Copper Plate Cleaning Process
Deciding on the process for cleaning the plates involved research, consultation with conservators, and trial and error. Our goal was to use methods and materials that would not damage the metal, were safe to use, and were effective in cleaning off the various materials that had accumulated on the plates. Copper is also a surprisingly soft metal and great care needed to be taken to avoid further scratching the plates.
In consultation with the Minnesota Historical Society, mineral spirits were used to remove the protective coating that had been applied to the plates. In addition, scrubbing the plate with a crumpled paper towel removed the hardened finish of the coating, allowing preservation staff to minimize, and in some cases eliminate, the use of mineral spirits. Under the coating, some plates had a considerable amount of ink remaining. In addition to being unsightly, the ink is damaging to the plates, causing pitting of the copper. The USGS recommended a non-scratching metal polish hto remove any remaining ink.
At the end of each day's work, the plates were degreased with a small amount of dish detergent, rinsed with water, and then wiped with ethyl alcohol. In preparation for display, staff decided to follow the advice of a conservator in Amsterdam and use finely powdered charcoal -- a non-destructive and easily removed substance -- to enhance the lines.
Many thanks go to Laurie Jedamus (Libraries' Preservation Department) for all of her work cleaning and preparing the plates.