Fire clay chemistry

Let the weather turn cold and it seems all the posts on Permies turn to rocket stoves! In all this recent flurry of activity, it seems there are lots of questions about what I can line my burn chamber with, what mortar can I use that will take the temperature, etc., so I thought I would start this thread to answer materials science questions having to do with high temperatures.

Recently I posted a reply on what refractory mortars are and gave a link to a recipe for making them. But I know that you Permies are a do-it-yourself bunch who would rather dig their own fire clay, and so we probably need to talk about what kind of clays are good for that.

The key thing to know here is that at high temperatures, clays will vitrify, that is turn to glass. This is a common practice in the crafting of pottery, as various clay slips are used that have just a slightly lower vitrification point and during firing form a smooth glassy surface on the pot being fired. This is also why fired bricks are much more resistant to the weather than unfired bricks -- they have a surface coating that keeps the rain from washing them away.

So now that we know how clays work, how can we find some to make a slip for the barrel of the rocket stove? The short answer is that you need to find a source of kaolin. Kaolin is the original material used by the Chinese when they discovered how to make porcelain -- what we know as "china". It's not that difficult to find, being a common clay mineral. All I have to do is walk outside my door, because most of Georgia below the Fall Line is underlain by kaolin, which got here by being washed down from the Appalachian mountains over the years. There's even a kaolin mine a couple miles down the road from me. In fact, if you live anywhere east of the Appalachians, you can probably dig some good kaolin type fire clay not far from your house.

For those of you that don't, you need not despair, there is probably a place where you can find some good fire clay, and you can use this thread to ask about your particular clay source.

so what are your thoughts on the fireclay from home depot / lowes?

brian hall wrote:so what are your thoughts on the fireclay from home depot / lowes?

It's always easier to put something in your shopping cart than it is to dig it yourself. I'm sure it works well for the purposes it is intended for, but this thread is for all the alternative building materials types out there. Part of permaculture means making good use of the materials around you, and not being reliant on the big box home center. So if you are living on top of a deposit of fire clay (as I am), it's important to know what you can do with it and how it fits into your building plans.

John Elliott: Excellent idea to open this thread.

Three questions regarding thermal stability of clay:
1. As more Kaolin (Al2 Si2 O5 (OH)4) clay contains as higher is the melting point. Is this true?
2. What about cracking during heating or cooling? How to avoid this? Is this somehow linked to Kaolin content?
3. If higher Kaolin content is better, can I simply mix any found clay with pure Kaolin to make it more heat resistant? Pure Kaolin is not cheap but you can buy it as a white powder.

One question regarding recognizing fire clay:
4. Is there an easy way to determine if a certain clay is fire resistant, if you do not have a kiln to simply heat it up and see how it behaves? This is probably the same question as: Is there a simple way to determine the Kaolin content of clay found in nature?

regards
Peter

1. As more Kaolin (Al2 Si2 O5 (OH)4) clay contains as higher is the melting point. Is this true?

Yes.

2. What about cracking during heating or cooling? How to avoid this? Is this somehow linked to Kaolin content?

Kaolin content may play a part in cracking, but there are many other factors. Probably the most important is not kaolin content, but rate of heating and cooling. Cracking happens when stresses build up due to differential thermal expansion in the piece. It happens in ceramics as well as glasses, which is why, when the glassblower is done making a piece, it goes straight to the annealing oven so that the stresses can be relieved and the temperature lowered slowly. Since pottery usually gets fired as a batch job, you take your chances that the kiln will not heat or cool too fast.

3. If higher Kaolin content is better, can I simply mix any found clay with pure Kaolin to make it more heat resistant? Pure Kaolin is not cheap but you can buy it as a white powder.

That's a reasonable thing to do. In this reference on clay types, you can see that the more impurities in the kaolin, the lower the melting point and the less heat resistant it is. Until you get down to red clay which has so much iron oxide in it that when you fire it, you end up with terra cotta (Italian for "cooked earth").

