Fireclay sand required for a diy tandoor

[Callie Beller Diesel]

Clay of a certain consistency will just do that. When it’s in a slurry state, it’s just going to be sticky. If you’re looking to make slabs out of it, you’re going to have to work with mucky hands and clean off later. Set yourself up so you’re just doing the messy parts at a time so you don’t have to go back and forth with any hand cleaning. If you do need to clean off, keep a bucket of water nearby, with a sponge in it to help wipe off your hands. I recommend a drywall sponge cut in half, but use what you have. I would not use sand in your hands: that’s only going to make more clumps.

[dexter7205]

Thank you my first attempt to make a sample for this tandoor has failed. I think this is due to my own negligence. In the original article the person is using a barrel. I did not have a barrel handy so i tried to use a plastic flowerpot. I oiled it with musturd oil from outside, put the flower pot upside down and applied the clay slurry directly without making slabs. It felt alright for couple days as it dried but broke when i tried to take off the plastic pot.

Is there an alternative we can think of to the barrel used on https://imperpro.ru/en/tandyr-svoimi-rukami-55-foto-sovety-po-stroitelstvu-i-ekspluatacii-delaem/? 

Edited May 10, 2022 by dexter7205

[PeterH]

17 minutes ago, dexter7205 said:

Thank you my first attempt to make a sample for this tandoor has failed. I think this is due to my own negligence. In the original article the person is using a barrel. I did not have a barrel handy so i tried to use a plastic flowerpot. I oiled it with musturd oil from outside, put the flower pot upside down and applied the clay slurry directly without making slabs. It felt alright for couple days as it dried but broke when i tried to take off the plastic pot.

Is there an alternative we can think of to the barrel used on https://imperpro.ru/en/tandyr-svoimi-rukami-55-foto-sovety-po-stroitelstvu-i-ekspluatacii-delaem/? 

That sounds like you applied the clay on the outside of the flower pot, rather than on the inside.

[Pyewackette]

what @PeterH said.  Clay shrinks as it dries.  If you put it on the OUTSIDE of a mold, it will crack as it shrinks because it gets smaller but the mold doesn't.

If you put it on the INSIDE, the mold still won't shrink, but the clay will and it should release naturally as it shrinks.  Though if the mold is "sticky" you may still get some cracking if it doesn't pull away evenly as it shrinks.  Should probably use a mold release, which doesn't have to be a chemically thing called "mold release" that you have to buy, but I'm not sure mustard oil is the best thing either.  Somebody who does molds would obviously know a lot better than me LOL!

[Callie Beller Diesel]

Even in the tutorial, which admittedly does leave out a lot of details, it says they applied the clay on the inside of the barrel.

“The clay mixture is applied inside the barrel, while the layer should be about 5-6 cm.“ 

They suggest soaking the barrel/mould with water so it swells.  Presumably would mean that as it dries, the wood would pull away from the clay as well as the clay shrinking. (I’m picturing the internal volume increasing somewhat as the wood dries).  Using fibre will make the clay resist some of its usual shrinkage, but it won’t eliminate it entirely. I don’t know that the wood barrel is critical to this process, and soaking it may be compensating for specific properties of the clay they’re using. It might also have something to do with the size of their tandoor.

Another good question to ask is how big are you making your tandoor? The size of the piece will affect what building techniques will be better suited to your circumstances.

[dexter7205]

Thank you! As @PeterH  and others mentioned my first mistake is adding clay to outside of pot. I thought() it would be easier to pull the pot out.

The first sample is about 6-8 inches. This is to test if the process works. I am going to use inside of the plastic pot i have and spray with vegetable oil. Google search suggested that.

The final tandoor i expect to be 17-20 inches tall and 17 inch wide(outside) bottom. 14-15 inches at the top. 1.5 inch thick may be or little less.

Again, thanks for answering my noob questions @Callie Beller Diesel @Pyewackette@PeterH.

Edited May 10, 2022 by dexter7205

[Pyewackette]

You know one of those wooden half-barrels they sell as planters might make a pretty good mold ... I think a full size one might be too big but sometimes you can find smaller ones.  They're not really usually half-whiskey barrels any more, they're usually purpose-made (and much flimsier as planters).

