: The combined impact energy and almost immediate heat absorption between the pellet causes the instantaneous conversion of the solid C02 into gas. The gas expands to nearly 800 times the volume of the pellet within a few milliseconds. This causes an “explosion” at the point of impact. This creates an “explosion shock front” that is a very effective lifting force that will carry particles away from the surface.

We continue to test and find new applications for CO2 cleaning.

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