Refractory Castable WFO - 2019 - Dome cast in 1 Day

[hasghar]

Hi
I recently created my first WFO using Castable Refractory for the inner dome and thought the process I followed may be of interest to others, especially as you can get the inner dome cast in 1 day
During my initial research I found useful information on this site which greatly helped with different aspects of the construction so I want to thank the community for their input. I realised at the time that using castable is not a popular method of construction hence I guess anything I can add may be of benefit to others too :-)

I could start at the beginning, but where’s the fun in that, lol. No, I think most here would be interested in the dome itself, so that’s where I will start. Will add a little section on the background and base at the end.

Pictures: https://photos.app.goo.gl/V1wSxkoZof22pEm69

Dome

• Inner Dome – Castable Refractory 70mm Build = 1 Day
  • Material
    • Dense Castable Refractory Cement – 1600cc rated 70mm thick
    • Cardboard, to build form guide
    • Thin easily bendable wood, to build form for base of mound -> HDF 3mm I think
    • Builders sand, for mound
    • Old newspaper

• Outer Dome – Insulation Layer ~100mm Build = 0.5 Days
  • Material
    • Ceramic Fire blanket 25mm
    • Chicken wire
    • Perlite
    • Cement
    • Thin easily bendable wood, to build form for outer base

I set out my dome circumference on the firebrick oven floor using a felt tip and string to give me lines to work to. I set out the outer circumference of the inner dome and, the inner circumference of inner dome Note: I actually set out all circumferences, including outdoor dome (inner & outer) too to make sure everything was going to fit!

I then took some old packaging cardboard and cut it into quadrants (0.5m radius) that I could join together and create a guide that I could use to form my sand mound. I put the cardboard form on the oven dome floor and weighed down with bricks. As an additional guide, I cut thin bendable wood into 100mm strip and used as a base board all around the bottom of the cardboard form. I thought this would give me a cleaner and stronger line to work to and build a better base. I then filled the whole shape with slightly damp sand. It took a lot of sand to fill in, apologies cant remember exact measure here but maybe around 200-250kg, but I also used bricks to reduce amount of sand needed. I smoothed the outer layer of sand down to form as good a sphere as I could get. I then covered the sand in damp newspaper, a few layers, and then sprayed down with a little more water. I cut another strip of the bendable wood to 100mm and placed this on my outer circumference of the inner dome, again to help guide the castable and give a good base to work with. That was me ready to cast.

The supplier of the castable material was excellent in providing information on how to mix and also in oven building in general; I don’t think I would have got the job done if it wasn’t for his patient and detailed information over emails/msgs. To mix the castable, he recommended to use 2 buckets and mix by drill with paddle (slow setting) or by hand, basically avoid a mixer as it will add air to the process and apparently air is the enemy! Add refractory to water and mix in the first clean bucket (the supplier actually supplied me the castable bags in steel buckets which I then used for the mixing). Then move the mix to the other bucket and mix some more to get all bits that the first mixing may have missed and to get the desired consistency. He recommended to add material to water rather than the other way around, so mix the first batch slightly wetter than desired and then add more castable to get desired viscosity. In this way you ensure all contents take on enough water and structural strength is not weakened by adding water afterwards. The supplier also advised on rodding the mix to expel trapped air as it is trapped air that causes the most cracking. He stated that he worked with moulds and would vibrate the mould which is much better. As I was casting over a mound, I did not have the option to vibrate, I also did not have any professional rodding equipment, so when I started casting I pressed on the mix by hand and then pushed/pressed down with hands many times to try and expel any trapped air. As stated, this is not the recommended or ideal process but it worked for me. I used an old drill bit with tape on it as a depth gauge and also used my measuring tape to try and ensure the cast was applied at 70mm across the structure. I cant say it was exactly 70mm across the structure as this is difficult to do manually, I guess I could have built a wooden form that could have been used to control the external depth, something that others should look into. However, the dome did come out pretty spherical so I think all in all it went ok.

