Iron Experiments - June 2007 - Post Smelt Excavation

Date: 10 June, 2007

Location: Vinderheima

Premise / Questions: 

We had received some questions about the debris field after a smelt. Before our double smelter on July 9 we laid down a clear layer of fresh sand. On the 10th the July Dr. Ron Ross and Neil Peterson of Wilfrid Laurier University brought out the grid squares and did a sketchmap of the debris field.

Tools and equipment:

• Plan frame - 1/2" copper pipe, soldered and pinned together designed by Dr Ross. The frame is 1 metre X 1 metre, subdivided by cord into 20 cm squares.
• Permatrace paper
• Graph paper
• String
• Coloured pencils
• Lead Pencils
• 100' tape
• Plumb bob
• Tent pegs (plastic, various colours)
• Compass
• Cameras
• Video
• scales for weights/mass and measuring sizes

This description was written by Ken Cook. This dig was his introduction to some of these techniques and he recorded the techniques used quite well. This makes this text a good introduction for other interested parties.

This is work which involves a high attention for detail, an ability to draw and the patience to record the details as found. Since any small detail may be the key to un-locking an archeological mystery, it behooves the archeologist to record everything, even if it seems irrelevant.

Dr Ron and Neil used those really cool plan frames, which made layout very easy within the 1 meter squares.

The first thing Ron did was layout a base line for the site. Normally, the baseline runs North - South if possible. In this case Ron laid out the baseline so that it ran along the axis of the smelting site and then took a bearing with his compass to get the relation of the baseline to true North.

Next, he laid out 1 meter intervals along the baseline and marked them with the plastic tent pegs. These tent pegs then became the reference points against which the plan frames were set.

The next step involved setting up the tracing paper so that the baseline ran down the centre of the paper. The co-ordinate system set-up by this method runs on the military grid system. In this case, everything EAST of the baseline was a positive number. Everything WEST of the baseline was a negative number. The baseline has a ZERO point. Hence everything within the excavation now has a co-ordinate which locates it precisely with respect to the baseline and the EAST/WEST direction. All measurements are in Metric.

The baseline represents SITE NORTH, and this is not always TRUE NORTH. TRUE can be obtained by survey techniques.

RECORDING the DATA:

Ron and Neil talked about a KEY or LEGEND. This KEY is how objects are marked on the plan diagramme of the site plan. It is very important to be consistent when recording objects within the plan and maintaining the correct KEY for the same type of objects. i.e.: always mark slag debris with "DOTS" etc.

Object resolution is also a concern since the record has to be detailed enough in order to capture the information you want. It is my understanding that things 1 cm and larger are drawn as discrete objects, while things less than 1 cm are shown as areas, usually with descriptive notes attached.

The recording of data can be done in lead pencil only, but it makes sense to record data in colour in order to separate types of data and also make patterns more readily apparent.

The scope of the "DIG" needs to set with regard to the nature of the site, the material that needs to be included in the analysis and the material provides context only.

Context material sets the "tone" of the dig I think, in other words, it is rather like a signpost letting you know when and where the next town is. Material for analysis is the meat and potatoes of the dig, and this is where most of the physical effort is expended.

Cameras are a must, they can provide both context and analysis material for the site in general and for the 1m X 1m square plans. They can help identify patterns and may show objects and patterns which are not readily apparent to naked eye. A full suite of pictures were taken with the digital camera recording each 20cmx20cm square. These pictures are not reproduced here. Extreme care must be take to ensure that such pictures are accurately labled with the grid location and scale.

Copious notes are a must, both for context setting and for data recording. Temperature, humidity, geology, geography, altitude, latitude soil types, pH, climate and climate patterns (if known) are just some of the things which can set context and provide data for analysis of the site. For the prupose of this "dig" this data was not collected as the raw material was deposited the day before and would not be impacted by these factors - yet.

