The cementation process is an obsolete technology for making steel by carburization of iron. Unlike modern steelmaking, it increased the amount of carbon in the iron. It was apparently developed before the 17th century. Derwentcote Steel Furnace, built in 1720, is the earliest surviving example of a cementation furnace. Another example in the UK is the cementation furnace in Doncaster Street, Sheffield. Origins The process was described in a treatise published in Prague in 1574. It was again invented by Johann Nussbaum of Magdeburg, who began operations at Nuremberg (with partners) in 1601. The process was patented in England by William Ellyot and Mathias Meysey in 1614. At that date, the "invention" could consist merely of the introduction of a new industry or product, or even a mere monopoly. They evidently soon transferred the patent to Sir Basil Brooke, but he was forced to surrender it in 1619. A clause in the patent prohibiting the import of steel was found to be undesirable because he could not supply as much good steel as was needed. Brooke's furnaces were probably in his manor of Madeley at Coalbrookdale (which certainly existed before the English Civil War) where two cementation furnaces have been excavated.<ref>P. Belford and R. A. Ross, 'English steelmaking in the seventeenth century: excavation of two cementation furnaces at Coalbrookdale' Historical Metallurgy 41(2) (2007), 105-123.</ref> He probably used bar iron from the Forest of Dean, where he was a partner in farming the King's ironworks in two periods. By 1631, it was recognised that Swedish iron was the best raw material and then or later particularly certain marks (brands) such as double bullet (so called from the mark OO) from Österby and hoop L from Leufsta (now Lövsta), whose mark consisted of an L in a circle, both belonging to Louis De Geer and his descendants. These were among the first ironworks in Sweden to use the Walloon process of fining iron, producing what was known in England as oregrounds iron. It was so called from the Swedish port of Öregrund, north of Stockholm, in whose hinterland most of the ironworks lay. The ore used came ultimately from the Dannemora mine. Process The process begins with wrought iron and charcoal. It uses one or more long stone pots inside a furnace. Typically, in Sheffield, each pot was 14 feet by 4 feet and 3.5 feet deep. Iron bars and charcoal are