Sod vs Turf?

I've never been to Iceland.

(Although at least my primary smelting partner, Neil Peterson, has.)

One of the important materials for the Viking Age iron furnaces at Hals, Iceland, is the use of cut ground cover - 'turf' for the structure of the furnace.

Of course after 1000 years and with only an field report to base measurements on, exact details are hard to determine. The original report had the excavation diagrams as quite small scale, and only in grey scale. The text describes the turf blocks seen in the cross section cut along the edge of furnace shaft III as being about 10 cm thick. (1) Recently Kevin Smith had provided us with larger sized versions, also in colour and with a more complete key to the elements represented (2). The version seen above has been modified and printed to graph paper (seen lightly in the scan) at one square at roughly 10 cm. The lower diagram is my own interpretation of how that furnace would have been originally constructed. More or less taking the line of the shaft, and pushing it back to vertical, shifting the individual turf strips against this line. (This would be the guide for the construction of the furnace used for the June 20 and September 4 tests.)

 

One of the certain problems with attempting to design any experimental archaeology project is always 'how close can you get'. Our long standing work related to Hals (since 2007 / now 10 smelts) has included many elements which may be at best approximations :

- use of DD1 (Fe2O3 / red oxide) ore analog : not natural bog (FeO-OH) ore

- (most recently) simulated clay mix : not local Icelandic clay

- stones of Ontario granite or gniess : not Icelandic basalt

- sand of Ontario granite origin : not from Icelandic basalt

- charcoal of either white oak or hard maple ; not Icelandic birch 

- local Wareham cut grass sod : not natural Icelandic turf

In recent discussions, the question of terminology over this last aspect arose. It is obvious that that environmental conditions around West Central Iceland at Hals would be significantly different than those at Wareham, in Central Ontario. Growth cycles are not comparable, as well as the types of natural vegetation. This is compounded by the fact that the yard at Wareham was started as an artificially laid course of commercial grass sod, laid over a new graded surface about 35 years ago. This has been left largely untended, so whatever natural plants growing locally have slowly encroached. One specific change has been the dominance of 'twitch grass' in many areas, which chokes out other plants (itself an invasive species). 

 

June furnace, before start of August repair.

 

The overall result is that any cut blocks of the upper grass layer are more dirt than root mass. This is important related back to use of this local grass sod for furnace construction. The roots are what holds the dirt in place, securing the structure.  When this organic material is exposed to the heat of a working furnace, the roots simply burn away. The now hot dirt quickly looses any structure, and will fall apart and flow almost like water. Eventually, and especially if exposed to full temperature furnace gasses (at the range of 1150 - 1250 C), the silica components of the dirt can fuse into a fragile mass.  

In the image above, taken the day after the first full smelt of that furnace, you can clearly see the failure of the inner edges of the stacked grass sods, where heat had penetrated through the relatively thin (4.5 cm) inner clay liner.

I wondered how I might detail what I see as a major difference between the Ontario (semi wild) grass sods cut here at Wareham, to the Icelandic turf used at Hals. 

I still had a couple of pieces of the sod I had cut for use in the June build left over. 

From this I cut a block, as close as I could manage, at 20 x 20 cm, trimmed to 10 cm thick. The precise measurements are 20 x 19 cm, thickness tapers slightly from 11 to 10 cm. I used hand shears to trim the grass fairly close to the ground surface. 

 

Cut block : 3/4 view

 

View of the front edge

The weight of this block, as damp and 'living' is roughly 5 kg (5093 gms including the metal tray it is sitting on)

The intent here is to leave this block exposed to the sun for a length of time to start to remove the contained water. 

- Next the piece will be heated (likely by placing the tray on the top surface of my wood stove) to remove any final moisture. 

- At this point the material will be weighed again (so giving some idea of the water weight).

- When the block is 'bone dry', it should be possible to knock off the dirt from the root mass. This will allow for photographing the root web, and most importantly weighing just the contained dirt for comparison back to the starting amount. 

This all should allow for some kind of comparison 'root to dirt' measurement. 

This exercise is more likely to be only of qualitative (rather than precise quantitative) value. The hope is that this test might proved some data towards what up to now has been a gap in my personal experience with Icelandic conditions. 

1) Smith, K.P., 2005, "Ore, Fire, Hammer, Sickle: Iron Production in Viking Age and Early Medieval Iceland", AVISTA Studies in the History of Medieval Technology, Science, and Art, Volume 4, USA

Also available as PDF on line : https://www.academia.edu/191535/Ore_Fire_Hammer_Sickle_Iron_Production_in_Viking_Age_and_Early_Medieval_Iceland

2) A overview of both the Hals site and these experiments is currently under preparation, co authored by Smith, Markewitz and Peterson (‘Now with 70% Less Clay! Experiments with Viking Age Icelandic Turf walled Iron Smelting Furnaces’) A short video overview was presented at the recent EAC 12 virtual conference, available on line : https://youtu.be/7Ltz5NG2BP0