Guanajuato Build

[Blairt]

I'm able to source a couple of insulating options for under the oven. I've attached the PDFs for what is available locally and highlighted the ones that they have in stock (HD board + K-23 & K-25). Hopefully these load correctly. I'd appreciate the expert's opinions on the pro's and con's of the three choices available. MORGAN Board Products TDS.pdf Morgan IFB Datasheet.pdf

[Jubilado]

With all my tools in hand, materials finally located and ready to purchase, and my paper design hopefully complete, I thought I would post again and run some design and construction issues up the flag pole.

I hope to start in a week or two and I plan to post photos of my progress along with inquiries.

Floor insulation will be 2x2" Foamglas with 2" calsil on top, total 6". If I could have obtained 3" calsil I would have gone with 7". I found that it is easier to find calsil if you use the brand names. Some sales people connect better that way. I think home oven builders get different sales people than commercial and industrial customers. It was quite an adventure to locate sources for calsil and foamglas in Mexico.

Herringbone oven floor extending beneath the dome. This just seems easier with less precise cuts needed. Also, (probably of little consequence) there is more bearing surface to transfer the dome weight onto the calsil. I don't think I will wear out the floor bricks in my lifetime and, if necessary, I could veneer with the porous basalt stone slabs available here.

The vent arch and landing are completely isolated from the oven dome. There is a 2" insulated separation. This gap will have an insulated stainless channel on the landing. I am undecided what insulation material to use on the arch gap - foamglas, calsil, or perlcrete, perhaps with refractory cement or perlcrete veneer. I'll make a decision when I get these materials in hand and with serious consideration of input from this forum. I've never worked with these materials. The concern is spalling down onto the landing and keeping the insulation in place. This area will be serviceable; so I think there is little risk in the experiment. The most complicated solution might be to fit a stainless channel over the entire insulation perimeter.

There has been a fair amount of forum discussion regarding a thermal break. My thoughts are based on my experience with industrial furnace design. The relatively wide break will lower the heat loss and also mitigate high temperature gradient and thermal strain. A narrower gap would allow for movement, but do less for the temperature gradient. The 2" width can be compared to the 6" or 8" dome insulation thickness. In my oven 2" it fits well into the design with the 6" chimney and excess 2" thick insulation materials.

The vent arch is only 6.5" long. I am keeping it short for oven access, while the landing extends out, 11" total, (9"brick, plus the 2" thermal break area). Since I have no wfo baking experience, I wonder what others might think of this.
The chimney will be custom made 6" square double-wall, insulated with perlite. It bears on the vent arch and cantilevers onto the oven arch with insulation islating it from the oven arch. I plan to use high temperature caulk to seal at the brick services and lateral bracing above the oven.

[Jubilado]

I started construction I picked up foamglas and enough brick to start the floor, with the remainder of brick to be shipped in. I plan to document here to get input or help others.

I found that using aluminum angle, which I had on-hand, to be a good tool for inscribing circles. It was easy to drill a hole for a carbide tipped scribe, which cut handily into the foamgass. A box knife cuts the foamglas easily, even after it dulls after the first cut. The 2" foamglas thickness is nice to work with.

Has anyone used silicone adhesive with foamgas? I was considering sticking the foamglas pieces to one another and the base layer to the slab with RTV silicone, mostly so they don' move during construction. I was considering using silicone to stick decorative tile to the foamglas under the landing, also. The foamglas and calsil will be 48" diameter, protruding about 1.75" beyond the oven base. This is just to utilize the material. First two layers of ceramic fiber will rest on the calsil.

My dome will set on the oven floor. After seeing the variation in thickness of the floor brick, lippage maybe 1/16", less than 3/32" by eyeball, I was considering using a very thin layer of refractory mortar here to level it out under the dome. It is often recommended on this forum that the dome floats, but I do not see why it would be a problem to use mortar here. So I am wondering if anyone has specific experience here and would care to comment. I will put down porous basalt stone on the oven floor to take care of brick height variation.

Rob

[Jubilado]

Turns out that the refractory mortar I purchased is intended for very narrow joints. After more searching and reading on the forum I see confirmation and discussion of the refractory mortars and homebrew. So, I am considering whipping up some home brew, which might be interesting because local sand here is not silica sand and varies widely. It seems the finer sands used for top coat stucco would be appropriate for home brew.

