Hot blast refers to the preheating of air blown into a blast furnace or other metallurgical process. As first developed it worked by alternately storing heat from the furnace flue gas in a firebrick lined vessel with multiple chambers, then blowing combustion air through the hot chamber. This is known as regenerative heating. This has the result of considerably reducing the fuel consumed in the process. This was invented and patented for iron furnaces by James Beaumont Neilson in 1828 at Wilsontown Ironworks in Scotland, but was later applied in other contexts, including late bloomeries. Later the carbon monoxide in the flue gas was burned to provide additional heat.
Invention and spread 
James Beaumont Neilson, previously foreman at Glasgow gas works invented the system of preheating the blast for a furnace. He found that by increasing the temperature to 300 degrees Fahrenheit, he could reduce the fuel consumption from 8.06 tons to 5.16 tons with further reductions with higher temperatures. He with partners including Charles Macintosh patented this in 1828. Initially the heating vessel was made of wrought iron plates, but these oxidized, and he substituted a cast iron vessel.
Spread to other parts of Britain was relatively slow. However by 1840, 58 ironmasters had taken out licenses, yielding a royalty income of £30,000 per year for Neilson and his partners. However they had to engage in substantial litigation, ultimately successful, over the following years to enforce the patent against infringers. By the time the patent expired there were 80 licenses. In 1843, just after it expired, 42 of the 80 furnaces in south Staffordshire were using hot blast, and uptake in south Wales was even slower.
Other advantages were that raw coal could be used instead of coke and in Scotland, the relatively poor "black band" ironstone could be profitably smelted. It also increased the daily output of each furnace, in the case of Calder ironworks from 5.6 tons per day in 1828 to 8.2 in 1833, which made Scotland the lowest cost region in Britain in the 1830s.
Nevertheless, early hot blast stoves were troublesome, as expansion and contraction were liable to cause breakages in the pipes. This was to some extent remedied by supporting the pipes on rollers. It was also necessary to devise new methods of connecting the blast pipes to the tuyeres, as leather could not longer be used for making the connection.
Ultimately this principle was applied even more efficiently in Regenerative heat exchanger, such as the Cowper stove (which preheats the blast in blast furnaces to this day), and in the Open hearth furnace (for making steel) by the Siemens-Martin process.
Anthracite in ironmaking 
The invention of the hot blast process was of particular importance to the iron industry in the United States. At the time the process was invented, good coking coal was not yet being mined in the U.S., and iron furnaces were compelled to use charcoal. This meant that any given iron furnace required vast tracts of forested land for charcoal production, and generally went out of blast when the nearby woods had been felled. Attempts to use anthracite as a fuel had all ended in failure, as the coal resisted ignition under cold blast conditions. In 1831, Dr. Frederick W. Gessenhainer filed for a U.S. patent on the use of hot blast and anthracite to smelt iron. He produced a small quantity of anthracite iron by this method at Valley Furnace near Pottsville, Pennsylvania in 1836, but due to breakdowns and his illness and death in 1838, he was not able to carry the process into large-scale production.
Indendependently, George Crane and David Thomas, of the Yniscedwyn Works in Wales, conceived of the same idea, and Crane filed for a British patent in 1836. They began producing iron by the new process on February 5, 1837. Crane subsequently bought Gessenhainer's patent and patented additions to it, controlling the use of the process in both Britain and the U.S. While Crane remained in Wales, Thomas would move to the U.S. on behalf of the Lehigh Coal and Navigation Company and found the Lehigh Crane Iron Company to make use of the process.
For steel the hot blast temperature can be from 900 °C to 1300 °C (1600 °F to 2300 °F) depending on the stove design and condition. The temperatures they deal with may be 2000 °C to 2300 °C (3600 °F to 4200 °F). Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy which is necessary to increase productivity.
- [David. S.] (1969). The Unbound Prometheus: Technological Change and Industrial Development in Western Europe from 1750 to the Present. Cambridge, New York: Press Syndicate of the University of Cambridge. p. 92. ISBN 0-521-09418-6
- W.K.V. Gale, British iron and steel industry (David and Charles, Newton Abbot 1967), 55-8.
- "Neilson, James Beaumont (1792–1865)". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/19866. (subscription or UK public library membership required)
- C.K. Hyde, Technological change and the British iron industry 1700-1870 (Princeton University Press, 1977), 154-5.
- C.K. Hyde, Technological change and the British iron industry 1700-1870 (Princeton University Press, 1977), 151.
- W.K.V. Gale, The Black Country iron industry (David and Charles, Newton Abbot 1966), 71-5.
- W.K.V. Gale, British iron and steel industry (David and Charles, Newton Abbot 1967), 98-100.
- Bartholomew, Craig L.; Metz, Lance E. (1988). Bartholomew, Ann (ed.), ed. The Anthracite Industry of the Lehigh Valley. Center for Canal History and Technology. ISBN 0-930973-08-9.