Should I mix anything with my clay wall layer?

[mattyana2003]

I am building a clay oven in my backyard and would appreciate some help. I have built the foundation and sand form and am ready to put the clay layer over the form which will be 6" thick with a cow dung/clay/sand stucco over the outside which will be 1" thick. I have bought Buff Raku clay from a pottery supply which has a high grog (ground fire brick) content. I am not sure weather to mix any sand with this and if so in what ratio. I have heard varying reports as to weather this is necessary or not because of my high grog content clay and that the sand will keep falling into the food as the oven ages. I intend to wet the clay down to a more pliable consistency before use and would prefer not to add sand as this is more work. Will pure groggy clay crack easier than the same clay mixed with sand?
Please help,
Mattyana

[Archena]

I'm not sure - I've always thought sand was the filler and that straw was the binding agent when used. I don't see why sand should increase binding.

[dmun]

Grog, at least when I was a potter, was fired clay, ground in a ball mill then sifted to the proper size granule. I don't think it has any specific alumina content like refractory products, unless it's something special. It's intended to add strength to the fired clay body, not provide any temperature resistance.

Manure? In an oven? I know this is traditional, but I hope you are going to cure this with long, hot fires before you put any food in the oven. Won't it, you know, smell?

[Archena]

No, manure only smells when fresh - it's urine that won't lose its smell. Dried manure has little or no odor and doesn't regain it when rehydrated.

Don't ask how I know that...

[RTflorida]

Its amazing what you retain from watching the Discovery Channel
We certainly don't think it is from firsthand experience

[dusty]

I don't even WANT to know!

dusty

[asudavew]

Originally Posted by Archena

No, manure only smells when fresh - it's urine that won't lose its smell. Dried manure has little or no odor and doesn't regain it when rehydrated.

Don't ask how I know that...

So how do you know?

Someone had to ask..

[rcspott]

A little info

Handy Home Projects: final plaster layer for the mud oven - clay manure mix

Rich

[Guerito]

asudavew,

My wife knows. Born in Mexico; her family was poor, and everyone worked on the ranch. When she was little, they would burn dried cow pies as fuel for their oven. Each family that worked on the ranch had their specific section where they could gather the dried cow pies. No odor!

Guertio

[Carl]

I'm going to use a recipe from John Conrad's book "Contemporary Ceramic Formulas" to make my oven walls. The book contains a whole chapter on refractories, and has a range of recipes to make refractory materials, including firebricks and crucibles.

The recipe I'm using will be 70% firebrick grog, 25% fireclay, 5% ballclay. The firebrick grog is just ground up firebricks, so this stuff is as close as I can get to 'mould your own firebrick'. The alumina content of the grog is high, so it should hold heat just like firebrick. I'll report back here once I've done it - I'm working on a roof for my oven right now so all the clay doesn't wash away in the rain...

As I understand it from Kiko's book, the thermal expansion rates of sand and clay differ, so they can lead to sand separating from the clay when heating and cooling. That's why I'm using grog, rather than sand. Sounds like your clay is well grogged - can you find out what percentage grog it is?

Here's a link to someone who's done similar with a grog, clay and fireclay mix: Backyard Boulangerie I'm copying parts of this design, while trying to add a few improvements too.

Carl