File:Reverberatory furnace for tin ores roasting.png

English: Most of the tin ores in Cornwall have to be roasted, or calcined, before they are fit for the smelting-house, although in some mines the admixture with other minerals is so trifling, that this operation is considered unnecessary. The furnace {figs. 420, 421) in which the roasting is carried on, is about 10 feet long, 5 feet inches wide in the middle, and 3 feet wide near the mouth. The fireplace, it will be observed, is situated at the back, the flames playing through the oven and ascending the chimney, which is above the furnace door. The man is represented in fig. 421 as stirring the ore with a long iron rake. The ore, before it is submitted to the action of the fire, is thoroughly dried in a circular pit, placed immediately above the oven, into which it is let down through the opening when it is considered to be ready for calcining. Beneath the oven and connected with it by an opening through which the ore when sufficiently roasted is made to pass, is an arched opening about 4 feet wide, termed the " wrinkle." Here the ore is collected, whilst another charge is being placed in the furnace. About 7 cwt. or 8 cwt. of ore is the quantity usually roasted at one time. Whilst undergoing this operation, dense fumes of arsenic and sulphur escape with the smoke from the fire, and pass through large flues, divided into several chambers, {fig. 422,) where the former is collected. The flue is often 70 yards long, and the greatest deposit of arsenic takes place at about 15 yards from the oven or furnace. Instead of being at once completely roasted, the "whits" from the stamps are sometimes first "rag" (or partially) burnt, for about six or eight hours. The object of this partial burning is to save time and expense, nearly three-fourths of it being thrown away after dressing it from the first burning.