Everyone who knows me knows that my favorite place in the world is New York City. “Liberty Enlightening the World” or The Statue of Liberty is my favorite landmark in NYC, well technically New York Harbor. The statue is currently closed for renovation after celebrating its 125th anniversary in October 2011, but what does this have to do with particle properties?
Well the last time the statue was closed for renovation was in 1984. As part of this, multiple layers of paint and coal tar needed removed from the inside of the statues copper skin to allow for repairs, but without causing further damage to the 3mm thick, soft metal surface. One of the methods used to achieve this was soda blasting.
This is where sodium bicarbonate (baking soda) is delivered to a surface to be cleaned using compressed air. It sounds like sand blasting however instead of relying solely on the abrasion process from the particles; the soda blasting is also based on the physical properties of the sodium bicarbonate. The crystals are sharp so it does have some abrasive qualities but it is also relatively soft, so it is less damaging to the substrate to be cleaned than sand blasting.
The main property of interest and use is that it is friable which means that on impact with the surface it will fracture into smaller pieces that aids the cleaning action but also softens the impact. Both of these properties are influenced by the particle size of the crystals used. The abrasiveness can vary from gentle to moderate to aggressive with increasing crystal size so it is important to understand and control the media in use depending on the application. So in the case of Lady Liberty, they may have had to be careful not to use a crystal size that was too large to prevent potential damage to the soft copper, but also the use of a much smaller particle size would have increased the time taken for the cleaning. They must have optimized the method for their situation as the statue was opened again in time for the planned centennial celebrations in October 1986.