As I mentioned before, attention to detail is very important, so is not having a set of blinkers on when recording data. Drawing skills are important as well, these can be developed over time and with practice.

The final cleaned up version of our sketch map

Some Discussion Points:

This text is taken from our own observations and discussions with Kevin Smith at the Haffenreffer Museum. It has been edited to make it a single text block rather than attempting to attribute specific thoughts or questions.

• Archaeological Remains of Smelters
  
  After 4-5 repeated smelts the base of the furnace stack itself (beyond about 20 cm below tuyere level) was still essentially unconsolidated, poorly fired or unfired clay cobb. It is unlikely that anything this soft this would preserve through 1000 annual freeze/thaw cycles and yet archaeologists are always looking for the bottoms of the furnaces, expecting them to be preserved. It makes perfect sense that the best preserved parts of the furnace wall are going to be above and around the tuyere which will appear in the archaeological record as loose plates as they fall over. It is also important to realize that the base may well be ephemeral from an archaeological perspective. We should expect to see unaltered earth, then maybe disturbed earth, then the ring of raw cobb. Inside this would be whatever remains from the extraction process and walls.
  
  If you extracted the bloom from the top, the slag bowl might just be left in place. That would leave you with the classic shaped bowl. Below that would likely be some combination of ash, unburned or partially burned charcoal and maybe charcoal fines. The looser this lower packing is, the more likely there would be 'icicles' of slag from the bottom of the slag bowl. There might also be the same shapes of bright white cast iron. We have also seen plates of this metal forming between the slag on top of the fines layer we use.
  
  If you extracted the bloom from the bottom, you would first have dug out all the lower packing material through the tap arch to leave a void. Then normally you would try to puncture the slag bowl to let some of the liquid run into the space created. This serves to help heat and loosen the structure of the slag bowl. Next you either pound on the top or work to free the edges of the slag bowl from the top. With some levering from the bottom, you work to loosen the entire slag bowl and get it to drop down into the space. Then the whole slag mass, with bloom still in place, is pulled out in one large piece. In the past we have dragged this forward a couple of feet, Then most of the loose slag is removed by striking with chisel ended heavy iron rods. (This is how Skip and Lee extract their blooms) So this method pulls the entire core out of the smelter. Nothing would remain of the slag bowl save some fragments still attached around the edges. There would be some mix of slag / ash / burned charcoal, very low in the smelter base. This at the level of the raw cobb. I would expect a much more disturbed artifact - likely one that pretty much results in loose pile of mixed elements as it collapses. A trail of slag debris outwards from the tap arch would also be an indication of this technique. Unfortunately it could also indicate a large number of smelts on a single location with the slag coming from tappings.
• Patching Walls
  The patches in the walls finally burned/melted through with subsequent influx of external packing material and consequent production of excess glassy slag. And interesting, as well, to find that 4-5 reuses was the limit for the utility of the patched stack. This stack been standing for two summers / one winter. It would be interesting to know how many times in one summer a single cobb-built stack could be used before it became necessary to tear it down and rebuild it, as the "typical" structure of the Norse smelting sites is based on a series of overlapping furnace bases representing the reconstruction of a furnace on more-or-less the same site over and over again...yet it's not that clear how much time (or how many smelts) each base in the "stack" represents.
  
  We are finding typically that our smelters loose something in the range of 2 - 4 cm of wall thickness each use. This effect is most drastic just above the tuyere. (We have found that tuyere angle effects this - remember that this smelt had that angle at only 10° due to winter slumping. The ideal has been found to be 22.5°. The lower angle suggested that excessive melting would in fact take place.) Also significant for this smelt was the double smelt. In the past we have had a gap between smelts that allowed us to repair and patch up the lost wall thickness.
  
  The repair before this experiment was done using a clay that is known to have a lower firing / melting point (local 'Blue Mountain Red'). This likely was an important factor.
  