Key thing I want to call out here is the inner dome, from sand mound to casting, only took me 1 day to actually implement… of course, there were many days of planning that went into this!

Notes:

• supplier stated 60mm would have been an adequate thickness for the inner but I added 10mm for additional strength because I am a worrier!
• The recommendation was that the less number of batches/casts the better, however the mix has a limited working time so there is a trade-off to be made. More people, then you can probably do it all in one cast. If its just you working alone, as with my scenario, then you are probably going to need a number of batches. As the top of the dome defends against the most heat, the supplier recommended that the top third should be done in 1 cast if possible.

Supplier stated that the castable takes around 14 days to dry out properly, though 7 days may be enough, and that Curing is a critical stage and should not be rushed. He advised that any refractory concrete is vulnerable to thermal shock when not void of moisture hence the longer left to dry the better. I was thinking of drying out the inner dome naturally and then by building up small fires over a number of days as recommended on FornoBravo site, and then later adding the outer insulation layer of perlite & cement mix, thinking being that if there was going to be any crack and the structure failing, then I wouldn’t have wasted more material on the outer dome layer. However, when I ran this past the supplier, he recommended against this approach, saying that the outer layer would have water of its own that would seep into the inner layer, and it made more sense to do the two layers and then dry out the structure as a whole. So this is what I did.

• Inner dome cast (1 day), then left for 2 weeks to dry out naturally
• Fie blanket & Chicken wire added over inner dome
• Outer dome cast with perlite & cement mix (0.5 days) and left for 1 week to dry out naturally
• Fire dry structure over 2 weeks as per curing recommendations on FornoBravo site for their own ovens e.g. 5 burnings, building up from 300F for x hours on first firing and moving up to 500F
  • https://www.fornobravo.com/pompeii-oven/curing/

Notes:

• I mixed the perlite and cement in a wheelbarrow with a garden hoe as apparently a mixer shouldn’t be used as it breaks the perlite up too much. It was easy enough, just time consuming.
• I bought a laser thermometer off ebay to gauge temperatures for drying out
• Although you can cast the inner dome in 1 day, please keep in mind the whole process took weeks and weeks, mostly drying time.

To Do
Waterproof Dome - Outer Layer – Porcelain Mosaic (to do)
Waiting for good weather to get started on this one so I have just covered with tarp for now. I have left over brown porcelain tiles from a bathroom fit so I am going to break those and mosaic the outer dome. I also have some white porcelain which I will break and get the kids to layer in their names in white into the bigger brown background. Will add some pictures into the album when done.

As I am over my word count, I will add further sections in separate posts:
- background and base
- faq

[hasghar]

Background & Base not dome specific – so ignore if you want :-)
So this whole project came about because I had a couple of pallets of left over facing bricks from another garden project and needed something to do to get away from the wife and kids . I had put in a few patio areas around the house, and there was one behind the garage that wasn’t getting much use so I thought it would make a good spot for a brick bbq. I looked up ideas online and extended the plan to put in a prep area. When I drew up plans for the bbq and prep area, I realised I would have space and bricks leftover, so I decided to extend further and put in something else – a WFO made sense. There are so many beautiful designs out there, it also seemed an interesting project to undertake, and the structures themselves seemed to add a little something to the spaces they were in. So I added the base of a WFO into my plans.
As I wanted to get the bbq up and running for summer I cracked on with the project, with only a little consideration given to the overall dimensions of the oven dome. I thought I would make the base as big as I can and work the dome bit out later, this was a mistake. If I had done my dome research properly back then I would have built the base for the dome deeper and less wide as that’s what was needed for a 0.5m inner radius dome which I eventually decided to build. I just wanted the bbq done so raced on with rough measures, and then worked out later the base wasn’t quite deep enough for what I wanted. Anyways, later on I had to adjust the design of the dome to work with the base I had built.

