The use of dry ice blasting to clean public art

Jun 10 2024
Public Art | June 10, 2024

A few years ago, the Belkin Art Gallery at the University of British Columbia required our specialized services for the conservation of a large, outdoor sculpture. The sculpture, Gerhard Class’s 1968 Tuning Fork, is a pair of 7m tall Cor-Ten steel pillars artistically twisted into the shape of a tuning fork, which sits just outside the front doors to UBC’s School of Music. Unfortunately, the artwork was tagged with some unwanted graffiti and cleaning attempts in-house were not successful.  The sculpture also had several years of algae, moss, and lichen growths on the surface. Our goal from the conservation treatment was to remove the disfiguring graffiti, remove the graffiti cleaning attempt remnants, and remove biological growths to better improve the appearance of the artwork.

Cor-Ten steel is a type of weathering steel which is popular for use in public art.  It is an alloy of iron and carbon mixed with a selection of various other elements, such as copper and phosphorus. These alloys were designed to corrode in a particular manner to produce stable corrosion products, called a patina, that not only gives the metal its distinctive red-brown-purple colour, but the patina layer also protects the underlying metals from further deterioration!  With proper care, Cor-Ten steel sculptures can last for generations.

Gerhard Class, Tuning Fork, 1968

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

However, this protective and aesthetically sought after patina layer is quite fragile and most physical contact with the surface of the artwork will dislodge the layer and lead to further damage. Sadly, this occurred prior to our conservation work as an overzealous maintenance worker, with nothing but good intentions, attempted to remove the graffiti causing a large area of overcleaning that damaged the desired patina. After examining the artwork and the damage done, our new goal was to re-integrate the overcleaned area with the surrounding area while also cleaning off the plant growth and any remaining graffiti.

Spray paint graffiti on the surface patina of the sculpture

In-house cleaning attempts to remove graffiti changed the patina and a visible patch on the surface

Biological growth on the sculpture at the top

 

 

 

 

 

 

 

 

 

 

To remove the graffiti and biological growth while reintegrating the damaged patina, we had to weigh the pros and cons of several different treatment pathways.  High pressurized washing would remove the remaining graffiti and plant growth, but would potentially remove significant layers of the patina, further altering the surface appearance.   Scrubbing the remnant paint and plant growth would create very visible markings in the patina and could very easily lead to overcleaning. Artificially patinating the overcleaned area could re-integrate the appearance of the artwork, but many recent studies had shown that the artificial patinas on Cor-Ten steel do not age in the same way as naturally patinated weathering steel and would inevitably become visible again. The artwork needed to be able to develop a stable patina again.  Given the unique characteristics of the material, we realized that we needed to remove the uppermost layers of patina and leave underlayers of original metal patina, and dry ice blasting would be the best way to achieve these goals.  Properly done for artwork, the procedures can be gentle, effective, safe and customized to get the desired results.

Ice blasting illustration. Credit: Wickens.

 

 

 

 

 

 

 

 

 

 

Dry ice blasting is a technique that uses rice-sized pellets of frozen carbon dioxide fired through a pressurized nozzle to blast off unwanted material from the surface of various materials. The technology works via three chemical and physical mechanisms. First, the physical force of the dry ice hitting the surface blasts off loosely bound materials. Next, as the dry ice hits the artwork, it rapidly cools the surface to near -80°C, freezing the unwanted surface materials, making them brittle and causing them to contract and crack. Lastly, the dry ice sublimates, turning from a solid to gas, which creates micro-explosions of rapidly expanding gas that blast off the now frozen material. As you move across the surface of the artwork, all of these mechanisms are occurring simultaneously, making the method very efficient and effective at removing only a few millimeters of material at the surface of the artwork.

Ice blasting is used to clean sculpture.

 

 

 

 

 

 

 

 

 

 

 

 

The technique can be modulated with skill and experience; controlling the size of pellets, the pressure behind the nozzle, the distance of the nozzle from the surface, and as well as the angle at which you are firing are several ways you can get different intensities and results from the blasting.  Additionally, because the dry ice blasting media is instantaneously converted into a gas, there is no cleanup required like you would need for other blasting methods, such as sandblasting.  But, the fact that dry ice blasting can produce different results with different settings and procedures means that this can only be done through a conservator experienced with this technique, working with a skilled ice-blasting technician.

 

 

 

 

 

 

After initial testing, this method of resurfacing the artwork proved highly effective and efficient! The blasting successfully removed the biogrowth without damaging the underlying patina and the treatment of the overcleaned area re-integrated the appearance with the rest of the sculpture, resulting in a more uniform surface that will continue to patinate as a whole unit into the future.

 

Before, biological growth and uneven patina.

After ice blasting, dirt, graffiti, biological growth and uneven superficial patina are removed, revealing an even underlying patina.

 

 

 

 

 

 

 

 

 

 

 

public art

Area of sculpture with a patina disturbed by in-house cleaning attempt.

After ice blasting, the superficial patina has been removed and a more even underlayer of patina is revealed to allow for the object to re-patinate naturally and evenly.

 

 

 

 

 

 

 

 

 

 

 

 

Daniel Schwartz, Conservator of Objects and Public Art