In 1865, George Duncan purchased the Ripley & Company glass factory at 10th and Carson Streets in the South Side neighborhood of Pittsburgh, Pennsylvania from D.C. Ripley, a former business partner of Duncan. The factory was located adjacent to the Monongahela River, which provided easy access to transportation. A new company, George Duncan & Sons, was formed by Duncan, his sons Harry B. Duncan and James E. Duncan, and Augustus H. Heisey, who was married to George's daughter Susan. In 1874, John Ernest Miller, a 20-year veteran of the glass manufacturing industry, joined the company as a designer. He was previously the foreman of the mould shop at King, Son and Company. The addition of Miller proved to be fortuitous, because Miller's designs over the next 52 years became internationally famous. In 1877, George Duncan died and James E. Duncan took control of the company.
In 1890, the United States Glass Company formed a glass trust that included the George Duncan & Sons operation. In 1892, their factory was destroyed by fire, relieving the company of their trust relationship. After the fire, son-in-law Augustus Heisey left the company to begin his own glass manufacturing operation in Newark, Ohio. James E. Duncan built a new factory on Jefferson Avenue in Washington, Pennsylvania, where access to railroads was convenient, and the natural gas which was needed to power the furnaces was inexpensive. Construction of the plant, which featured a 16 Pot Deep Eye furnace, was finished on January 3, 1893. The first pattern produced in the new facility was the well-known Mitchell pattern. The new operation was named George Duncan's Sons & Co.
On November 15, 1900, after James Duncan's death, the firm was officially incorporated as Duncan and Miller Glass Company, with John Ernest Miller joining the Duncan family as stockholders. The company abruptly ceased operations on June 13, 1955. By then, modern assembly lines and glassmaking machinery caused hand made glass to be uneconomical. The remaining inventory was sold at greatly reduced prices, causing glass collectors to travel hundreds of miles to purchase the final pieces. The molds and equipment were sold to the US Glass Company, where their Duncan Division in Tiffin, Ohio continued to produce Duncan-ware pieces. A number of Duncan employees also joined US Glass Company in Tiffin. The Duncan factory was sold to Andy Bros., but before they could move in, a fire destroyed the building on June 29, 1956. Production of the Duncan-ware glass ceased in 1980.
The glass making processes and technologies at Duncan & Miller did not significantly differ from contemporary glassmakers in Western Pennsylvania and neighboring Ohio and West Virginia. Duncan differentiated itself through exceptional artistry and design. This was accomplished by employing highly skilled workers, and using unique batch formulas to produce colorful glass. The production process for each piece usually required 10 craftsmen.