Swedish law required bars of iron to have the forges mark stamped into it for quality control reasons. In Britain, the iron was known by these marks', and the quality of each brand was well-known to the buyers in London, Sheffield, Birmingham and elsewhere.  It was divided into two grades:
* First oregrounds came from Österby (double bullet), Leufsta (now Lövsta - hoop L), and Åkerby (PL crown).  Later Gimo joined them.
* Second oregrounds came from the other forges, including Forsmark, Harg, Vattholma, and Ullfors.
Its special property was its purity.  The manganese content of the Dannemora ore caused impurities, which would otherwise have remained in the iron, to react preferentially with the manganese and to be carried off into the slag.  This level of purity meant that the iron was particularly suitable for conversion to steel by being re-carburized, using the cementation process.  This made it particularly suitable for making steel, oregrounds iron was an indispensable raw material for metal manufactures, particularly the Sheffield cutlery industry.  Substantial quantities were also (until about 1808) bought for use by the British Navy.
This and other uses absorbed substantially the whole output of the industry.  The trade in oregrounds iron was controlled from the 1730s to the 1850s by a cartel of merchants, of whom the longest enduring members were the Sykes family of Hull.  Other participants were resident in (or controlling imports through) London and Bristol.  These merchants advanced money to Swedish exporting houses, which in turn advanced it to the ironmasters, thus buying up the output of the forges several years in advance.