Saturday, March 23, 2019

Materials Test - Lyndhurst at CAMELOT

Although the Bloomery Iron Smelt undertook last Fall at the CAMELOT conference was primarily a public demonstration of the physical process, the concepts of Experimental Archaeology were also illustrated.

Additional Descriptions:
Ore Analog addition - main sequence
There was also a small experimental materials test integrated into the two day demonstration.

Regular readers may remember that I took part as a 'digger' volunteer at the ongoing excavations at Lyndhurst, Ontario, late August 2018. (1)
This illustration does not quite match the actual ground contours!

The original Lansdowne Iron Works is the first full scale commercial iron processing in what was then known as Upper Canada, dated to 1802- 1811. The complex included a blast furnace (producing cast iron) on the east bank, and a bloomery iron furnace on the west bank.

One of the main reasons for establishing a major iron processing facility has always been the local availability of the main raw materials : suitable iron ore and abundant wood for charcoal. A third element is the type and quality of the materials needed to construct the furnace itself. (2)

Of course limestone, which dominates the geography of the lower end of Southern Ontario, is not really suitable for construction of iron smelting furnaces. (3)
Lyndhurst however, is in the region of Ontario where the granites of the Northern Shield transition into the limestone covering Southern Ontario and New York state.

There was a thick deposit of clay also underlaying the modern backfill layer which has been placed over the ground level were the 1800 bloomery furnace was built. Clays, often modified with the additions of sand or organic materials has proved a very effective building material for furnace construction.

So - as part of my time at Lyndhurst, I gathered some of the clay from the furnace level, and some small fist sized stones from local rock cuts.

The raw clay was a medium grey colour, with what appeared a fairly high sand / silica content. Manipulating it, it felt more solid than 'slimy' (if that makes any sense to you). 
Lyndhurst stone sample
I had gathered two similar sized pieces of stone. The one used for the CAMELOT test was roughly 6 cm thick, 7 cm wide, on its highest point 8 cm tall. The thickness was consistent overall. (4)

Normally, the hottest part of our normal furnace build and operations is a rough oval that circles the tuyere. This extends roughly 10 cm to either side, 10 cm below and 15 cm above the centre of the tuyere. A ceramic tube tuyere was used for this smelt. (5)
Outside, Tuyere in place. Stone L / Clay R
Interior view (before tuyere installed). Clay (upper) is more obvious here.
You can see here that this test may not be quite as conclusive as it might be, as the two samples are placed just outside, and below, the 'hot spot' described above.
The surrounding wall material was the standard mix of high temperature clay / course sand / shredded horse manure. 

View of the interior of the furnace - cleared after the full smelt.
- In the image above, you can see how the tuyere has eroded back towards the furnace wall. 
- The stone sample shows less reduction than the regular furnace wall material that surrounded it originally.
- There is a bit more erosion to the sample clay than the regular wall material. 

The addition of sand and organics to the standard wall material mix is done specifically to reduce erosion and the effects of high temperatures. This does suggest that the Lyndhurst clay would prove effective for furnace construction.
During dismantling - The slag attached to the stone, and heat effects clearly seen.
The stone itself showed almost no damage at all - at least on a single smelt sequence. Again this suggests that the locally available stone would prove resistance to damage and effective for at least bloomery furnace construction.


At least as 'proof of concept' both the clay and rock samples gathered have endured a full bloomery iron smelt, at least within the dynamics of limited size and specific position. 
Ideally this limited test should be followed up with full builds, clay cylinder and stone block. 

1) The report on my observations of the dig - and my interpretation of the ground at Lyndhurst is still pending!
The excavations (test pits really) I helped with were on the west bank - bloomery iron furnace side.
One of the problems with close inspection of the original furnace construction is that the site of the bloomery furnace has been reused for two large scale commercial operations since. First was a flour mill, which involved the building of a more massive stone walled structure, which appears to have been constructed on top of the remains of the iron furnace. (This itself subjected by a major fire in 1811, which destroyed the entire complex.) Later there was a lumber mill constructed over the remains of the abandoned grist mill. This lumber mill was in operation into at least the 1950's, when another major fire destroyed that operation. Considerable earth fill has been added over each of these sequences in an effort to level the ground. The test pit that appeared to extend down to the level of the bloomery furnace was a good 2 metres deep below modern ground level.

2) This is apparent when you consider the wide variation of furnace builds seen throughout the Norse world. Although the slag tapping, 'short shaft' design is fairly constant, the builds range from clay (Denmark), stone slab / block (Norway), to grass sod (Iceland). See the extensive experimental series.

3) The working temperatures inside an bloomery furnace are in the 1150 - 1250 C range (or higher in a cast iron producing blast furnace). Limestones will decompose into lime (CaO + CO2) at about 900 C. You * might * be able to work around this by either using massive thickness and allowing for aggressive erosion, or using a sacrificial clay liner. (Note that is is very offhand - I have no direct experience here!)

4) I freely admit that my knowledge of stone types is limited at best!
The stones gathered appeared to be some metamorphic type.
(DARC does include a professional geologist, and I need to get him to examine the rock used.)

5) This with a fairly durable ceramic or the very fixed copper tuyeres in standard use. Tuyere set 5 cm proud of the inner surface, roughly 20 degrees down. Important is that this is also with a high volume air blast.
Past experiments has proven changing these settings can dramatically change the amount of erosion to wall materials.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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