The Thereza Christina Maria collection is composed of 21,742 photos assembled by Emperor Pedro II (1825-91) throughout his life and donated by him to the National Library of Brazil. The collection covers a wide variety of subjects. It documents the achievements of Brazil and Brazilians in the 19th century and also includes many photographs of Europe, Africa, and North America. In 1868, photographer Augusto Riedel accompanied Luis Augusto, Duke of Saxe, son-in-law of Emperor Pedro II, on an expedition into the interior of Brazil. During a visit to the Minas Gerais town of Morro Velho, the group witnessed the process of amalgamation, used to recover gold from mined ore. This process consists of dissolving the ore in liquid mercury and then distilling the amalgam to separate out the gold.