If you are going to dig your own, color is a good guide and the whiter it is, the more kaolin it will have in it and the more refractory it will be. In my area, the dirt on top has more impurities in it and can have streaks of orange in it. To get really good kaolin, you have to dig down a couple feet and you get the nice, white stuff.

John: Thank you for answers and for the interesting link.

I found a lot of clay here nearby but only the red/brown one. good for terra cotta. That's why I am thinking about mixing it with pure Kaolin.

Yes, cracking during heating or cooling is a very complex process. It is an interaction of the coefficient of thermal expansion, the thermal conductivity, tensile ductility of the work piece and of course how fast and uniform you heat or cool the workpiece. Many electrically heated kilns offer the possibility to precisely program the temperature over time curve. With a wood fired kiln the operator has to have the experience how to control temperature by adding more or less wood or open one or the other air intake but if you operate a RMH you just put some wood in the feed opening and light it up and add more and more wood. The combustion chamber and the heat riser will heat up quite fast and probably not very uniform. May be this is the reason why many people build the combustion chamber out of bricks i.e. relatively small pieces of clay. The chance that such a small piece is heated uniform is much higher than for a large piece (e.g. the whole chamber). And if in spite of this a crack appears, it will not go through the whole wall, but will stop at the boundary of the brick.
I do not know how for instance the “dragon heater” people avoid cracks in the cast large components of their combustion chamber.

regards
Peter

I have some clay that is almost blue looking, what does that mean? When it drys on your clothes it does look whiter

Jerry Ward wrote:I have some clay that is almost blue looking, what does that mean? When it drys on your clothes it does look whiter

Blue is very often due to the presence of copper compounds. Copper sulfate (CuSO4) is blue in aqueous solution, but as a ground up solid can appear white. Since the most common oxides in the soil, silica (SiO2), alumina (Al2O3), and titania (TiO2) are white, the color usually comes from transition metals, of which iron is the most abundant, followed by copper, nickel, and cobalt.

Does this mean it is a good choice?

Jerry Ward wrote:Does this mean it is a good choice?

Color is most likely independent of the characteristics that you are looking for, like "does it fire well, without cracking". So sure, go ahead and try it.

Jerry Ward :Ernie and Erica Wisner report that Blue,blue grey clay contains so universally a very low Alumina Silicate % that it is condemned out of hand. My one conversation
with a potter suggested that the best you should hope for is a terra cotta, If a wet batch feels soapy slippery when handling its the kiss of death ! Not much good news here .
Big AL

allen lumley wrote: Jerry Ward :Ernie and Erica Wisner report that Blue,blue grey clay contains so universally a very low Alumina Silicate % that it is condemned out of hand. My one conversation
with a potter suggested that the best you should hope for is a terra cotta, If a wet batch feels soapy slippery when handling its the kiss of death ! Not much good news here .
Big AL

That blue grey "clay" doesn't sound much like clay. It certainly sounds different from kaolin with some copper bearing minerals in it, which is where I was coming from.

Jerry Ward : John Eliot is right, and I missed the point rather badly, when in doubt, test brick it, even if it is a total failure you will learn more about the brick making process !
From somewhere it is my understanding that this is one of the oldest sediments, the true primordial ooze !

For the Good of the Craft ! Think like Fire, Flow like a Gas, Don't be the Marshmallow ! As always questions and comments are solicited and Welcome ! PYRO - Logically Big AL !

Thanks for the great information on this thread guys!! I feel a little more confident identifying our clay here in Ohio with this info. We're on a pretty tight budget, and prefer to harvest what nature has provided so this is great!

Thanks again,

Jennifer

John Elliott: I live in NE Georgia. Is there a place I can go (in/near Sandersville) to dig my own kaolin out of the ground? I'd like some for future plastering projects. And I can't find it here in NE Ga. It would cost me less for gas, than buying it somewhere, if I can dig it for free. thanks

Guys, there's plenty of discutions about cob, clay, geopolymers and the likes at Donkey's board.