Formula assistance / help

[Lou M]

Hello, I'm looking for formulas or help in determining the rough amount of castable refractory materials I would need to build and oven dome and chimney / Doorway for a DIY oven build I am about to embark.
My dome diameter will be molded from a 75 CM gym ball. The door opening will be 22 -24 wide by a depth of 12-14" by the height needed determined by the dome to door ratio, 63% everyone talks about. The thickness will be 2-1/2-3" cast for the dome and slightly more for the doorway and chimney.

So the two big measurements needed to move forward would be the the amount of castable materials I would need to build the dome with determining the height needed to get the correct height for a doorway 22 inches wide.
I do plan on using vermiculite mix for the dome exterior after a fireproof wrap and mesh exterior and a final thin smooth coat covering for tile on the exterior.

I would really appreciate the experts assistance on this.

thank you,
Lou M

[david s]

Each 25 kg bag of my castable refractory will make up around 14 litres of wet mix. For a dome of your size and 3" thick I get 81 litres for the hemisphere. That comes to 5.8 bags The amount not required for the oven mouth is around the same as is required for around the mouth opening. Not sure if you intend to make the flue gallery the same thickness so haven't included that in the calculation, but guess you'd need about another bag and a half. So around 8x 25 kg bags in total.Casting in one piece over a gym ball requires some fairly extensive formwork to support the weight and you will need to move the whole casting onto the supporting slab once de-moulded. The weight is substantial and won't be easy. It is far easier imo to build a sandcastle hemisphere and cast in situ. But you may have a better plan.

[Lou M]

Thanks David,
I plan on making a fibre glass mold made from a gym ball. The mold will be in 6 peices of which will interlock into each other. 4 will be around the ball then 1 for entrance and 1 for chimney. I"ll be pouring each mold separately. Allow several weeks to dry then assembly and bonding each section with castable cement on assembly.
Its a hybrid DIY with some manufactured ideas and some DIY innovation.
Thanks again for the help.

[Lou M]

Question about the homebrew recommendations mix vs. Castable premade bag material. Will the workability in pouring the castable vs. homebrew into a mold be drastically different or am i looking at a similar consistency for the mold pouring.
like mentioned in other post it could come down to dollars and cents and also ease of acquiring these products locally or paying for outrageous shipping fees.
Would love some insight from those that have worked with these materials.

thanks
Lou M

[david s]

There are many different castable refractories made by many different refractory suppliers. The type I use has similar characteristics regarding workability, but very different working times. In temperatures exceeding 20C I always use chilled water which increases the working time. I also do not exceed half a bag (25kg) of castable in any one batch and ensure to wash barrow and tools scrupulously between batches.
The home-brew is far more forgiving in terms of working time but needs at least a week of damp curing, whereas castable refractory achieves full strength in 24 hrs, although I always give it 48 before uncovering and remoulding. You should add some polypropylene micro fibres to the home-brew to assist in the removal of water more safely. The castable refractory already has fibres in the mix (at least the stuff I use has them).

[Lou M]

So it may be that Homebrew will be the direction i take. Is there a thread or link which has the exact recipe and or type of ingredients recommended so that i can reserch quantities and availability from local sources. Price is more a factor as ive seen prices for castable refractory cement everywhere but an afordable price.
thank you again for your wisdom and inspiration from this forum group.
Lou M