Mining and smelting, metals and mineral resources
1 Ores to metals
In the process from a piece of rock to a metal tool, we generally differentiate between two main steps: mining, or the extraction of the piece of rock from the ground, and smelting, or the chemical transformation of metal compounds to metals that usually involves firing at high temperatures.
The division may appear arbitrary, and the two steps are in fact linked by a third: the ore dressing, the preparation of the extracted pieces of rock for smelting. It makes sense in that the two main steps require different fields of expertise and were often separately organized: Mining required practical mineralogical experience to recognize the presence of ores, and technologies to extract them. Smelting requires metallurgy, commonly by operating high-temperature furnaces fed with ore, fuel and additives to produce the environment for a chemical process in which metal compounds are reduced to metals. To be more specific, the following outlines of the most common work steps that provide an idea of what was involved and where the exploitation of metals and minerals meet and overlap.
1.1 Mining
Mining was commonly a labour-intensive process. It does not necessarily involve underground mining. Processes of decomposition may leave metal oxides or even native metals in surface soils or debris. Gold washing and the famous karst ore, deposits where ore can be worked by washing the soil between limestone formations as certain sites (e.g. Gejiu, Beiya, probably many more), are deposits that can be worked from the surface or in small open pits. A historic technology of flushing our mountain flanks may have been developed for working such ores. This involved the construction of water reservoirs on top of a mineralized slope, the burning off of the vegetation, and the flushing out of gullies. We have been able to confirm the existence of this technology, but are not sure whether it was used for prospecting or extraction.
Mining pits resulted from working down, typically into “iron hoods.” These are also the point where stone age and metal age technologies most evidently meet, as we know of the existence of stone-are flintstone mines (flintstone not exposed to oxidation is less brittle, hence pit mining made perfect sense).
Fully developed underground mining involving networks of shafts descending into the mountain and level or inclined galleries, with ventilation and drainage systems. It permitted reaching a few hundred metres into the mountain and meant a major investment.
Mining involved special skills and body technologies for spending entire working days in dark and confined spaces, performing hard and dirty work, which was unhealthy even under the best conditions. Mining before modern equipment always involved the lethal risk of rockfalls, especially in an earthquake region of much folded and often instable rock. Even in of the absence of any accidents the fine rock dust, often needle-like and not infrequently toxic, almost inevitably damaged the lungs, while the work itself hurt the joints. Mined ore had to be carried. In underground mines, ore carriers worked with bags and usually walked on all fours, with two short wooden stilts in their hands. Outside the mountain, short distances were commonly performed by human porters, who worked with carrying frames or poles.
1.2 Ore dressing
The preparation of the ore involved two aspects: removing gangue parts of the extracted material that contained none of the targeted metal or was too low in metal content to be worth smelting, and crushing the ore to a grain size most suitable for the smelting process. Much or all of this work was usually performed by hand.
People sat at the mines sorting the extracted material and chipping off gangue parts working with a pounding rock or a hammer.
Once the sorted ore was carried to the dressing site, others crushed it by hand with rocks or large hammers, under a water-driven hammer that worked like a pestle, or ground it finely in grinding stones or by ox-driven stone grinding wheels. The technologies depended on their availability, the scale of the mine, the availability of labour and water power, and the grain needed for the following smelting process. Grinding was usually followed by flotation that removed the lighter gangue fraction. People loaded small amounts of ground ore on bamboo sieves and gently moved it in flowing or stagnant water, using different combinations of movement and water flow to loose light and fine fractions and thus condense the metal content.
Flotation and crushing was often joint with roasting, a low-heat firing process necessary with sulphide ores to remove the sulphur and other substances that interfered with the smelting reactions. Roasting commonly took place in low open stacks and required little fuel once the process was started. The burning sulphur largely maintained the process, producing massive amounts of sulphur dioxide and other fumes.
Crushing and flotation usually had to be repeated after roasting. With certain ores, several rounds of these processes were necessary. In short, ore dressing was a work step that was crucial to get the ore into the right state and grain for smelting, usually aiming at reducing gangue as much as possible and at transforming the metal components into oxides as far as possible. It required less skills than the other steps and hence appears relatively minor, but could be labour intensive.
