Materials for Aristotle Furnaces?

Raymond asked :

I have a 25 lbs block of china white clay (ceramic clay) and because of this am wondering if that will work for building the furnace.  I understand that I have to ad a couple of materials to make it a refractory, mostly sand and ash, but will it work for a base material?

One consideration at the start is if you think the furnace is a permanent or a limited use structure.

Although both bloomery and smaller re-melting furnaces are fairly durable, the truth is that they will have a limited life span. Some people are attempting to use more elaborate materials in an attempt to fight this basic reality.

My own experience is that you can certainly build a furnace that will run through dozens of use cycles. The Aristotle furnace I have on hand right now has been used currently through four working sessions - each session averaging about 5 - 6 individual 'puck' productions. These furnaces take maybe two hours to build, against a potential 8 - 10 plus working hours at temperature.
Not so bad for something made out of a half bag of clay and some found materials.

Clay * of any kind * is your starting ingredient. I have worked with everything from locally dug 'river' clay through to high temperature 'foundry' type clay bodies. At the temperatures these furnaces can produce (can exceed 1300 C) * any * of these clay bodies can (and do) melt.

I personally like to start with a dry powered commercial potter's clay. This mainly because its just easier to mix the other potential ingredients. A full bag costs me $13, enough for two Aristotle furnaces. (usually use 2 - 3 bags for a full sized bloomery furnace). I will try to purchase a clay with a higher melting point when I purchase, but I have had just as good results with a standard ball clay. The stuff I get does not have any grog in it, so I can't say if that adds anything to durability.

Adding sand increases the resistance of the clay to expanding as it heats. One of the primary sources of cracking is the difference in expansion rate between the inner and outer wall surfaces of the furnace.  Again, at the operating temperatures created, even the sand will melt to glass. The sand does have a higher fusing temperature however, so the net effect is to limit the impact of temperature on the walls.
The balance is that adding sand can make the clay a bit less 'sticky' and a bit harder to form. As Lee Sauder has certainly demonstrated, extra care in building using a high sand / clay mix will result in a more durable furnace.

I have long been an advocate for using some kind of an organic component to furnace materials. Addition of an organic does three things:
- Acts as a structural re-enforcement while the furnace is drying and along the exterior portions of the walls. This like rebar (straw in bloomeries) or like fiberglass in bondo (manure in Aristotles).
- Allows an internal space for steam expansion as the thick walls undergo their initial heating. Steam expansion resulting in cracking is a major problem.
- As the interior surfaces bake to ceramic under furnace temperatures, the organics burn off, leaving a slightly porous material. These small hollow spaces increase the heat resistance of the clay as it fires into a ceramic.

For the Aristotle, my chosen organic additive is shredded dry horse manure. Horses by nature are not efficient consumers of grass, so what passes through their systems are quite uniformly shredded particles of grass. Typically the pieces are about 1/4 inch / 5 mm long. You want to get 'last years' horse pucks, the ones dried and that have turned a light beige colour. If you rub these between your hands, they will come apart easily into a perfect organic additive. (It really is like an organic version of the small fiberglass pieces in bondo.)

I have tried a couple of different mixes employing plain beach sand (dug from a local natural deposit), horse manure and clay.  I will mix by eye and volume.

For the smaller Aristotle, my standard is 50 % clay / 50 % shredded horse manure.

As I mentioned, adding sand will stablize the wall material, which I have found is not as much of a problem with the thinner total wall thickness and smaller total size of the Aristotle.
But truthfully, at the small size of the Aristotle, you are more likely to eventually * break * the walls before you ever have problems with erosion through them.

For your first working furnace, my best advise is 'just do it' ! Don't worry too much about the durability of the furnace itself. Its quick and easy enough to build another one.

Gus Gissing starting to extract a finished 'puck'. This furnace has gone through about 24 + use cycles.

(Sorry for droning a bit - I was shedding horse manure yesterday in preparation for demonstrating the Aristotle at CanIRON 9 over the upcoming July long weekend. Lee Sauder will be demonstrating his bloomery furnace there as well. )