Kawau Copper Mine operation and ore smelting
The Kawau copper lode is contained in silicified greywacke and dips downward at a high angle. Above ground water level the sulphide is well weathered and oxidised to sulphate, the blue copper sulphate being easily seen in the exposure on the spur.
Blue copper sulphate visible in the flooded drift entrance adjacent to the Cornish Pump House built on the south side of the island.
Lower down in the lode there is a transition through an enriched zone where copper from the weathered ore above has been transported and re-deposited, and below that again is the relatively unaltered primary ore of lower grade. Workings commenced in the enriched zone which about at the top of the permanent ground water region, and mine development then proceeded on three levels. As the mining progressed toward the unaltered primary ore the Kawau Company found in January 1847 that the sulphides underwent an exothermic reaction aboard ship. The shipments of ore, released from the underground pressure and exposed to the air heated and swelled dangerously on the way to smelting works in Wales. As a result some Kawau ore shipments were stockpiled in Australia and others disposed of at sea to save the vessel.
To overcome the difficulties a decision to build a smelting works was made in November 1848 with construction proceeding during 1849 in a bay on the north side of Bon Accord Harbour. Both the Cornish pumphouse at the mine and the smelting works building were made from sandstone blocks quarried at Matakana on the mainland.
The arched seaward side sandstone block wall of the of the smelting house can be seen in the photograph to the left.
A party of smelters was engaged in Swansea in Wales and sent out to Sydney where they worked for six months at Port Jackson, experimenting with Kawau ore stockpiled there before continuing on to Kawau. At the Kawau works the sulphide ore was smelted and cast into copper regulus blocks and the slag was also cast into blocks. The casting was done in pits in the floor of the smelting house building. The copper regulus blocks of about two cubic feet in volume were then shipped overseas for further refining. Some of the slag blocks were later used to construct pillars at the landward end of the Mansion House Bay jetty in Bon Accord Harbour, and for foreshore retaining structures. It is thought that others may have been dispersed as ships ballast.
Operations at the smelting works produced quantities of sulphur dioxide fumes as the sulphide ore was smelted and to avoid the acrid fumes the Welsh workers and their families moved further down the harbour to a place they nostalgically named Swansea Bay.
After the mine closed about 1855 Mr Alexander Harris continued to stay on as caretaker and to run a store and post office. He was there when Sir George Grey purchased Kawau in 1862 and his daughter Elsie became housekeeper for Sir George Grey.