Building an Aristotle re-melting furnace

 This photo essay details the construction of a small re-melting furnace for iron materials, originally named the 'Aristotle' furnace by Skip Williams and Lee Sauder, who originated the design in 2008. It was tested extensively at Sauder's 'Smeltfest' workshop by the Early Iron Group in 2009 (over 30 cycles).

There are two PDF descriptions available :

Lee Sauder (2010), Aristotle's Steel, paper for the Historic Metallurgy Conference

Darrell Markewitz (2012), The Aristotle Re-melting Furnace, the Wareham Forge 

In simple terms, the Aristotle is a small, table top furnace made of clay, about 10 cm internal diameter and 30 cm tall. Through a burning cycle that consumes roughly 2 kg of charcoal over about 25 - 30 minutes, it will convert 600 - 800 gms of iron alloy bar into a 'puck' of modified carbon content at 500 - 600 gms. Depending on the set up of the base distance below the air blast, it may either add or subtract carbon from the starting material.

Base: 

Ideally the furnace should be built on a piece of heat resistant material, both to prevent damage to the working surface, but also to allow or ease of moving. In the past I have used refractory plates, stone slabs and the piece of glazed ceramic tile seen here. Once fired, the furnace will be stable enough to be lifted with out a supporting plate. 

Form:

A length of standard black PVC drain pipe at 4 inches interior diameter works ideal as a form. Because of the thickness of the plastic walls, this will give you a finished (wet) interior diameter of 11.5 cm. Alternately, you can use a 2 l plastic pop bottle, fill this with water first and cap it off. Wrap a sheet of newsprint around the form first to keep the clay from sticking (making removal of the form almost impossible). I use a sheet from our local weekly for this - which is 30 cm tall. If you use a full sized newspaper, mark a line at the desired 30 cm height.

Plastic pipe form, paper wrapped, on the tile base.

Clay :

You can use straight clay for the build, but better would be the addition of plain beach / playground sand. Sand adds resistance to high temperatures, but at the cost of making your mix less sticky, so more care needs with blending the individual additions of material. We have had excellent results with a combination of equal amounts by volume of clay, sand, and dry (last season's) horse manure, shredded in the hands. For this specific build, I used a combination of locally dug clay (a low firing red earthenware) with pulverized walls from previous iron smelting furnaces, half and half by volume. (1)

Dry clay (red) and furnace wall (grey) before mixing.

The first step is bending the loose dry material as evenly as possible through your fingers. I used an 800 ml (crushed tomato) can for measuring, using a total of 4 cans (so 3200 ml) of each of the two main ingredients. I have a large plastic tub with low walls, about 60 cm diameter, that I use for mixing clay for furnace builds. This is large enough to allow 'wedging' the mixture as water is gradually (!) added. This could easily be done (with a bit of care) on a spread out garbage bag or piece of plastic tarp. 

Blended material, first water added as a well in the centre.

Anyone who has ever made bread from scratch will be familiar with the basic technique required. Make a depressed 'well' in the centre of the mound of dry ingredients. Pour a quantity of water into that well, and start to fold the mass together from the outsides. My initial addition was two of the same cans = 1600 ml. This proved not enough moisture, so again I pushed the material towards the middle and added a third can full of water, so the initial total was 2400 ml. As you bend the material under your hands, make sure you you are incorporating the dry mix that will be remaining underneath and to the edges. The material is pushed together, then you squish down under your palms, pushing from the shoulders. (This method just like kneading bread dough, called 'wedging' for clay work.) 

You are likely to require some additional water as you do this. You are well advised to just cup a small amount in your hand and sprinkle over the top, NOT to add a large volume at once. If you add too much, your mixture will become far too wet and impossible to effectively build with. Unfortunately, knowing just what the best exact consistency is a matter of experience. A mix too dry will be difficult to blend together as the furnace builds, too wet and the material sticks to the hands and slumps as you try to work upwards. I judge this as I press the mix under my hands, ideally the clay pulls fairly cleanly away from my palms. I ended up adding smaller amounts of water as described several times before I had a consistency I liked.

The final blend of clay mix, ready to build.

I would end up with three fairly similar sized balls of material, each about 15 cm in diameter. This would prove almost the perfect amount for the furnace build, with a bit left over. 

Build:

Placing the furnace base.

 

The first step is to make a base pad for the furnace. A ball of material about 10 cm in diameter was pulled off and flattened into a thick pancake, about 16 - 18 cm wide and 3 cm thick. This placed on the centre of the supporting plate, then the pipe form positioned on top. 

Ball / 'brick' / added to the wall.

It may be worth noting that the steps described next also apply to building larger sized smelting or casting furnaces : see 'Stacking Up'.

Pull off individual balls of material (2), for this build I found a size like a large orange or small grapefruit (7 - 8 cm) worked well. Again wedge the ball between your hands to ensure it is consistent. Then take it and form it into a rough 'brick' shape, which will be roughly 15 cm long, 7 cm tall and about 3 cm thick. Tap the edges roughly square, then bend to curve it slightly. Each brick is then added to the base / each other as the walls are built up. Care is taken to blend the seams between individual bricks well together. Pressure needs to be applied with one hand over the outside surface while force is being exerted to the seams. Otherwise the whole structure will be pushed down and outwards, rather than being built higher with consistent wall thickness. (Again, experience with clay building comes to play!)

End of the second course.

As you build upwards, it is important that you both make sure the lines between individual bricks are well blended, but as well keep placing your hands flat against opposing sides and press both inwards and slightly upwards. This will keep the diameter of the furnace uniform, and keep the soft clay from being pushed downwards, thickening the lower walls while reducing your desired height.

