Furnace Building

... some illustrated notes.

 

The following from preparations for the CAMELOT Iron Smelt demonstration on SUNDAY : 

 

The furnace for this demo will be a standard 'Norse short shaft' / Clay cobb, cylindrical, roughly 25 cm ID at a height of about 70-75 cm, set on a block plinth.

 

As is the case where some protection to the underlaying surface (grass lawn in this case) is required, a set of four 60 x 60 standard concrete paving slabs are put down as a hard base. This gives a fireproof surface at 120 x 120 cm.

To ease the later extraction processes, a set of four, half thick, standard concrete blocks are laid to form a hollow square. This creates a box roughly 55 cm OD, with a central hole about 25 x 25 cm. This space is filled with charcoal fines (left over screenings from the charcoal sizing process). Excess hot slag will burn down into the fines, creating a kind of limited 'slag pit' effect. The holes in the blocks are filled here with a 50/50 sand and wood ash mix. This effectively creates a flat top surface to support the furnace itself. A thin layer of the sand mix (about .5 cm) was spread over the central fines to help preserve these during the drying fire to come later.

Gus Gissing assisting - image by Neil Peterson

The standard mix used for furnace construction is 1/1/1 of dry powdered clay / course sand / dry shredded horse manure. In this case the clay used is a high temperature 'EPK' (although in the past we have found most potter's clays resist internal temperatures well). The proportions are measured by eye, based on volume, then mixed together through the fingers. Water is then added to a well formed in the mixture, and blended in by hand to make a suitable 'dough'. (1)

Using a softer (higher water content) mix makes for both easier mixing, and better blending of the individual cobb balls / bricks. But - a stiffer mix means the walls are less likely to start to slump as the furnace is built higher.(2)

image by Neil Peterson

A metal internal form is used in the wall building. This is wrapped with a double thick layer of newspaper (3). This does result in some ease in building the walls up, but extra care needs to be taken to fully blend the individual clay 'bricks' as these are added. The most important result of using a metal form is to ensure a standardized furnace interior diameter. (Duplication being critical in an experimental process.)

You can see in the image how I brace the side of a new addition at the same time you I wedge it downwards to blend it into place. If you do not do this, the net result is that you continually force the soft clay down - expanding the base wall thickness, without actually adding much to the wall height.

image by Neil Peterson

With the first course of the build completed, and the internal form removed. The breaks between individual clay 'bricks' are clear. The next step is to work down the inside to ensure full blending between additions (remove all the gaps seen).

At this point, the clay cylinder remains somewhat soft, and thus unstable. How to easily both support the work so far - and continue building upwards?

A pair of solutions:

1) as each section is constructed higher, a packing material consisting of sand / ash / charcoal fines is added to the interior to stabilize the interior diameter.

2) heavy rope is coiled around the outside to keep the whole construction from sagging outwards at the same time. (4)

The interior packing also helps dry out the clay initially. 

 

The metal form can be placed on to the top surface of the packing, preparing for the next course of the build.

 

 Repeat...

 

This shows the final build. At this point the furnace varies a bit along the upper edge, from 72 - 75 cm.

The heavy rope has been re-tied to a more evenly spaced wrap. Charcoal fines have been used to top off the internal packing. The furnace has been left exposed to daily sun to promote drying. The ideal would be to leave the clay to dry in this manner to 'leather hard' before removing the interior packing and the rope binding. At that point (1 - 3 days normally) the cylinder would have the tuyere mounting hole and tapping / extraction arch cut in.

 

Ideally the furnace would be left to naturally (gently) dry to the extent possible. A slow drying process is most likely to result in the minimum of cracking.  

In practice however, this time is most often just not possible. (It is most typical for a furnace to be constructed and initial drying fire applied later that same day, with its use for a smelt the following day.)

In this case, the build was on Friday, with return to the working site for the full smelting sequence on Sunday.

With thanks to 'Gus' Gissing, for his heavy labour help in the build, and Neil Peterson, for image taking and 'general supervision'.

1) For the recent 'Mother of All Furnaces' build, a small cement type mixer was used for the dry mix and initial water addition. This proved to work quite well. As the hand mixing with water is the most labour intensive part of the whole build, this innovation was greatly welcomed! 

 

2) In the past, the exact consistency of this mix has been found to largely be a matter of personal preference. Neil Peterson generally likes a stiffer (drier) mix, I like a softer consistency.

3) Important Tip! 

Without the paper liner, the cobb mix simply adheres to the metal form - making it almost impossible to lift / remove. (Lee Sauder uses an internal form of thin wood splints, which he then burns away in the initial drying fire.) In the past, our internal forms have been bundled grass and tied up sticks - both worked successfully. There are two main limits with a fixed internal form :

a) The clay shrinks 10 - 15% as it dries. if the form is too rigid (or left in place too long) the most likely result is extensive cracking. 

b) With the form left in place, there is no way to smooth or blend in the seams between individual 'bricks' on the interior surface.

Archaeology has found traces of both use of interior 'bundled stick' and exterior withy forms being used - found of impressions in preserved wall pieces.

4) I can't imagine the use of the interior packing material would leave any visible trace afterwords (some spillage on the ground?)

The rope definately leaves traces on the clay. As the exterior of the furnace does not heat enough to actually sinter into ceramic, these impressions most likely only to be rarely preserved.