Summer Solstice Iron Smelt : REMAINS

The objective of the regular early summer experiment was further investigation of the 'Slag Pit' style furnace - tentatively described as 'Celtic Iron Age'.

Refer back to an earlier postings for the :
build / experiment outline
conduct of the smelt itself

The upper furnace was a re-use of my 'production build' from May 2012. It not only has run through several uses, it had been sitting outside (semi-protected) for 6 years (!). When I attempted to lift the furnace body off, the metal outer can pulled free - and the interior walls completely broke apart. (1)

The details here start after the broken furnace wall pieces had been cleared away.

Note on the images :

It was blindingly bright the morning I excavated and recorded. Many of the images suffer from exposure problems because of this. For that reason, relative brightness and contrast has been altered via Photoshop on many.

There has been an attempt to mark the location of the tuyere on several (use of the scale, or via an added arrow).

Base of Furnace - broken walls removed

In the (admittedly poor) image above, you can see the shell of fused wall and slag remaining along the 'hot spot' at the tuyere (far left - indicated by scale). In this image, there is still a lot of loose, small fragments from the broken walls covering the top of the slag bowl. You can see that this bowl is sitting higher to the tuyere side, not extending completely over to the side furthest from the 'heat zone'. 

Cleaned furnace base, showing bloom and it's position

The next image shows the furnace base, with all the loose wall pieces removed and as much as possible of the fine debris cleared off the slag bowl. The bloom is shown on its side (so top side is towards the bottom of the image). Tuyere marked with an arrow. Matching up the X (centre of bloom - bottom surface) and Y (extended 'tail') indicates the location of the bloom as it formed. The slightly scalloped edge of the top left of the bloom (so closest to tuyere) suggests that into the end of the smelt, the air blast effected the deposit of iron and shape of the bloom. Slightly oxidizing to the front, the heat zone pushed further back moving higher up from base level.

Slag block as lifted clear of the pit

Close up of the slag block - X indicates bloom / arrow the tuyere

The next step was to pull the slag block out of the pit. 

The way the heat had eventually effected the clay cap, and the seeping of hot slag into the pit was clear. The slag had formed into a roughly conical shape, wider and deeper directly below the tuyere 'hot spot'. Individual 'icicles' of slag became more distinct towards the bottom. In fact these extended down even further into the pit area, but broke clear when the slag mass was lifted. The main solid slag mass extended about 15 cm below ground / furnace base level.

There was also a clear colour and consistency shift to the slag as it moved further down away from base level. The upper bowl was dark and solid, clearly the most iron rich. Slag excavated from the lower pit tended to be a lighter colour (tinges of green) and contain more bubbles.

Fragments of the clay cap

From the rear area, about ground level, it was possible to pull out some larger fragments remaining of the initial clay cap (covering and separating the vegetation packing the pit). It was easy to distinguish the upper (burned black) and lower (cleaner) surfaces. The lower surfaces had clear impressions of the plant stems and leaves the soft clay had been packed against. 

Towards the front, the clay had ether been melted away, or was completely enveloped by the slag.

'Icicles' of slag, running down from the upper bowl

Carbonized stem fragments, about bottom of slag block (1/4 inch grid)

Individual slag flows had penetrated further down into the pit. These proved not firmly enough attached, or too physically weak, so broke free as the upper mass was lifted clear. As individual runnels of slag were exposed, some were found to contain carbonized stem pieces. (2) As well, individual stem lengths had been transformed into short charcoal sticks. At this depth (15 cm +) there were no remains of the leaves found.

Moving towards the bottom of the pit, slag was found to be fragmentary, lighter and containing much more bubbles. Although clearly heat effected (surface carbonization), individual leaves remained. These were found to be damp and still pliable. 

There was no slag penetration below 30 cm. (Although partially blocked with stones left in place, the deepest part of the pit extended to 35 cm below ground level.)

(1) My best guess here is that moisture had penetrated the baked and partially sintered clay / sand construction. Even despite the wood splint pre-heat, water expanding into steam resulted in the structure pretty much fragmenting. 

There may be some indication here why the Norse would have built structures over furnaces (consider the 'Furnace Hut' at L'Anse aux Meadows). Not for the protection of the *workers* - but for the protection of the furnaces themselves.

(2) Of interest here is the potential preservation of vegetable packing materials fully inside glassy slag. At Heltborg in 2008, one of the interesting presentations was on research being undertaken on the organic materials preserved in ancient Danish slag blocks. The glass slag is virtually indestructible. Vegetation trapped inside the slag can produce information on environment / dating / crop types, even time of year for the smelt.