The origins and inspirations of zinc smelting
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- Craddock, P.T. J Mater Sci (2009) 44: 2181. doi:10.1007/s10853-008-2942-1
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The smelting of zinc was one of the most challenging technologies of the pre-modern world; indeed in the application of scientific techniques to industrial processes it anticipated modern industrial practice. Very different approaches to the problem evolved in different parts of the world. As the process required the condensation of the metal from a vapour, it is likely that some of the then current distillation practices might have provided an inspiration for the process adopted. Specifically, the techniques employed for the production of mercury may also have provided methodologies that could be adapted.
The production of metallic zinc must represent one of the most advanced technical processes of the pre-modern world. Indeed the development of these processes can be seen as forming a continuous link with the Industrial Revolution which is regarded rather narrowly by some as being exclusively a European-North American phenomenon of the 18th and 19th centuries AD.
The problems confronted in zinc smelting are three-fold. A great deal of energy is required to reduce the ores to metal. The zinc is produced as a vapour with a boiling point of 907 °C (although in practice the vapour has to be cooled to below 600 °C if it is to be quickly condensed, leaving only quite a narrow temperature range between the condensing and 420 °C solidification temperatures). Thirdly, this vapour is very reactive, being intensely reducing, such that an overpressure of carbon monoxide is necessary throughout the system. In each of the three regions to be considered it seems that the model for the condensation of the zinc vapour was based on the existing systems for the distillation of aqueous media, or more specifically mercury, suitably adapted to meet the much more challenging conditions required to smelt zinc.
Other metals, notably copper, also occur in the vicinity and much of China’s cash coinage was produced here. Zhou  has suggested that the production of brass by the direct method for coin production was rapidly expanding into a large-scale industry about 500 years ago and the development of the production of metallic zinc, which was used almost exclusively for brass making, should be seen as a progression in the brass-making technology.
The ceramic retorts were charged with small lumps of ore and briquettes of the special low-sulphur coal that occurs in the region. The collecting dish was put in place but the condenser lid left off and the furnace was fired for some hours to calcine the ore. The lid was then put in place, the temperature of the furnace was raised and the smelting commenced, the process running for between 12 and 18 h, the whole process taking about a day.
In India the distillation of zinc began about 1000 years ago utilising the traditional Indian distillation method of adhaspatana yantram, which is distillation by descending . Some of the very early Indian descriptions of mercury production given in works such as the Rasaratnakara (7–8th century AD) or the Rasahridya (12th century AD) describe how a pot containing the mercury ore is to be inverted over another pot containing water in which the descending mercury vapour condensed .
Once the sea routes to the East were opened up to European vessels in the Post-Medieval period, zinc was amongst the products brought back to Europe. As early as 1513, an Italian agent in Lisbon reported that a Portuguese ship loaded with 200 pieces of ‘Indian tin’ had just arrived from the East and that the English had better look to protect their tin trade. Outside of India, there was as yet no word for zinc, and in the lands bordering on the Indian Ocean, it was commonly referred to as Indian tin. Indian zinc continued to be imported into Europe, but was soon overtaken in international trade by Chinese zinc, usually transported in Dutch vessels . In Europe, zinc commanded a high price for making top quality brass, and the very limited amount collected from the flues at Rammelsberg was used for this purpose, as already noted, mainly by the brass founders of Nürnberg.
The process was in use in South America and at Almadén through the 18th and 19th centuries and was reasonably well publicised in Diderot’s Encylopedie etc. In addition, the Almadén mines had been carefully studied by the great Spanish scientist Augstín de Betancourt, who was to spend much of the late 18th and early 19th centuries travelling through Europe at just the time when Dony was developing his process with the encouragement of Napoleon. Thus it is possible that the lines of inclined horizontal condensers of the Bustamante process could have inspired the inclined horizontal retorts with their condenser prolongs of the Dony process.
The zinc distillation technology in India and China would seem to have been completely independent developments and in Europe, once the reality of the production of zinc on an industrial scale had been established, probably inspired by the Indian process, there were rapid developments of a variety of processes. The rising popularity of brass concentrated the search for a more efficient use of the sublimed zinc ore, leading to the production of the metal of metallic zinc as the feedstock in brass production. There was also the growing realisation that brass made by mixing the two metals was superior. The problems encountered establishing viable high-temperature distillation under very reducing conditions on an industrial scale were met by the brass makers across the Old World in different in a variety of ways. The form of the furnaces was variously based on pottery kilns, glass or brass making furnaces. The principle of the distillation technique was usually adapted from the existing local prevalent distillation technology, especially for the production of mercury where such a technology existed.
However, there was a very considerable extension of the operating parameters necessary to produce metallic zinc and the diversity of solutions that were developed must command an admiration for the metal smelters across the world and through the ages.