[dexter7205]

Thanks @Pyewackette ! i used a cut out cylinder i had on hand. I dried the template in sun for a few days and then put it in the garbage can i used for previous tandoor.

I might have heated it too fast so it broke. See picture. When it cools down, can i fix it with more clay and let all of it dry. Is this piece gone now, no good? 

Also provide tips on slow heating. I have hay/bale, dried wood from a tree and coal on hand. 

 

 

[Pyewackette]

@dexter7205 I'm no expert  but given the requirement that it survive and operate in FIRE I would say there is no fixing it.  The stresses of uneven woodfired *or charcoal* heating and cooling are already significant, a repair just wouldn't survive, I don't think.

Edited May 17, 2022 by Pyewackette

[dexter7205]

Thanks going to throw this and trying again. If there are tips how to avoid this let me know.

 

I was thinking of reasons why it exploded and cracked, It could have been due to the vegetable oil i used. Unfortunately the next sample i did in evening had vegetable oil as well. Let's see how that goes.

Edited May 17, 2022 by dexter7205

[Callie Beller Diesel]

Your sample most likely cracked because it’s in direct flame, not because of any vegetable oil. 

Ordinarily for a functional piece or even a sculpture, a crack that large would be a deal breaker and we’d advise folks to discard and try again. Cracks like that can be filled, but rarely fully eliminated, and will likely produce structural issues in a piece that needs to support itself.  

BUT.

You’re building an insulating wall that doesn’t have to be engineered to hold more than the weight of a little bread dough, or some cooking skewers leaning on it. You’re building the tandoor inside a garbage can insulated with vermiculite, I believe? This might be an instance where you can try patching it without it mattering. You have yourself a sample that you can now experiment with to see how well that would work. Take advantage of that. A crack that size may need to be filled more than once. The fibre in your clay slurry will help with this. 

When you’re firing clay, the things that are most likely to make it crack in the firing (as opposed to other points in the process)  is uneven heating, or having a partially dried piece. Clay shrinks and expands as it cools and heats, much like water freezing into ice and melting again. If one face or surface of the piece heats so that it’s hotter than the other, the difference in shrinkage rates causes stress cracks.

If there’s too much water in the clay and the piece is heated past boiling, the water vapour escapes at higher velocity, and this is the origins of the “air bubbles in your piece makes it explode in the kiln” trope. It’s not the air bubble, but the water inside that’s the problem. But I digress. The net result is still destruction.

When you’re firing this tandoor for the first time, you’re simulating a kind of super low (for us) kiln firing. You’re hardening the clay, you’re not really turning it all the way into ceramic.

If I was firing a pot that had the thickness and dimensions you talked about, I’d wait until it was bone dry, or as close as I could get it to that point, and then do a hold in my kiln at about 80*C (about 175F) for 4 hours to make sure any physical water was gone. I’d then increase my kiln temperature no faster than 100 *C (200ish F) per hour, up to 500 C (932 F). So that’s 8 hours right there. I don’t know that you’d get hotter than that for tandoor cooking, but if you do, hold again at that point for another half an hour just to hedge your bets. 

Approximating that with an open fire is pretty tricky but is possible. The scale of your piece dictates the method you’d use for this. If it was just your sample piece, you could stick it in a keep warm setting in your oven for a few hours before finishing the firing in your barrel, while avoiding exposing the piece to direct flame. For your actual tandoor, you have to account for the difference in mass and the practicality of any available equipment. Think about heating the air inside the tandoor, rather than the tandoor wall itself. The heat energy will then work its way through the piece.

You either need to tend a really tiny wood fire with kindling sticks for a longer period of time, or use something like charcoal briquettes that do a long slow heat release,  or a weed burner turned on to the lowest possible  setting for several hours. You want to avoid direct flame on the walls if possible.  After it doesn’t feel steamy anymore,  then ramp it up to tandoor cooking temperatures at that same 200F rate. 

Subsequent uses of the tandoor don’t have to be as careful as the first one, but you've done that part before.