Detailed plan of the Merton patent roasting furnace; the mill is to the right.  This site has been largely intact until the last decade, when vandalism to the furnace has caused some damage.  However, the remains are good enough to restore to original condition if required.  Some of the metal work is missing, but the majority is either on site or has been removed for safe-keeping.

The Merton furnace was introduced in the early 1900s.  It was a compound structure, typically comprising a building containing one or more sets of three hearths stacked one above the other, usually about one metre apart.  A cast-iron spindle ran upwards through the centre of each set of hearths, turning a rabble or rake arm in each hearth.  Ore was poured through a hopper into the top hearth, raked and roasted and dropped through a grate into the next hearth down then repeated for each hearth in the set.  From the bottom hearth the ore was either raked out or poured automatically into wagons.

Within the UK this type of furnace found more use in the production of sulphuric acid, from the roasting of pyrite although several were erected in South Wales for roasting sphalerite.  A four-hearth furnace was built at Coniston Mine in the Lake District.  It is thought that about six of these furnaces were built in Cornwall.  Of these only the Boscaswell furnace exists above ground level; it is probably one of the most important industrial remains in the west of the county.