Tin mined from lodes is usually impure, with pyrites and arsenopyrites reducing its value to the smelter. Mines removed these impurities by burning the ore; the pyrites etc decomposing in oxygen to arsenious oxide and sulphur dioxide which could be carried off in fumes (see Earl 1968, 92). By the mid-19th century burning, or calcining was done in Cornwall on Brunton Flat-bed calciners. These replaced the more simple form of which this Bosigran building is a late example, presumably being constructed after the 1861 stamping mill (94497) was established. It is shown on the 1876 OS 25 map but was not there when the 1842 Tithe Map was made. The calciner itself is the smaller western room (3.6m E-W by 3.0m); the eastern room (4.4m NS x 4.0m) being where the person raking out the roasted ore worked. This eastern room was possibly originally the southern of the two Bosigran grist mills (94496) although no sign of a wheel pit survives (the Tithe Map suggests it would have been on the south side). It is certainly structurally primary in relation to the calciner; walls stand to 2.9m high (south) although the northern has tumbled. A small keeping place (0.5m wide, 0.4m high, 0.5m deep) is centrally placed in the internal east wall. A sill 0.1m wide, 0.9m above the floor, runs along the inside of the south wall and may have supported a wooden floor, or bench. The calciner comprises a low (c 0.5m high) brick chamber or furnace 3m long with gently arced sides 1.8m apart at the west end, broadening to 2.0m in the centre and narrowing to c 1.3m at the east end where a hatch 0.6m wide 0.4m high gave the raker access. This hatch was closed with a metal door; the holes of its hinges and latches etc survive. The fire, on a griddle, was lit at the west end and the heat was drawn through the furnace by a stone-lined flue which commenced at the east end of the building and ran along its north side and along the ground to the stack. The ore was introduced to the furnace through a small hole in its roof (which does not survive); it was brought into the calciner by the west door (1.0m wide). The ore was heated until dull red and the fumes given off ceased to be white (ie until all the arsenic was removed). The raker periodically stirred the ore in the furnace and, when it was finished, raked it out through the hatch. The chimney, 12m to the south-west of the mill, is 2m in diameter at its base and still stands to c 6m. The flue entrance is visible at the base, to the south-east. It is now a completely stone built chimney and W. Berryman (Upper Porthmeor) believed it was originally stone-topped (ie no brick topping).