Fire Clay Substitute

[Karyn B]

Looking for Fire Clay. In order to level my oven floor, I will need to mix sand and fireclay according to FB instructions. Finding fireclay seems a bit challenging as I do not need a 50lb bag. Any suggestions would be awesome. My location is Northern California. Building a 42" oven.

[david s]

You should have a plentiful supply of clay under the topsoil in your own garden. Dry it, pulverise it and sieve it. Alternatively you could just use sand alone or perhaps mix some sieved wood ash into it.

[Gulf]

Hi Karyn B,

Welcome to the forum. If you are brick building, what type of mortar are you planning to use for the construction? Fire clay is a component of the "home brew". Home brew is by far cheaper to make than any of the refractory mortars that the brick yard will recommend. I also agree with David. The clay just a few feet under where we walk is free for the taking. It just needs a little processing.

[wotavidone]

I made some relatively pure clay. First of all, I found in a local wetland a very pure looking outcropping of what I thought was clay. I wet that in a bucket, stirred it up until I had a slurry called slip. I skimmed off the floating bits of organic matter, leaves, dead ants etc., then decanted the slip off, leaving the little bits of sand, fine gravel etc, in the bottom of the bucket. When I was satisfied I had a clean clay slurry, I poured it into a bucket lined with a piece of old cotton bed sheet. I tied the ends of the bedsheet up and was able to lift the "bag" of clay slurry out of the bucket and hang it to drain. Eventually ended up with a dried ball of what my consultant geologist called silty clay, which I pulverised back to powder and used in my mortar.
The oven is still standing 7 years after I started laying bricks.
But, according to my company's "Refractory Primer" this isn't fire clay. Our furnace lining guys define fire clay as clay that has been fired and crushed.
One advantage I can think of for using pre-fired clay is that it is pre-shrunk.

[Gulf]

No, it is not fire clay. But, it has been used for normal brick laying and oven construction for hundreds of years. I believe that it actually reacts better with the lime than fire clay. On the shrinkage, I think that you have a point. I processed cuttings from the wet saw. It's neither clay nor fire clay. It's closer to grog. I used it as a replacement for fire clay in the "home brew" for some very large joints and had no shrinkage at all.

[Gulf]

I ran across that water extraction method for processing clay a few years ago. Among a hundred or so other things it's on my to do list to try .

[wotavidone]

No, it is not fire clay. But, it has been used for normal brick laying and oven construction for hundreds of years. I believe that it actually reacts better with the lime than fire clay. On the shrinkage, I think that you have a point. I processed cuttings from the wet saw. It's neither clay nor fire clay. It's closer to grog. I used it as a replacement for fire clay in the "home brew" for some very large joints and had no shrinkage at all.

Yeah, ordinary "native" clay has stood me in good stead all this time. And you are right IMO about the cuttings from your wet saw. From a dry saw, yep the dust would be fire clay. From a wet saw, the cementitious qualities are probably done and dusted by the water used. However, shrinkage is not a problem. Raw clay can be 10% shrinkage or more. Fired clay even if re-hydrated a little when wet-sawed, won't have that much shrinkage. But it doesn't matter much, as you say ordinary clay plus lime equals strong mortar with enough heat resistance for our application.

[wotavidone]

I ran across that water extraction method for processing clay a few years ago. Among a hundred or so other things it's on my to do list to try .

I've got a shipload of things I'm going to do when I retire, too!

That is pretty much the method I used.
It isn't quick, but most of the time is waiting for things to settle. In my case the actual hands-on time required was justified because the nearest source for ready to use clay was 230km away and $1 per kilo. I guess I put in three hours of actual hands on time for a 5 gallon bucket of clay - no travel, no freight, no dollars.

[david s]

The OP just wanted to know what to substitute the clay with for the sand/clay underfloor leveller. This thread has taken on a whole new direction.
The term "fire clay" is used in the US for any cheap powdered clay used in the building industry generally as a mortar improver. To a potter the term "fire clay is a totally different animal. It is a clay that is designed to withstand extremely high temperatures, (very low in impurities and fluxes, large clay particle size and high in alumina), expensive and certainly way beyond the needs of WFO builders, in terms of service temperature. A powdered clay has extremely small particles and is not cementious, i.e. it does not react chemically at ambient temperature (that begins at 573 C), so it should really be considered an aggregate in a mortar or concrete mix. Because the unfired clay particles are so small they induce shrinkage (typically on its own up to around 10% wet to dry) particularly if used in a large proportion in the mix. Once fired and crushed the particle size is increased enormously and the shrinkage proportionally reduced. This stuff is called grog not fireclay. (well it could be fired and crushed fireclay). The slurry left by the brick saw is also fired clay but the blade cuts it so fine that the particle size is far smaller than crushed fired clay (grog). However the particles of the saw slurry are still way larger than most unfired clays and this would explain why it works so well as a mortar addition with a lower shrinkage.
ps. stay away from Bentonite as it has extremely small clay particle size and has a very high shrinkage rate.

[wotavidone]

You are near enough correct, without going on a long thesis about mix water, platelets, chemically bound water in unfired clays, hydration reactions that take place when you wet fired clays and the effects of particle size on those reactions, it being a waste of time looking for high alumina clays since pizza ovens never get hot enough to form mullite anyway, etc.
In the end, ordinary clay will work in a pizza oven, and may even be preferable.
And fire clay to a metallurgist is a different animal again.

[Karyn B]

Wow!!!! What a fantastic group of knowledge! Thank you sooo much! I am now comfortable in either making my own or using the dust from cutting my fire bricks. Now, to figure out what tool to use to make the curving cut of my oven floor bricks. Thank you so much!!!

[UtahBeehiver]

Fire clay is very cheap. HC Muddox is a CA company tha makes fire ckay. Any brick or mason supply store should stock it. 50 bag $10-15 so unless you have a lot of time this is a non cost issue in the overall oven cost.

[JRPizza]

Wow!!!! What a fantastic group of knowledge! Thank you sooo much! I am now comfortable in either making my own or using the dust from cutting my fire bricks. Now, to figure out what tool to use to make the curving cut of my oven floor bricks. Thank you so much!!!

If you have a brick saw cutting the outside radius on your floor bricks is a piece of cake. Once you have the curve drawn on the bricks it only takes a few tangent cuts to make a fairly round edge. Also, I don't know about the rest of the builders, but I didn't use any sand/clay mixture for leveling my floor bricks. l just selected bricks of uniform thickness (there can be quite a bit of slight dimensional variation) and placed them directly on my insulating board. I did use fireclay but that was for the homebrew mortar mix.

[Karyn B]

Hi JRPizza. But what is the tool you used to cut the curve? Forgive my lack of tool knowledge...but I cook a mean pizza!

[UtahBeehiver]

I'm sure it was a wet saw. If you are building an enclosure around the oven then you could build dome on fire brick floor vs around floor then not have to worry about cutting so precise. Even if doing an igloo you can do the same.