I was considering whether sand, brick swarf, or fireclay could be added to refractory mortar to allow wider gaps and filler application. Has anyone tried this?

Any advice would be greatly appreciated.

[david s]

If you add sand to the mortar you will be weakening it by reducing the cement proportion, so not a good idea. Any sand should be ok for mortar , but coarse grains can make application difficult. You can simply sieve out the coarse material. I think the reason refractory mortars recommend it to be only used for thin joints is because it doesn’t contain burnout fibres. You could try adding these yourself. They must be well mixed in for good dispersal which takes around double the mixing time that you’d normally use.

[Jubilado]

My dome brick work is complete and now I am considering the vent arch, landing, and door while my chimney and thermal break are being fabricated and also waiting for the rain to subside. Also, I'm trying to find time to get forum posts together.

I would greatly appreciate comments and reports of experience.

Landing - I had been figuring on using firebrick for the landing, but now considering alternates. I think this area is more a food prep area than part of the oven and the porous brick seems would have sanitary drawbacks. With my 2" thermal break I am not concerned with conduction from the oven, but there will be radiant heat.

Does the firebrick landing floor work well? Cleanliness, wear?

I see on the forum that many people use stone, often granite. Any cracking problems? Does it get very hot?

I also wonder about stainless steel sheet metal for the landing floor. I would back it will calsil. Has anyone had experienced with stainless steel? Polished stainless would reflect radiated heat and thin material would not conduct much heat, even though it has a high thermal conductivity.

Thanks for looking.
Rob





Alt

[fox]

Yes i have used ss for the landing floor sitting on top of ceramic fibre board, i have also built the whole gallery from insulating bricks and lined the floor and sides with ss.

[Jubilado]

Thanks fox for the comment. I trust that you are happy with the stainless galley.

After your comment and some additional thought the stainless is the way I will go. It will be sanitary, function as a thermal break in the floor with calsil support, and also not absorb any rain (unlike the fire brick) that might blow in and filter down to the calsil below.

[fox]

Just bear in mind that stainless will expand when warm or hot and if restricted it will warp or bow up so leave a little room for expansion.

[david s]

Thin stainless warps like crazy.

[fox]

Only if it get to hot and is very thin.
This ss has been in place for a few years with no issues …

[Jubilado]

Very nice work, Fox.

I plan to use 18 or 20 guage, depending on what my fabricator has on hand - likely 430ss. The metal will be unrestrained on the two sides in and out of the oven. The other edges border the vent arch brick and I will leave 1/16-3/32" on both sides. I calculate about 1/32" expansion with 400 series stainless over 20" with 500F rise. Installed it will be like an inverted, insulated cake pan that can be slid out and replaced.

400 series stainless should be more stable with a lower expansion coefficient and higher conductivity than 300 series. Thanks for stimulating my thoughts, because I was figuring on 304 previously.

Rob

[Jubilado]

Hello Forum,

I am getting ready to insulate the dome whenever the rain here lets up. I noticed that the Forno Bravo commercial ovens have less ceramic fiber thickness on the sides of the dome than the top (4" versus 6"). Although I am not sure how the sides and top of a hemisphere are defined, I wonder if anyone on the forum has temperature profiled the dome. I have seen temperature graphs here but I do not recall one that shows temperature variability in the dome.

Heat rises. However, with so much heat transfer by radiation in the oven, a significantly higher upper dome temperature is not a given.

Thanks for viewing.

Rob

[Jubilado]

My oven is complete, cured, and I've fired it about 4 or 5 times to prepare pizza and other food. I am quite pleased with the result.
This post is to document what I have done as a reference for others, especially since I have tweaked some of the methods commonly used on this forum. I have obtained so, so much information from this forum and I hope others will find my post useful, as well.

