What Links Carbon Intensive Steel and Energy Saving Insulation?

By Richard Howard

The global steel industry accounts for 6-7% of global greenhouse gas emissions [1], making it one of the most polluting industries on earth. However, this carbon intensive industrial behemoth also gave the world mineral wool, which is used as one of the most effective energy saving measures: thermal insulation. So, how did this apparent contradiction come about?

In order to extract iron from iron ore, a continuous stream of coal or natural gas is fed into a furnace, along with the crushed ore, limestone, and hot air. The ensuing chemical reaction produces a shower of molten metal, which flows out of the furnace as a river of iron covered by a layer of molten impurities, known as ‘slag’.

By blowing a stream of hot air across the molten slag as it exited the furnace, early steel manufacturers found they could quickly cool it and produce a wispy fibrous material [2, 3], which we now know as the earliest example of mineral wool.

Nowadays, mineral wool is made quite differently. These early production methods produced a substance with good thermal insulation properties and continued being used for approximately 20 years in industrial applications. However, these early iterations lacked structural integrity. They could easily be crushed into a powder with little force and became unstable when moist. 

Enter Charles Corydon Hall, an American chemical engineer, who in 1897 invented a method of producing a stable mineral wool material using the locally sourced limestone near his home in Alexandria, Indiana. Hall had previously worked in the steel industry and had observed the mineral wool process during a business trip to Pittsburgh earlier in 1897. Hall left the steel industry the same year to commercialise his new mineral wool production process.

The new process heated limestone rock in a steel cupola furnace, before using jets of hot air to extract the fibrous material. Crucially his mineral wool did not contain any sulphur contamination, improving the stability of the material over the material produced as a by-product of the steel making process. Hall successfully scaled up his process, marketed his product as building insulation, and subsequently became known as ‘the father of the rock wool industry’ [4, 5, 6].

Increasing the energy efficiency of buildings is one of the cornerstones of the fight against climate change [7]. One of the cheapest ways of doing this is to increase the level of insulation, slowing heat lost, and reducing the energy required to heat the building. And in order to meet our net-zero goals many more houses will have to be retrofitted with extra insulation. So some might consider it a sweet irony that the mineral wool industry was many years ago born out of the carbon intensive iron and steel industry.


  1. CDP, Nerves of Steel, 2016 https://6fefcbb86e61af1b2fc4-c70d8ead6ced550b4d987d7c03fcdd1d.ssl.cf3.rackcdn.com/cms/reports/documents/000/001/195/original/CDP_Steel_2016_FINAL.pdf.pdf?1479377027
  2. Swapna Mukherjee, Applied Mineralogy: Applications in Industry and Environment, 2011, Springer
  3. US Patent, US103650A, 1870, https://patents.google.com/patent/USRE6895E/en
  4. http://alexandriahistoricalsociety.com/?q=node/18
  5. The Seattle star (Seattle, Wash.), 05 April 1899. Chronicling America: Historic American Newspapers. Lib. of Congress. https://chroniclingamerica.loc.gov/lccn/sn87093407/1899-04-05/ed-1/seq-1/
  6. The Alexandria Times-Tribune (Alexandria, Indiana), 21 Aug 1935, https://www.newspapers.com/paper/the-alexandria-times-tribune/3383/
  7. Allwood, J., Azevedo, J., Clare, A., Cleaver, C., Cullen, J., Dunant, C., Fellin, T., et al. (2019). Absolute Zero. https://doi.org/10.17863/CAM.46075

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