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The Custom Assay Office, also known as Critchett and Ferguson, began in El Paso around 1902 and continued until 1950. Otis A. Critchett (1876? – 1950?) and George W. Ferguson (1872 – 1940) worked as assayers, metallurgists, and chemists for individuals and several mining companies in Texas, New Mexico, and Mexico. Critchett and Ferguson (whose sister, Belle, married Critchett in 1901) represented ore shippers, conducted assays and analysis of metals and ores, and did sampling work of car lots at the El Paso smelter. The Critchett and Ferguson Assayers records date 1902 – 1918, bulk 1911 – 1918. Types of records include correspondence, hardcover bound copy books, bills and invoices, and other printed material. Arranged in three series (Correspondence, Account/assay record books, and Other business records), these records document the business activities and assay research and results of the Custom Assay Office, run by Otis A. Critchett and George W. Ferguson.