Insulation
I used 4" of foamglas plus 2" of Calcium silicate board on the base. This was on top of an existing concrete counter, 36" high. So the oven floor is about 44.5" from the floor. I find this height very good, but it could be improved by another 2" in height to lower the amount of stooping to work the oven. There is no roof over the oven so the foamglas was chosen in consideration of the possibility of water intrusion.
The base insulation protrudes from the outside of the oven by about 6". This provides a land for the ceramic blanket; which, in my case, is 6" thick. I think this is a good use of the base insulation that would otherwise likely be scrap. It makes installing the blanket easier and keeps the blanket away from any moisture that might be present on the concrete hearth. I attached cut pieces of foamglas with silicone where needed to get the right size circular shape (it would have been easier if I had this concept earlier). Silicone adheres well to foamglass and also well to dust-free calsil board.
The 6" of 8 lb ceramic fiber blanket covered by about 1-1/2" percrete and then a render layer. I contracted out the stucco work. Perlcrete was novel to the masons and I could only negotiate about a 4-6:1 ratio. So good thing that there is a lot of fiber. The fiber amounted to 3 rolls.

Dome
My build has alternating courses of rectangular and #2 arch brick. These formed the 90cm inside diameter vertically very well, with horizontal joints of zero clearance (buttered mortar only) inside and about 3/16" on the outside joint. One soldier course was laid broad side down, with the dome radius beginning at the midpoint of the next course. The dome rests on top of the herringbone brick floor.
My concept was to use single diagonal cuts in the brick to avoid radial tapering. The cut brick would be laid by alternately placing the short cut and long cut bricks on the ID of the oven. Cutting the brick length diagonally to about 4" x 5" worked very well. This turned out to be a very efficient construction method for level courses, which I continued up until the inverted vee joint reached about 3/16". A wider gap would jeopardize the pre-mixed refractory mortar I used.
The single cuts worked well with rectangular brick, but the geometry doesn't work with the arch brick. I wish I would have figured that out on paper. So I radially tapered the first course arch bricks to maintain thin joints. Then I did the math and realized how many bricks I would chew up. Rather than buy more bricks I decided to accept wider triangular openings on the outside of the oven, which were filled with a commercial castable refractory mix (refractory mortar, small pea gravel, and fiber reinforcement).
I messed up on the arch, failing to taper the incline to match the course pitch. I actually had the bricks marked for cutting, but I got into a tizzy and had a brain fart. I laboriously overcame this by forming and casting and then cutting bricks. It would have been much easier to have followed the method of Beehiver and others on this site. I was concerned as to how the arch cuts would intersect with the alternating arch and rectangular brick. I believe that the dome/arch intersections would have been easily constructed with shim cut brick on the arch to match the dome row level. Anyway the arch-dome area came out good enough in the end - I'm glad you can see it though. All roads lead to the top of the dome.
I plugged the dome with two flat bricks cut to form the tapered 14 sided shape created by the next-to-last course. A template helped, but it took two prototypes to get it right. I cast in above the plug, with the casting filling the adjacent dome joints.
All told, no cracks in the dome that I can see.

Outer Arch/Landing
The outer arch is completely isolated from the dome. Two partially circular 2" calsil board form a thermal break. The calsil board runs from the base calsil up to the rectangular stainless steel flue. Refractory cement (caulk) seals the calsil break on the inner arch side and high-temperature silicone RTV seals it on the outer arch side. A thin stainless steel channel protects the lower calsil from mechanical damage and sealed with high temperature silicone RTV.
The landing is formed stainless steel, butted to the hearth brick with a narrow gap (filled with brick dust) and isolated from the outer arch with gap filled with clear silicone caulk.
The (thoroughly dried) base bricks of the outer arch are attached with RTV silicone to the dust-free calsil base insulation. An experiment, I suppose, but it allows for expansion.

Chimney
I had a double-wall stainless flue and rain cap formed. 6" square ID and 8" square OD. The 1" gap walls is filled with perlite. The outer wall terminates with an arch shape that forms a saddle on the front and back. The front rests directly and the outer arch for support. The back side sets on about 1" of calsil board mortared to the inner arch. It's about 50" tall and draws well.

[Jubilado]

I added a vent to the oven to allow any residual moisture to escape from the insulated area. I bored a hole through the top stucco and perlcrete layers and inserted a 3/4" cpvc pipe, which had been perforated with a number of 3/8" holes for breathing. An 1-1/2" cap was epoxied to the top of the pipe (which had also been perforated) and the assembly sealed with epoxy into the stucco/perlcrete layer. Obviously, it would have been easier to install a vent during the rendering process.
The need for the vent became evident as I could feel the moisture present through the open bore hole.
The vent will allow the residual moisture from the perlcrete and stucco to eventually exhaust.

Rob