Thursday, November 01, 2007

Recording Smelting Slags

In the recent set of conversations backgrounding the Icelandic smelter series, Kevin Smith had asked me if we had been keeping any records of the amount of slag produced in each experiment. Truth is that although we have numbers for ore and bloom, we generally have no records for the amount of actual slag created. This is certainly a significant measurement, as there are very few metal blooms found - these were just too valuable considering the effort that had gone into creating them. Slag, on the other hand, is nothing more than a waste product, and an extremely durable one at that. There are literally tons of various slags, even within a single major historic iron producing site. Hals in Iceland, for example, Smith estimates there is some 5000 kg of waste slag. *

Slag remains are also used by modern researchers to estimate the probable yields of individual ancient smelts:
- First the ore utilized will be examined. Normally it is expected that there will be some 'slop' of ore to be found right close to the shaft of the furnace. (In our own work, we always end up dropping some ore material by accident as it is added to the top of the furnace.) By analyzing the relative iron content of that ore, an idea of the starting ratio of iron and other waste products (which will go to the slag) can be gathered. As has been pointed out by other contributers to the Early Iron discussion, there can be a couple of reasons why the ore materials found around a smelting area could be misleading. Ideally an experienced worker can make a good judgment of the suitability of an individual piece of ore at the gathering location. In many cases however, our own experience has shown that it is only after ore is roasted and broken for size that good quality may become apparent. Much of the ore found around a smelter site may actually represent this discarded material. Little (if any) of the actual ore utilized for the smelt itself may remain.
- Next the slag itself is analyzed for the remaining iron content. This can only supply the roughest of estimates for a number of reasons. The slag found will certainly vary considerably. The quality of the slag will change over the course of a smelt, from the viscous bubbly iron poor slag at the first stages, eventually becoming a thin hard iron rich material in the latter stages. Slag from any given point in the several hour process of the smelt can be quite different in composition. Even inside a large slag block from a single smelt event, there is certainly differences in iron content remaining at various points within the mass.
- Not only the ratio of iron and silica from the ore effect both volume and nature of the slag, but the materials and set up of the furnace itself have a major effect. Different wall materials will erode at quite different rates, and of course melted furnace walls are a major component of slag. Generally only the very base levels of a furnace will remain to be examined. so at best the amount of
- The mechanics of a single smelt will greatly effect the way slag may be scattered over a working area. In most cases these processes will change the visual appearance of the slag materials. Tap slag will have distinctive flow patterns for example. As hot slag is always a problem to the operators, discarded slags may be tossed some distance away from the working area. Our own experience has shown that a large amount of material may be pulled away from the slag bowl inside the smelter when the bloom is extracted, especially if a bottom extraction method is used. This material usually has a certain amount of partially sintered ore with it. (What Sauder & Williams call 'mother'.) This loose material is going to be found not at the smelter, but at the area where the initial consolidation of the hot bloom is to be carried out. This location is certain to be close by to the smelter, but may in fact be removed by a number of metres (and thus may not be uncovered by the excavation at all)

In our own experiments, we have generally been working with ores that run in the range of 60 - 70 % iron content. Our yields vary considerably, but run from about 30 - 40 % metallic bloom against ore.

For the last two smelts, we have attempted to recover as much of the slag produced as possible. This can hardly be considered a representative sample, more (and more detailed) observations need to be made.

EVENT Icelandic ONE Icelandic TWO
DATE 10/8/07 10/27/07
SMELTER Norse short shaft Norse short shaft
CONSTRUCTION clay slab / stone plate stone slab
NOTE start low, high majority low air volumes
ORE TYPE hematite hematite
WEIGHT  12.3 12.3
BLOOM 6 5 (see note)
TYPE dense lens badly sintered
SLAG  8.5 3.5
TYPE complete bowl broken pieces
TAPPING none incontinent
OTHER no mother measured all included

Note : On Icelandic TWO, the 5 kg listed includes all materials recovered
- 1.7 kg small un-sintered fragments
- 2.3 kg badly sintered pieces (larger than 6 mm)
- 1 kg roughly golf ball sized denser pieces (considered true bloom)
Overall the material produced from this smelt proved too fragmented to forge, with an extremely high carbon content.

* With my recent focus on Hals in Iceland, I would be remiss if I did not provide readers with the reference for further details:
'Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland'
Kevin Smith
Kevin has become a good friend and a close advisor to our experimental work over the years. He has contributed considerable depth to my understanding of the archaeology of iron smelting through our ongoing personal communications.

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