Diamond Shamrock was one of the major oil-refining and marketing companies in Texas in 1993, with more than 6,000 employees and over $2.5 billion in revenue. It traces its origins back to three companies: Diamond Alkali, Shamrock Oil and Gas, and Sigmor Corporation. A group of glass manufacturers founded Diamond Alkali in Pittsburgh, Pennsylvania, in 1910 to produce soda ash, an important component in the glass industry. A large industrial plant was built at Painesville, Ohio, in 1912, and in the 1920s, under the leadership of T. R. Evans, Diamond Alkali developed into an important chemical company. It diversified into the production of calcium carbonates, coke, cement, chlorine, and a variety of other products. Under the presidency of T. R.'s son, Ray Evans, the company moved to decentralize after World War II. In 1946, in Deer Park, Houston, Texas, construction was started on a new plant to produce chlorine and caustic soda. Diamond Alkali changed its headquarters from Pittsburgh to Cleveland, Ohio, in 1948. A third plant was erected in Muscle Shoals, Alabama, in the 1950s, and Diamond Alkali further extended its range of products into plastics and agricultural chemicals. In the 1960s the company opened another facility in Delaware City, Delaware, and purchased several additional chemical companies, including the Chemical Process Company of Redwood City, California, and the Nopco Chemical Company of New Jersey. In spite of its growth, Diamond Alkali feared absorption by a larger chemical firm or by one of the major oil conglomerates, and in 1967 it merged with Shamrock Oil and Gas of Amarillo to increase its corporate strength.
The second progenitor of Diamond Shamrock, the Shamrock Oil and Gas Company, was founded on August 9, 1929, by John Sheerin, who named the company for a symbol of his native Ireland. The company was headquartered in Amarillo and financed by the Fownes family of Pennsylvania. In its early years Shamrock Oil and Gas lost some $9 million. Its first refinery was built in 1933 at Sunray, in Moore County, Texas, and that same year the company opened its first gas station, also at Sunray. In 1938 James Harold Dunn, a young engineer at the Lone Star Gas Corporation, joined Shamrock Oil and Gas as vice president and general manager, and the next year the company made its first small profit. In 1939–40 Shamrock and the Lone Star Gas Corporation jointly built a plant at Murchison in Henderson County to recycle natural gas. By 1941 there were 162 Shamrock service stations in the southwestern states. During the mid-1940s Shamrock became a major producer of natural gas. The company paid its first dividend in 1943, and was listed on the New York Stock Exchange in 1944. Dunn became company president in 1945 and served in that post for ten years; he was succeeded by C. A. Cash in 1955. In 1959 Shamrock opened its first catalytic cracking unit at the McKee complex at Sunray.
In 1960 Shamrock Oil and Gas increased the number of stations using its gasoline by purchasing much of the Sigmor chain of service stations-the third major component of the modern Diamond Shamrock Corporation. Sigfried (Sig) Moore operated the Midway chain of stations in the 1930s and 1940s. When, in 1943, Moore loaned one of his employees, Thomas E. Turner, the money to start up a business of his own, Turner adopted the name Sigmor for the chain of stores he built up in the 1940s and 1950s. Sigmor was incorporated in 1952, then restructured in 1959 so that each station could incorporate individually. Most of the Sigmor chain was purchased in 1960 by Shamrock, which then leased it back to Turner, who continued to run it. In the 1960s Shamrock expanded its production facilities at McKee and constructed pipelines throughout the Southwest.
In 1967 Shamrock Oil and Gas Company and Diamond Alkali merged to form the Diamond Shamrock Corporation. At the time of the merger, the products of the company were about 20 percent oil and gas and 80 percent chemical. The company expanded its oil and gas exploration efforts in the early 1970s and built several new chemical plants, including the Battleground plant in Houston. In 1978 the company headquarters were moved to Dallas. That same year Sigmor purchased its stations back from Diamond Shamrock, though it continued to market Diamond Shamrock products. In 1980 Diamond Shamrock employed some 12,400 people in thirty-seven countries and was a leader in energy, technology, and chemical markets.
In January 1983, Sigmor, by then one of the largest independent service-station chains in the country, merged with Diamond Shamrock. Diamond Shamrock acquired some 600 retail outlets by the merger, as well as the Three Rivers oil refinery, constructed by Sigmor in the mid-1970s in Live Oak County midway between San Antonio and Corpus Christi. In 1987 the Diamond Shamrock Refining and Marketing Company was separated from Diamond Shamrock Corporation, the parent company, and became an independent company headquartered in San Antonio. As part of the same reorganization, Diamond Shamrock Corporation changed its name to Maxus Energy Corporation, and severed all legal ties to Diamond Shamrock Refining and Marketing Company. In 1990 Diamond Shamrock Refining and Marketing Company, Incorporated, simplified its name to Diamond Shamrock, Incorporated. In 1993 Diamond Shamrock operated two major petroleum-refinery complexes: the Three Rivers plant and the McKee complex at Sunray. The two complexes had a combined capacity of nearly 200,000 barrels a day. An extensive network of some 4,400 miles of company-owned pipelines connected the refineries to supply points and markets in Texas, Colorado, New Mexico, and Oklahoma. Some 2,000 outlets carried the Diamond Shamrock brand, including 776 company-operated Corner Stores in Texas, Colorado, Louisiana, and New Mexico.