Coplay can be called the birthplace of the modern American cement industry. Its two most important developments, portland cement and the horizontal rotary cement kiln, were introduced there in the latter part of the 19th century. In 1875, David O. Saylor's Coplay Cement Company began producing his patented portland cement, which was stronger and more versatile than the natural cement he had made there for nearly a decade and used the same types of limestone, but with rock ground nearly to dust and burned at much higher temperatures. This was the first time portland cement was manufactured in the United States. During the next decade, sixteen new portland cement mills were built around the Lehigh Valley and the rest of the U.S., but Saylor's Portland Cement was the standard against which all the rest were measured. In 1889, Jose de Navarro built the first successful rotary cement kiln just north of the Coplay Cement mills. The plant, known as the Old Valley Mill, went into production on November 8 under the company name Keystone Cement and turned out an average of 200 barrels of cement a day. The horizontal rotary kiln was far more efficient than vertical kilns, producing more cement in less time and requiring less fuel and labor, and it had replaced vertical kilns everywhere by the early 20th century. Navarro's company was renamed the Atlas Cement Company of New York, and became the Atlas Portland Cement Company in 1899. The first Atlas cement plant in Northampton began operations in 1896, and the Old Valley Mill was shut down in 1907. By then, the Lehigh Valley was producing 70% of all the Portland cement used in this country.
