One of our excellent archaeological volunteers posed this question on our facebook page:
“from the research figures of the ore-processing capacity in the Altogether Archaeology project document, the Smelting Mill must have produced a lot of slag over its lifetime (a quick calculation suggests something like 78,000 tons, assuming a 50% ore-slag rate). Do we have any idea where that slag all went? I don’t see any mention of a slag heap in the vicinity of the Arches.”
Our experts replied:
The lead ore (PbS) was separated from the unwanted material before it arrived at the smelting mill. It is probable that there was not a great deal of other material mixed with the ores. The sulphur mainly went up the chimney and a small amount of lime was added to the ore hearth in order to improve the smelting process. This process does not produce a great amount of slag. The slag from the ore hearths was usually crushed and then smelted again in a slag hearth. This process seems to have produced smaller amounts of slag, which was often smelted again to recover small amounts of lead. The final slags were then dumped nearby. Photographs of other 19th century smelting mills show relatively small slag dumps. On all the smelt mill sites, the slag appears to have been removed for re-smelting. Some of the old photos show slag dumps adjacent to mills, but none of them appear to be of the same size now. In the archives we’ve found records of slag being spread on the local roads to repair them. So – slag was cheap and available pot hole filler!