For those interested in the base, here are my notes on the process:

• To save on facing brick the inner layer of the base is actually made from breeze block.
• I built the concrete slab that would support the dome structure. I shuttered with wood, then added iron rebar for strength and poured in my mixed concrete. The core depth was approx. 100m, but I dropped to 75mm at front to tie in with my brick lines. I covered with tarp and let it dry out slowly over a week or two.
• I then insulated on top of the concrete. I put a layer of vermiculite mixed with cement 60mm thick and let this set for a week.
• I then cut fire blanket (25mm, see pic) and added this in.
• I then covered the blanket with more vermiculite mix and took total depth of insulation up to around 100-110mm.
• This gave me the foundation for my firebricks. To lay the firebrick, I mixed a dry sand and cement mix and screeded it onto the base, levelling out as required. I then placed the firebricks tight together on top of this, cutting at edges as required. Apologies, I didn’t take any pics of this stage.

Base Material

• Concrete Mix - Cement, Sharp Sand, Aggregate
• Vermiculite
• Fire blanket (think it was 25mm thick)
• Sand & cement screed
• Firebrick base

Q&A
Why did I decide to use castable refractory as opposed fire bricks?
I researched various methods for building the dome and the most popular was building the inner dome out of cut firebricks and then insulating over with either perlite/vermiculite, or a second layer of regular bricks as an outer shell. Because of the size of the base I had already made, I didn’t feel I had enough space for this approach as I would have needed at least 225mm+ which I did not have (100mm fire brick + fire blanket + brick/perlite layer). But to be fair, I also didn’t feel comfortable or competent enough to make the dome out of cut firebricks, especially the vaulted part of the top of the structure. So I researched on and found that you could also build the dome out of clay/straw, or refractory cement, using a wet sand mound as a structure to cast over. This seemed simpler to me and I thought it would also work out cheaper so that’s what I did.
In review, I think it was definitely quicker and easier to do, I did the mound and cast over it in 1 day, I don’t think I could have done this with a firebrick inner dome. But I am not sure it was cheaper as I went through a lot more castable refractory than I originally thought I would: 8 x 25kg bags in all, but that’s also because I built a big dome, 1m internal diameter. Not sure I needed it that big to be honest, if doing again would drop to 0.9m.

Why did I use vermiculite for the base but perlite for the dome insulation layer?
I had used vermiculite for the base insulation but read perlite was a more popular choice and slightly cheaper so worked with that as I needed a lot more volume for the dome. In review, I think the vermiculite provides a smoother more even finish than the perlite so I will use that for the oven door when I build it. The perlite pieces are a little thicker and it looks like a cement rice crispy cake when on so you have more roughness to the finish. As I am covering the outer dome anyway, doesn’t matter, but a point to note.

Why did I go with that particular Chimney design?
Reading up, the general advice seemed to be make the chimney as big as you can and towards the front so you have a strong draw and none of the smoke comes out at you. So rather than a circular 100-150mm pipe based chimney I went with a bigger more integrated structure. It also serves as a façade, so in case I fk up, sorry mess up, the dome behind (don’t get it nice and round), the façade will hide some of the mess away. I drew out various designs on paper until I got something that worked for me. I didn’t have the same bricks I used for the base but had these clay bricks lying at a relatives so used that for most of the chimney. When those ran out, I bought some that looked similar from a merchant.

Where’s the arch opening to the Inner Dome?
I also chickened out of building an arch as the inner dome entry ☹ So I improvised and had the refractory come into the firebrick opening and then added perlite between that and the eventual concrete lintel I put in to support the chimney structure above (lintel at back, and lintel at front). In that way the refractory would be exposed to the heat, then the perlite, and the concrete lintel would be kept away to areas with less heat exposure. I think I could have built the arch, and should have, but I wanted to crack on and didn’t want to take the extra time needed. You will see what I mean from the pics.

