I know some peoples obtained most of their iron from bogs and lakes. I suppose it was less efficient than mining, but required almost no investment and there were much more bogs around than iron vein outcrops. Is there more presice information on historical mining economy? E.g. how much ore could be harvested by one worker from a bog in a day, vs how much could be mined from a vein..
Medieval English mining appears to have averaged about 15-20kg of ore extracted per working day per miner (refs 1,2).
Based on modern experimental archaeology, it's possible to collect 10kg of bog iron in an hour. Carrying the ore can be a larger part of the labour involved than the actual harvesting of the bog iron.
A third source of ore, iron sand, was also used. Based on the traditional Indian iron industry, 4 workers could collect all the iron sand needed to operate a small smelter for a year in 4 months (3). When the smelter was working (during 8 months of the year), it would use about 100kg of ore in a day. Assuming that the number of working days per month was the same during smelting and collecting iron sand, each worker would collect about 50kg per day. The labour cost for the smelting would be double that for collecting the iron sand (the same 4 workers operating the smelter for 8 months and collecting iron sand for 4 months). The other cost was charcoal, which was about the same as the cost of operating the smelter. Thus, the labour cost of obtaining the ore was 20% of the total cost of producing the iron, operating the smelter 40%, and obtaining the charcoal 40%.
Similarly, when using bog iron, more labour would be required to obtain the charcoal than the ore.
References:
Blanchard, Ian. “Labour Productivity and Work Psychology in the English Mining Industry, 1400-1600.” The Economic History Review, vol. 31, no. 1, 1978, pp. 1–24.
John Blair and Nigel Ramsay (eds), English Medieval Industries, A&C Black, 1991
Francis Buchanan, A Journey from Madras Through the Countries of Mysore, Canara, and Malabar, for the Express Purpose of Investigating the State of Agriculture, Arts and Commerce, the Religion, Manners, and Customs, Asian Educational Services, 1988 (reprint; originally published London, 1807).
There were bloomeries in Delaware that smelted bog iron in the early 19th century. And there’s a document saying 6 men dug out 574 tons of ore in 5 months, that is about 750 kg of bog ore in a day per worker.
An estimate of Collins' operations may be gained from the field account of ore raised at Smith's beds between May and September, 1821. This tally booklet, now in the Collins Papers at the Delaware Hall of Records, states that six different men hauled away 574 tons, 11 hundredweight of ore from these beds, which apparently lay near Milton.
source:“The Delmarva Bog Iron Industry” by Edward F. HeiteNortheast Historical Archaeology vol. 3: Early American Iron Making1974