  The heat profile in the smelter is a torus. This washes back over the walls above the tuyere (relates to angle again). The distribution should be roughly symmetrical. What happened was that the excessive erosion was primarily on the right side of the tuyere - the area which was inset into the bank and had a layer of ash and sand packing against it. Again I would say this is significant. Free standing clay smelters can radiate heat off their surface and thus avoid excessive heat build up. There is some balance between clay properties and ideal wall thickness.
  
  In our area it is pretty dry from mid June through to mid August there is always a lot of rain In September and October. The smelter had a piece of sheet steel over the top through all this time, but the exposed surface (roughly half) was open to winter snows. When the snow melted the interior base actually had several inches of standing water in it (this at the level of the raw clay cobb.)
  
  So it may be that a single cobb wall smelter, especially if free standing, would withstand even more uses - providing that it was fully patched between smelts. This smelter had started to develop some fairly serious top to bottom cracks - several of which were leaking jets of hot gases. The cobb mix helps to tie together even cracked furnaces. My best guess is that after 5 - 6 uses of the furnace that it might just be too damaged to continue using the same structure. The damage done during the extraction process is actually more significant to the ability to re-use the structure. Most of the patching is done at the base around the tap arch - most especially if a bottom extraction is used. Skip and Lee have never gotten any more than two uses of a Flue Tyle furnace - and there is almost no wall erosion with that system. The base at the tap arch is the problem.
  
  Depending on your access to clay (problematic in Iceland) building a new smelter is not a big deal. At best two days for two people (that is including digging and preparing the clay). Given the physical difficulties of top extraction (heat related), and the vast reduction in charcoal required for a second 'hot swap' smelt, it may just have been more efficient overall just to re-build entirely.
• Soil Patterns
  Icelandic archaeologists tend to look for burned/oxidized/intensely fused soil beneath and around the furnace base to convince themselves that they have found a furnace base. Yet, if the lower 15-20 cm of the furnace stack itself, below the tuyere, is still unfired clay and the charcoal charge beneath the tuyere remains unburned (and I well remember finding unburned paper in the base of the furnace we ran in 2002), it seems questionable that the sediment/matrix/soil beneath the furnace will always be fused/heat-reddened/burned/fired and perhaps questionable, as well, that the matrix outside the cobb core will always be intensely thermally altered unless there has been a disaster, like having the wall melt through. I would suggest that there is minimal to zero effect on the condition of the underlaying soil.
  
  Remember however that when we dug in this smelter, we cut a larger hole then back filled the gap between the cobb cylinder and the undisturbed ground with that ash and sand insulation layer (also provided drainage !) We also raise our smelters up and create a base layer with charcoal fines. We did this initially to provide insulation from the damp soil underneath - which proved not a problem. It also allows the slag bowl to settle slightly below our estimate of its correct level. If you simply used bare earth for your base, then remember the slag bowl would form on top and transfer heat directly. I would think that you might end up with more of a flat bottom to the slag bowl if you worked directly on the soil. The distance of the tuyere from the base earth layer is likely the greatest effect.
  
  All this suggests (yet again) things that we may be doing that are different that what was done in the Viking Age. I will take some more images of the weathering smelter from 2004. What is happening there is that the sintered clay is becoming a standing ring surrounded by a wide smear of clay mud. As the ceramic is exposed, it cracks from freezing and largely falls into the hollow where the slag bowl was extracted. Any visible heat effect is limited to the walls of the smelter itself.
  
  We have always dragged off any slag that leaks out of the smelter away from our work area as soon as it cools enough that we can pick it up. This mainly to keep our work area clean and safe to move around it. This last smelt we had a huge amount of just waste glass slag. It mostly had enough iron contained to turn it black - but not much more of than that (not magnetic or we would have re-cycled it). Originally I wanted to also record our slag volumes and weights - but with the extra produced because of the hole in the smelter this information is not helpful to an understanding of ore to bloom conversions. Anyway, I'd guess we have at least enough of this material to fill two milk crates.