What would I do differently if doing again?
I would:

• plan all measurements up front, so I build the base appropriately (deeper instead of wider)
• make the internal diameter smaller, probs 0.9m instead of 1m
• make an arch to the inner dome
• take more pictures

[david s]

[QUOTE=hasghar;n418535]Hi
I recently created my first WFO using Castable Refractory for the inner dome and thought the process I followed may be of interest to others, especially as you can get the inner dome cast in 1 day
During my initial research I found useful information on this site which greatly helped with different aspects of the construction so I want to thank the community for their input. I realised at the time that using castable is not a popular method of construction hence I guess anything I can add may be of benefit to others too :-)

I could start at the beginning, but where’s the fun in that, lol. No, I think most here would be interested in the dome itself, so that’s where I will start. Will add a little section on the background and base at the end.

Pictures: https://photos.app.goo.gl/V1wSxkoZof22pEm69


Thanks for taking the time to document your build, it will be helpful for others.

A point of clarification re "curing" which gets some folk confused. The term curing regarding concrete is a process to hold in moisture for a sufficient time to allow for the chemical process of hydration to take place. Typically this should be done for a week to enhance strength. Concretes made with Portland cement actually continue to increase strength for years, but practically 28 days is considered the standard. Concretes made with calcium aluminate cement (castable refractory) achieve full strength in 24 hrs, but it is prudent to keep damp for 48 hrs. When folk talk about "oven curing" they are usually referring to the opposite procedure and that is driving free water, not used up in the hydration process, out of the material. This does create confusion and I prefer to call it "water elimination" as there is no chemical change taking place when driving mechanical water out at lower temperatures. Chemical water elimination begins at around 600C, north of the range of service of our ovens.

Regarding the vastly reduced working time of calcium aluminate cements, there is a trick you can use to extend working time and that is to use chilled water. We live in the tropics and during the summer I have to take great pains to ensure both the barrow and the castable have not been standing in the sun, and to add chilled water to the mix.

[hasghar]

Hi David
Appreciate the clarification. So I cured the concrete slab, but eliminated water from the dome :-)

[arnesr]

[*]I then insulated on top of the concrete. I put a layer of vermiculite mixed with cement 60mm thick and let this set for a week.[*]I then cut fire blanket (25mm, see pic) and added this in.[*]I then covered the blanket with more vermiculite mix and took total depth of insulation up to around 100-110mm.[*]This gave me the foundation for my firebricks.

Nice job!

I found your use of fire blanket in the base under your firebrick interesting. I don't think I have seen it done that way before. Did the blanket compress much?

I also built a cast oven this fall and I too used unconventional insulation in my base layer under the fire bricks. In my build I used a layer of pumice stone under a perlite concrete mix in an attempt to insulate my fire brick from the base support. So far so good, though I haven't used my oven much yet due to the cold weather here in Wisconsin.

https://community.fornobravo.com/for...over-sand-form

[hasghar]

Thanks
Apologies for the delayed response.
The blanket would likely have compressed some, not sure how much :-)
Your stones looks like a better option as they will not compress and looks like give a larger insulation depth too, though I dont have enough experience to fully comment.
I used the blanket as the one roll i bought for the dome insulation had enough in it to also add a layer for the base.

Looks like we followed a similar build in terms of casting over sand. I am intrigued with your homebrew, its something I feel should have considered more as would likely have been more economical.
-> 3 parts sand, one part fire clay, one part lime and one part portland cement
How was the strength of the mix and do you know what temperatures the mix can withstand?

Thanks

[david s]

The 3:1:1:1 homebrew mix is quite rich as the Portland cement and the lime are both cementious materials. Consider the powdered clay as a very fine aggregate. The Portland cement begins to break down north of 300 C, whereas the lime is good for 500 C. The idea is that where the Portland cement breaks down the lime takes over, that’s why the mix is so rich. (A standard mortar is 4:1 aggregate, cement)
The homebrew has proved to be a most suitable mortar for the temperature range of our ovens and apart from being really cheap, has better workability and working time than high temperature refractory mortars.