Crushing and flotation usually had to be repeated after roasting. With certain ores, several rounds of these processes were necessary. In short, ore dressing was a work step that was crucial to get the ore into the right state and grain for smelting, usually aiming at reducing gangue as much as possible and at transforming the metal components into oxides as far as possible. It required less skills than the other steps and hence appears relatively minor, but could be labour intensive.
1.3 Smelting
If we include all firing processes, the roasting process already described counts at the preparatory step. Smelting narrowly defined is the firing process that induces a reaction in which the metal is reduced and separated from the gangue. It typically takes place in a furnace in involves high temperatures. We should note that the required reaction typically required breaking up a metal oxide into its two components, the metal + oxygen, while liquefying the gangue fraction. It does not necessarily involve reaching the melting point of the targeted metal.
Most processes reached the required temperatures by a high input of fuel and ventilation by bellows. Charcoal was the fuel of choice for most processes (the exception are closed retorts) because carbon acts as the reducing agent that binds the oxygen and because it keeps its shape reasonably well during burning, thus allowing the reactions to take place, the airflow to pass through, and the gases to escape. The production of metal usually consumed more charcoal (in weight!) than ore!
When everything went well, a smelting produced raw metal, which still contained impurities, such as other metals and slag inclusions. Depending on what the metal was going to be used for, it would be refined in another - usually smaller scale - smelting to remove impurities.
Some smelting processes produced mixtures of metals that had to undergo further treatment to separate the targeted component. The most important intermediary products were
- matte (a mixture of copper, iron and other sulfides). To obtain the copper, the matte had to be roasted and enriched to a state where smelting would oxidise most of the iron while most of the copper would be reduced to metallic copper.
- speiss (an arsenic – iron/copper alloy with other metals mixed in). This produce was in most cases a headache and unusable.
- rich lead (a mixture of lead and silver and/or gold). To obtain the silver (or gold), the product had to be cupellated, a process in which the lead was oxidised, leaving the noble metal.
Smelting masters aimed to liquefy gangue into slags that could be tapped and to reduce as much of the targeted metal as possible. Additionally, they kept a furnace running for as long as possible to optimise labour inputs and efficiency. To this aim, they added fluxes that reduced the viscosity of the slags and supported the reducing reaction.
Thanks to modern material science we now know much about the chemical processes that happen in a furnace, and you can look them up in neat formulas. Real processes, however, are inevitably messy and incomplete because ores are a mixture of substances and the furnace walls eventually start transforming as well. Smelting masters of the past, moreover, never studied material sciences and therefore never knew what was “really” going on inside their furnaces. Experience and watching signs of processes starting to produce undesired effect was everything. Smelting evidently relied on a lot of experience, luck, courage and rather dangerous work. It was tricky and mysterious, no wonder that is gave rise to stories of magic.
2 Metals and minerals
This project investigates metal mines and metallurgy. You may ask why the background materials includes some minerals that have nothing to do with metals. The main part of this page on mining and smelting contained part of the answer. It first and foremost tried to provide an idea of the work and the technologies involved. It also indicated that not all mining was not necessarily for metals and not all metallurgy needed mining.
This is the junction where minerals come in. There was an overlap in the expertise and the skills of finding and extracting metal and non-metal resources. In addition, the use of some metal and non-metal minerals overlapped. In some cases, historic use long preceded the awareness that a certain substance was a metal compound. Thus, cinnabar 朱砂 is a bright red mercury sulphide that has been used as a pigment and a drug long before it was linked with mercury Similarly, malachite (孔雀石 or 绿青, copper carbonate) is a green-blue stone or gem that was also used as a pigment, and orpiment 石黄 is a bright yellow arsenic sulphide also used as a pigment but far more important as a wood preservation agent against tropical insects.
There was a range of exploitable minerals. Some required smelting to be used, some had different uses, some occurred with or in the vicinity of metal deposits, some were more valuable than metals. Historic miners and metallurgists might have decided to abandon a metal deposit to mine a valuable mineral instead, or they may have moved from working native metals or minerals to more complex treatments to extract various products from a deposit. In order to keep a reasonably broad perspective on historic possibilities, the background information covers some minerals that played an important role in the regional economy in addition to the metals.
The following notes on specific metals and minerals outline what we know about their respective histories of exploitation:
(If you must know: The metals are listed roughly in the order of their history of human exploitation, while the other minerals are tacked on ordered by the scope of mining)
