The Welsh Process of extracting and refining copper was long, costly and tedious when compared with modern techniques, but it remained more or less unchallenged until the mid-19th Century. In 1851, the year of the Great Exhibition, considerable improvements in the smelters were announced; but it was not until Bessemer's invention of the blast furnace, together with the employment of large reverberatory furnaces, that output improved. According to Alexander, the ore at Swansea, after being broken into small lumps by hammers, was hand-sorted by girls; hence much good metal must have been wasted. The selected ore was then calcined no fewer than three times to reduce the sulphur content down to about 11 per cent. The calcine was then smelted with suitable fluxes, the aim being to produce a matte, i.e. a molten mixture of copper and iron sulphides, which was cast into pigs. These were slowly reheated until they yielded blister copper which was about 98.5 per cent pure. Blister copper is so named because its upper surface is full of blowholes or blisters due to entrapped oxygen. To expel this oxygen the cake was remelted; and branches or poles of green wood were thrust into the molten metal. Combustion of the green wood occurred and this reduced the oxygen content to a mere trifle. This important stage was called 'poling'. Finally the metal was cast into billets and cakes suitable for wrought work. This product is known as tough pitch copper. It is a remarkable fact that despite all the modern metallurgical improvements, no better way has yet been found to deoxidize blister copper than the old-fashioned and picturesque one of poling.