Hi! Thinking about building a cast refractory dome oven.

[apachebrew]

I'm thinking of building a wood fired oven in my backyard. I really like the idea of a precast segmented dome. I modeled one in my design software and am contemplating 3d printing molds from PETG so that I can at some point cast them from refractory. I think I'll make a DIY vibratory table when doing this. I'm about 100 miles from the nearest HWI distribution center. I emailed them about my project and they suggested KS-4V Plus for the castable. I'm thinking of installing the dome on 4" thick calcium silicate board, then 12x12x2.5" fire bricks for the floor. Refractory mortar the dome to the fire brick floor. (2) 2" layers of ceramic fiber insulation, 2" perlite render, and then a layer of finish stucco over that. The ID of the cooking surface would be 40". The shape of the dome wouldn't be perfectly hemispherical, rather someone elliptical with a 17" internal height at the center. 8" flue. I dont know if there's a lot of cost savings in this approach. I was kind of just wanting to see if I could do it, and then be able to cast a few domes for some friends too. Does the design/material selection look ok?

[david s]

Your plan looks pretty sound. What thickness are you planning on making the castings?
Because there is a huge temperature difference difference in the base and top of the dome during heat up and the corresponding difference in thermal expansion, relief from this stress can be achieved by making the top circular section much larger so it will receive the early heat first. If it is designed as a conical fit it can then expand a little and pop out at the top without stressing the side castings. Also,you should think about fibre reinforcing and burnout fibres. if you make your own castable you need to include burn out fibres. If you buy a proprietary castable, check to make sure the product contains them, if not add them.

[apachebrew]

Your plan looks pretty sound. What thickness are you planning on making the castings?
Because there is a huge temperature difference difference in the base and top of the dome during heat up and the corresponding difference in thermal expansion, relief from this stress can be achieved by making the top circular section much larger so it will receive the early heat first. If it is designed as a conical fit it can then expand a little and pop out at the top without stressing the side castings. Also,you should think about fibre reinforcing and burnout fibres. if you make your own castable you need to include burn out fibres. If you buy a proprietary castable, check to make sure the product contains them, if not add them.

Hi David,

I currently have it modeled at a constant 3" dome thickness. This could be easily modified to some variable thickness though. My thought was that the interlocking pieces would connect together to form the dome, and that I wouldn't apply any refractory mortar into the semicircle "groove" connections. I figure the only application of mortar where the dome pieces join together would be on the outside of the dome, almost like a wrapper or diaper.

Do you have a source for burn out fibers? Is the idea that they create tubular voids that allow for thermal expansion/contraction to avoid cracking? For the steel fibers, is the idea to act like mechanical tensile reinforcement? My only hesitation is I've read not to use them for cooking ovens (I assume the concern is the same one that you shouldn't use stainless brushes on a BBQ for fear of a loose strand getting stuck in food to be ingested?)

[david s]

Apologies for not replying, I missed your last post.
Only add the mortar from the outside after fitting the castings. You wouldn't want any bits of mortar falling onto your food.
For burnout fibres the easiest way to obtain them is to buy a two part pack of concrete polypropylene reinforcing fibres eg Sikafibre PPM 48/19 (48mm and the others 19mm) which provide the long and thick fibres for strength enhancement and the short extremely thin ones for early strength and shrinkage crack reduction. As polypropylene has a low melting point (160C) the large fibres are no good for an oven, use them for a concrete project. It is the very fine ones you need to add @1% by volume, well dispersed. They are for reducing the possibility of steam spalling, particularly when firing for the first time. On melting and then burning out they leave behind a network of very fine pipes through which moisture can find its way to the outer surface.
Regarding The stainless needles (actually called melt extract fibres if you are trying to source some), there is almost zero chance of any falling off the inside surface because they are imbedded in the mix. One end of one may protrude from the edge of a casting, but won't fall out. The worst is that these can cause an injury to you hand. The handling and placement of a mix containing stainless needles is more of a problem and care is needed when handling.
Another option for strength enhancement is to use AR (alkaline resistant) concrete reinforcing fibres as used widely by the concrete countertop guys. These are much easier to obtain, particularly in smaller quantities. They have a melting point well over a WFO service temperature (900C) and are quite soft and easy to handle.