are used: 1. The intermittent dome kilns that resemble in their operation common lime kilns. 2. Continuous kilns, of the Ditzsch or Schœfer patterns. 3. The Hoffman ring-kiln. These kilns are economical in fuel, but expensive in time and labor. The dome-kiln was the first used, but has now disappeared from the United States, and survives in Europe only in a few localities. The continuous kilns require highly skilled labor, and are used only to a limited extent either in the United States or Europe. The Hoffman ring-kiln is widely used in Europe, but has found few patrons in the United States. The second method of manufacture of Portland cement is by the use of the rotary furnace, into which the mix is fed as a dry powder or ad a wet mud. Although the rotary cement furnace was originally patented in England, by Frederic Ransome, in 1885, it has been improved in the United States to such an extent that it has become practically
an American invention. Ransome's patent required the use of gas for fuel and the first rotary kilns installed in the United States required gas, but gas was soon replaced by crude petroleum. This fuel took the place of gas entirely, and was exclusively used until it was replaced by pulverized coal, blown into the kilns and burned in a jet like gas or sprayed petroleum. The current or blast of air which carries the powdered coal into the kiln furnishes the oxygen for its complete combustion. This is a convenient method for burning the cheapest fuel on the market, and while it is not economical of fuel it