End of the basic build.

With the paper indicting a rough interior height of 30 cm, the interior form is gently pulled up and clear. The paper wrap should make this easy. 

Just after removing the form, note impression of the pipe lower edge.

Now, you want to check and see if the pipe has been pressed into the base clay. ( If you have used a pop bottle, there may be lumps into the base. ) You will want to smooth out any depressions. Ideally the base pad should be slightly bowl shaped, a bit lower in the centre than around the joint to the walls. You should also be checking the interior surface, to see if here are any flaws in the blending of the applied bricks. In this case you can see some flaws at about the three o'clock position. These should be blended smooth, again working with your second had flat and bracing the outside wall surface at the same location.

Showing the bevel at the top edge.

 I measured the important interior height at this point, and between being just short of the desired mark (by the paper) and the amount the pipe had sunk into the base, I found the distance to be 25 cm. As it is better to have the furnace a bit too tall than reducing performance by being too short, I decided to add another half wide course of clay to the top. I finished the top edge by slopping that with my thumb to about a 45 degree bevel. This is more effective than a flat edge, as it makes a slightly larger target for applying charcoal, and makes sure that all falls into the furnace.
 

Showing bracing rope wrapping.

Experience has shown that putting a spiral wrap of rope around a damp furnace will help keep the body from slumping after the build. In this case I used some on hand hay bale binder twine, with gaps at about 2 cm apart. 

The finished furnace was a total of 33 cm tall and 18 cm in diameter on the outsides. The interior working 'stack' was 28 cm, leaving the base at 3 cm thick, with an internal diameter of 11.5 cm. The walls were about 3 cm thick. 

The unit was put aside, to start with outside (exposed to wind) but under cover from the sun, at least to start. In this case the build was done on Monday, for a furnace required for six days later. This will allow time for considerable air drying (again the ideal situation. 

(three days later)

The furnace at this point was 'leather hard', so it could be lifted and read for the last step, cutting in the air ports.

You can see that this is a bit different than the illustrations on the two handouts. I have omitted the secondary air mixing chamber. That chamber serves an important function when you use a powered blower, for this demonstration furnace the air would be provided by a Norse blacksmithing sized twin chamber bellows. There would be no need to vent off excess air. 

Setting the tuyere hole.

 For the Aristotle, the ideal tuyere hole is 1 cm diameter. For this I used a 3/8 inch twist drill (by hand) which easily bores through the still damp clay. There is a trick here, as the ideal angle for the air blast is roughly 20 degrees. You can establish the angle by taking half of 90 = 45 and half of 45 = 22 ° as an approximate. There is an estimation on the location of the starting point on the outside, as you want the hole to hit the inside with it about 6 - 8 cm above the interior floor. You can see from the image, with wall thickness at roughly 3 cm, I started my angled cut at about 11 cm (taking account the base thickness as well).

Measuring the interior position of the tuyere.

You can see the final position of the tuyere hole is roughly 6 cm above the solid floor of the furnace. It is always better to have more depth than you need. The final depth below the air blast can be modified by putting some wood ash down first and tapping it tight. It will be the distance below the air blast that will determine the amount of carbon absorbed by the process. This position would also leave 22 cm as the 'stack' (above tuyere) height.

Positioning the air supply pipe.

As mentioned, the air supply for this demo was going to be a smaller bellows (see below). For that reason I wanted to mate up to a short piece of steel pipe, with an ID about the same size as the outside diameter of the existing bellows exhaust. This needed to slope downwards (exaggerated in the image) to match the height of the bellows unit. 

Finished mating port for the air supply.

I used the pipe itself as a simple drill to cut in the diameter I needed, about a cm in depth into the furnace wall. I then cut the clay away at the bottom of the port. Last I used a piece of 3/8 round rod to slightly taper the inside of the tuyere hole itself. This same rod will be used to clear the tuyere of any slag that might accumulate during the firing cycle later. 

The last step before using the furnace is to dry out the moisture remaining in the clay. This can be done by leaving it in the sun for several days, plus using a very small fire with twigs / kindling splits inside. Remaining moisture presents two problems. The most significant is that with the extremely high working temperature of the furnace (in the range of 1300 + C) and water will greatly expand into steam, potentially causing cracking to the walls. Water will also significantly rob energy, so for simple efficiency / reducing charcoal consumption alone, you want to work with a baked dry furnace. (3)

Previous demo of the same process / equipment.

1) Depending on the quality of your local clay, working this naturally sourced material may prove far more work than worth the cost of purchasing a dry powered potter's clay from a supplier. I had dug the clay, dried it, broken this up with a mallet, then pulled out any obvious stones. The material was then placed in a small cement mixer with dozen short metal bars inside for about 45 minutes to pulverize it. This was sifted through a 1/4" 'hardware cloth' screen to remove any larger pieces of gravel remaining. Given the clay bank was 30 km from Wareham, and all the work involved, against the current cost of a higher temperature potter's clay at $40 (CDN) for a 22.5 kg sack? (This would be for EKP, what we normally use for smelting furnace construction.) 

2) It is worth noting that the ideal method is to take the mixed clay material and form it into individual balls (as described) and leave them to sit, covered with a plastic sheet, overnight. This will even out the moisture inside each ball, which will result in more even drying of the finished build, so in the end reducing the development of cracking when fired. Time often limits the inclusion of this step.

3) Another way to fast dry the furnace is place it in your home oven at no more than 100 C for several hours to let the moisture bake out. Not ideal if you use the horse manure mix by the way!