This collection consists of pages of unbound printed forms on which daily assay samples are recorded, giving the percentages of copper, iron, etc. in each sample. Since the period covered by these records includes the World War I years, the data displayed may show an increase in mining activity in response to war needs.

Campo Seco, eight miles southwest of Mokelumne Hill in Calaveras County prospered briefly as a gold mining camp (1850-1860), but in later years was known principally for its production of copper and zinc, most notably from the Penn Mine. In 1887 the Penn Chemical Co. acquired a mining site and erected a smelter (1899) which served the Penn mines through 1919. The smelter was equipped with a crushing and grinding plant, eight roasting furnaces and a blast furnace. The smelter was shut down and dismantled when the price of copper declined following World War I. Gross returns from the smelting operation exceeded $7 million.

Collection is open for research.