The process was improved by David Davis and George Wightman Wallace, a consulting engineer of the NTU Company.
[2][3] In 1925, the NTU Company built a test plant at Sherman Cut near Casmalia, California.
[1][4] In 1925–1929, the process was tested by the United States Bureau of Mines in the Oil Shale Experiment Station at Anvil Point in Rifle, Colorado.
During the World War II, three NTU retorts were operated at Marangaroo, New South Wales, Australia.
The supply of fuel gas stopped after the upper quarter of the oil shale batch started to burn.
Operating the NTU retort with nominal capacity of 40 ton of raw oil shale, the full process cycle took about 40 hours.
The disadvantage of this process was a batch mode of operation not allowing continuous retorting, and therefore having small capacity being labor extensive at the same time.