Question regarding oven mix

[LaloBravo]

Hello, Forno Bravo forum members.

I want to make a 36" exercise ball oven.

I found a recipe for Firebrick on here (see comment by Eadwulf Greysteel): http://www.iforgeiron.com/topic/15207-firebrick-recipe/
TL; DR By volume, not weight: 1 part Perlite, 2 parts fire clay, 1 part sand, 1 part ash, 1 part Portland

I think it would be a good idea to just build the entire thing out of this mix and I hope somebody here would be knowledgeable and kind enough to tell me I'm an idiot while also explaining why that's a bad idea.

Furthermore, I was thinking about making a 1-inch-thick layer of fireclay, and then a 2-inch-thick layer of the firebrick mix, and again, if someone has any ideas on why this isn't workable, or what the pros/cons of doing this are, I'm open to suggestions.

I apologize if this is somehow a misinformed or too broad question. This is the first time I've made an oven and I'm quite excited by the prospect.
Thank you for your attention, and thanks in advance for any advice you guys have.

[fox]

You could use a exercise ball but a far more favoured way is to use a mound of sand shaped as you wish and covered with ‘home brew mix’.
Once the mix is dry the sand is dug out, there are hundreds of such projects featured on this forum.

[david s]

The high proportion of perlite in the mix reduces the strength of the casting as well as reducing its thermal mass. Both strength and thermal mass are desirable qualities for an oven, but perhaps less so for a forge.
A high proportion of clay leads to excessive shrinkage which may create cracks over a mould that it cannot shrink away from.
Portland cement begins to fail north of 300C, but in conjunction with lime temperatures up to 500C can be achieved. I think the theory is that if the Portland cement fails the lime takes over and if it fails the clay takes over. The homebrew recipe of 3:1:1:1 by volume of sand, Portland cement, hydrated lime and powdered clay has proved itself as an effective mortar for ages. Considering this I couldn’t see a reason why it wouldn’t work as a cheap castable mix for a cast oven. While the clay makes a pretty sticky mortar, with a slightly drier mix it makes a very workable castable with low shrinkage, good working time, sufficient refractory qualities for an oven, good strength and high thermal mass. I advised two customers to try it about 15 years ago, not knowing how it would stand the test of time. In both cases the ovens are still in service, but only used on an irregular basis. Check Cast Ovens in the Other Oven Types section.

[LaloBravo]

The high proportion of perlite in the mix reduces the strength of the casting as well as reducing its thermal mass. Both strength and thermal mass are desirable qualities for an oven, but perhaps less so for a forge.
A high proportion of clay leads to excessive shrinkage which may create cracks over a mould that it cannot shrink away from.
Portland cement begins to fail north of 300C, but in conjunction with lime temperatures up to 500C can be achieved. I think the theory is that if the Portland cement fails the lime takes over and if it fails the clay takes over. The homebrew recipe of 3:1:1:1 by volume of sand, Portland cement, hydrated lime and powdered clay has proved itself as an effective mortar for ages. Considering this I couldn’t see a reason why it wouldn’t work as a cheap castable mix for a cast oven. While the clay makes a pretty sticky mortar, with a slightly drier mix it makes a very workable castable with low shrinkage, good working time, sufficient refractory qualities for an oven, good strength and high thermal mass. I advised two customers to try it about 15 years ago, not knowing how it would stand the test of time. In both cases the ovens are still in service, but only used on an irregular basis. Check Cast Ovens in the Other Oven Types section.

Thank you so much, David. This is exactly the sort of advice that I was looking for. If you don't mind me asking, why is the sand proportion higher compared to other ingredients in the mix?

[Pizzarotic]

why is the sand proportion higher compared to other ingredients in the mix?

Think of the sand & clay as the refractory material and the cement and lime as the glue or bond to hold it together. Too much glue and not enough bulk makes it weak and brittle as does too little. The curing process is a chemical reaction so the mixing ratios are critical as is the water content. 3:1:1:1 is a well proven recipe that is very cheap and readily available everywhere