Within Peter Uerpmann's collection of European minerals we came across quite a large comparatively heavy specimen (110g) of slaggy matrix filled with bright metallic octahedrons of silver to 1mm stacked in pagoda-like parallel groups, labelled from Silberhütte Lautenthal, Harz, Germany.
The size and richness of the silver content intrigued me, surely silver was a highly prized mining and extraction product, how come such a large amount was present in the waste slag?
I thought I would undertake a little research into the matter.
Over the centuries Upper Harz evolved to be one of the most significant mining areas in Germany.
There were several metalliferous mines producing silver, copper,lead, iron and latterly zinc.
Economically silver was the most important and from C16th to the middle of C19th centuries around half Germany's silver was extracted in the Upper Harz.
Taxation on metal mining raised revenue, power and influence for the royal houses in Hanover and Brunswick-Wolfenbüttel.
The wealth generated drove a high degree of investment and innovation into the Harz mines as the Industrial Revolution fluorished.
As production increased mines followed veins deeper, with shafts going down to 600m by the beginning of the C19th, an engineering triumph for the time.
Smelting in the Harz
Smelting has been evidenced by archaeologists from as early as 300AD. The industry continued through to the 1990s, with development factored by resources, technology and the understanding of metallurgy and chemistry.
It is known that the earliest smelters were wind assisted and tended to be located near sources of wood, relocating when this resource was used up.
Later water power was used to process ore and drive bellows to provide oxygen.
Other techniques such as the provision of potassium chloride into the process to leach metals from the ores came later.
All of these are evidenced by amount of slag (waste) at sites in the area and also by the secondary or slag minerals minerals that have formed since.
A good example is that it is known potassium chloride was used at the Herzog Julietteshütte which has resulted in a proliferation of slags rich with minerals like botallackite, boleite and connellite.
Below is a list of the minerals known/found in the slag locations in Harz - reproduced with kind permission of UKJMM
Silberhütte Lautenthal Smelter
Silberhütte is the given location of our specimen, and indeed there are other samples already in the e-Rocks database, Mineralienatlas, Mindat etc, so we can be confident that origin is correct.
Location Map for Silberhütte Lautenthal and Surrounding Area
Smelting here is recorded as being active from 1227 to 1967, and as a result large dumps are situated to the North side of the village to the edge of the forest. These have been capped over with topsoil and the village of Lautenthal has expanded and houses are built over areas of the site.
According to Mineralienatlas (and UKJMM, though older) slags are still exposed at the edge of the dump by the forest.
UKJMM also states that the slags at Silberhütte Lautenthal have a high silver content.
The Unanswered Question
The origin of the interestingly crystallised silver seems certain, and there is some very good information about mining, smelting and slag minerals in Harz.
What seems to be missing is the answer to why there is such a large silver content at Silberhütte Lautenthal?
Secondary to this, based on the assumption that our slag may be from an earlier dump where extraction techniques had been wasteful, why did these not get reprocessed?
If someone knows the answer please let us know!
Reference: UK J. Mines & Min., 8, S.18-23. "Slag Minerals from the Rammelsberg, Harz Mountains, Germany" by W v.d.Berg & C v.Loon