Historic Stamp Mill Conservation— Empire State Historic Park

Grass Valley, CA

We performed condition assessments and treated an historic stamp mill located at the Empire State Historic Park in Grass Valley, California. The original stamp mill was relocated to the park at some point in the past to act as an educational tool for visitors.

During the assessment of the artifact, it was found that the feed inlets on the mortar boxes were corroded through completely, one of the mortar boxes was cracked near the feed inlet, and there was a significant amount of unstable iron corrosion present on most of the metal surfaces. In addition, the wooden components suffered from significant dry rot and insect infestation.

Once the structure had been inspected and documented, all of the metal components were cleaned with soft natural bristle brushes and a 3% solution of non-ionic detergent and water. The surfaces were then cleaned using waterjets at approximately 2,000 to 3,000 psi to help remove loose corrosion products. The metal surfaces were then treated with a 3% to 5% solution of tannic acid. Once fully dry, all surfaces received two coats of a microcrystalline wax and were then buffed using cactus brushes and clean cotton rags. A conservation treatment report, which included photo documentation and a continuing maintenance plan for the stamp mill was compiled for the client.