Clay Issues With Personal Project

[Davidpotter]

For about a year i have been trying to make a portable fire pit that would basically but just a large bowl with holes in the side for ventilation and sometimes i put a large hole in one side to put wood in. How ever booth designs that i have used cracked right away and fell apart from the rapid heating of having a fire in them. 

So, my question is, Does anyone know of a clay that can withstand this rapid heating and cooling that it would take without cracking?

It doesn't matter what type or cone it is, i just need to make one successful for my own sanity....

[neilestrick]

Most clays cannot handle that kind of uneven heat. You could try a groggy terra cotta, like chimeneas are made from (although those often crack, too), or a flameware body. Google for recipes.

[Chris Campbell]

Everything I can think  of is going to add weight which would make the portability an issue ... unless you were into lugging a solid mass around. For a really flexible fire pit I would recommend a shovel.

Good Luck with your quest!!

[oldlady]

try breaking up a number of ordinary bricks and using the crumbled stuff as insulation at the bottom.  then start the fire very slowly with just a little wood until the thing warms up enough to burn hotter.  crumbs not as big as an ice cube.  don't ever let the fire get too hot.

 

a layer of brick kept the woodstove at my old house going for 15 years with no damage to its bottom.  they were patio bricks, thinner than building bricks.  they looked really bad when i moved but the cast iron stove was fine.

[Jeffster]

I have made several vessels (bbq's and hibachi's to be specific) using high fire clay. You will want to find a clay that has been developed to be used as a Raku clay or one that also has the equivalent characteristics (grog and sand mixed in). I have had success with IMCO's Sculpture Mix (a buff colored clay body) with my projects. It is a high-fired (Cone 10) clay that I under-fired (between 2100 and 2200 Degrees F). IMCO is Industrial Minerals out of Sacramento,California so I'm doubtful you'll find it near you without special ordering it. Another solution is to find a similar clay with the same proportions of grog and sand mixed in it that also has the equivalent properties. Finally, Leslie Ceramics in Berkeley, California has a red colored clay body that is designed for 'Flameware' and they have appropriately named it that. This body is designed specifically to be used with fire. The clays I have mentioned are sold wet in 25lbs bags so depending on how much you need, shipping costs will be a consideration.

 

Besides the ability of the clay to handle heat and 'thermal shock' there are other things to consider. In my case, I have only constructed my slab walls to be 1/2" thick. The thicker you go, the slower you should heat up the piece with fire, i.e. start by building a small sized kindling fire and slowly add more wood over the course of an hour or so (for good measure). This will allow the clay body to heat up more evenly and expand with time (which will help prevent cracking). Lastly, as the outside air temperature is lower then add more time to make and a full-sized fire.

 

I found the other issue to prevent cracking is by designing the shape of the vessel that will expand uniformly. For example, a shape with a flat bottom and walls at a 90 degree angle (like the shape of the bottom of a drum barrel) will expand at different rates, and with clay, the bottom will heat up and have nowhere to expand outward while the tops of the walls will be relatively cooler... resulting in a different coefficient of expansion. Half of a round sphere (imagine the bottom half of a basketball shape), on the other hand, enables the clay at the bottom when heated to make it's way up the walls is a semi-uniform manner (much more effectively that walls at a 90 degree angle (as I have experienced). I used a grate to start my fire on (the kind you would find at the bottom of a Weber grill) within the piece and also made a hole in the bottom to allow air flow upward into the fire giving it the oxygen it needs (the hole can also be used as an 'ash-drop'). In addition to this, making a more complex shape, like chimera, I would highly recommend making the base as one piece, the sphere (which contains the fire) as a separate piece, and the chimney as an independent third piece. This design approach will allow all of the separate pieces to heat up independently of one another and not 'constrict' and the related pieces form expanding.

 

Jeffster

 

 

[Mark C.]

Jeffster has it right-flameware body.

All others cannot handle this uneven heating

The very nature of this made from clay will be an issue.

Mark

[Mart]

Jeffster is on the right track.

 

When I wanted to build a tandoor, I discovered that mid/low fire clay (under fired!), mixed with sand (or grog) and chopped up hay or tall grass (for fiber)) is the way to go.

BTW, most of Africa still cooks their food in clay pots. Guess what, those are usually round shaped and have no flat bottom as you see in metal pots and robots like Bender¹.

 

Look for fire clay

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¹ unit Nr 1729 actually has a shiny metal ass.