Steel Dome Oven

[firebug]

Re: Steel Dome Oven

Wiley.

You used 5 to 1 ratio for your cladding?

thanks...kevin

[Wiley]

Re: Steel Dome Oven

Kevin, I formed up as I went. I would bring to your attention the initial form which I made around the base of my WFO. This allowed me to get past the "vertical" (where the concrete is standing and supported upon itself ) and to be forming against the dome (cylinder). The further up the dome/cylinder the more the concrete is supported as opposed to the concrete standing on itself. Casting the ring around the base also allowed me to get a feel for how easily the material works and how soon it kicks off.

When forming the triangles I created them by eye and placed aluminium foil between the joins. I pressed the foil into the just formed triangles before it fully set off. This mandated the expansion joint occur where I wanted it to occur rather than form (crack) at some perhaps less desireable location. The size of the triangles was determined by the size batches I made...again learned by casting the initial supporting base mentioned above.

The stuff goes off quickly and one is advised to have a plan and have tools laid out etc. Not to panic you, but figure 20 minutes from mixing to unworkable, so not a problem with a reasonable size batch but no time to go looking for that trowel or aluminium foil. By the end I found I enjoyed working with the material, it easily and quickly assumed an initial set and allowed one to hold shapes with a trowel in one hand and shape with a trowel in the other; after a few moments the held material would stay in place (allowing it to be smoothed and faired into the rest) where with regular cement/concrete it would have slumped when released.

I also found that I had good results by mixing a batch and then cleaning up the wheel barrow (where I mixed it) with a hose and brush. Then starting a new batch. By the time it was mixed and ready to go the previous batch was quite hard and I could form against it without damage. You should have no difficulty alternately working one side and then the other of your cylinder.

Hope this helps, looking forward to some photos...
Bests,
Wiley

[firebug]

Re: Steel Dome Oven

Wiley,

I went back and looked at you pic's,and wanted to know if you formed up your cladding triangles? or did you form the triangles against the steel dome as you went. how long does it take to set up? I'm going to start on Friday and hope to be done on Sunday,Thursday (tomorrow) I'm going to build a fire and hope it vent's well. I used a 6 inch chimney from home depot.we will see thanks again...kevin

[firebug]

Re: Steel Dome Oven

Wiley

Makes sense,1/4 basically has the caricature of sand,and now that i think about it i think the basalt supplier said 1/4 inch minus that will save me a step thanks,I followed up on the lime, I guess you only want to use lime with normal cement the lime help's keeping the cement from burning out.so if I use fondu I won't need it....thanks again.

Kevin

[Wiley]

Re: Steel Dome Oven

Kevin, I'm not as sharp as I once was (was I ever?), however, something about your mix sent me searching for something I remember reading. Back in 2008 there was a thread about casting and a proper mix; David S. in Australia had this addition:

Lime and calcium aluminate together are not good. I've tried this and the lime acts as an accelerant to make your brew go off much faster. The calcium aluminate is also quite temperature dependant. On a cold day it takes ages to go off (use warm water) On a hot day it'll go off really fast (use chilled water or some ice) don't mix product that's been sitting in the sun. It's also quite thixotropic ie It'll start going hard but if you give it more mixing or vibration it liquifies again weird stuff.

So you might want to keep that info in mind when you start mixing your brew. Calcium aluminate goes off pretty fast on it's own. Also I would see if your source for basalt has it in something smaller than 3/4 inch. My thinking is you would want something more like "xx minus". The minus in crushed rock terminology means it will have finer/smaller particles of the same material intermixed in the load. Having the fines of the same material would make for a final mix with more consistant characteristics throughout, as opposed to adding sand of unknown composition. My personal choice was 1/4 minus as a simple one step mix. I didn't have to add or measure out anything but the Fondu to the stuff. It's your WFO and so you will do as you see fit (as to be expected and hoped).
Bests,
Wiley

[Wiley]

Re: Steel Dome Oven

Hi Kevin,
Finally I have a working and hopefully safe, virus free computer system. I just got back online using this machine this evening and of course the first place to check...Forno Bravo!

Regarding your question: If I am understanding correctly, you are asking whether you should place some kaowool between the steel and the basalt fondu concrete mix. I would say no. I would suggest you simply place a layer of aluminum foil between the two. The kaowool goes on after the cladding.

The aluminum foil wil create a slip surface so the steel can expand and contract and whatever movement should be accommodated. I suggest that when you apply the concrete that you create some larger polygonals so that the pieces of heat sink can act as individual plates. We've discussed this before, the idea is sort of like a snake skin. The actual scales in a snake skin are rigid. The expansion needed as the snake respirates is accommodated between the individual scales. The plates are held to the steel liner by the steel spikes.

Great to learn that your project is coming along...some pictures would be nice:-) That seems like a good price for Fondu, compared to traditional portland cements it is fairly expensive but IMHO, worth the cost.

Hope this helps,
Wiley

[firebug]

Re: Steel Dome Oven

Hey Wiley,


Trailer is built ,oven is built,I need a vacation.I also welded spikes all over the dome so it can be one with the cladding.should I put kaowool down between metal dome and cladding for expansion? how does this recipe look for my heat sink cladding?aka refractory concrete.

3 part 3/4 inch basalt
2 part sand
2 part calcium aluminate cement fondu
1/2 part lime

this recipe comes from traditional oven.It seems correct.

I can purchase 94 lb bag of fondu for $45 10 min from my house I thought this was a pretty good deal.
and 1/2 ton of basalt for $20 also near my house.

Thanks for all your help and input...kevin

[Wiley]

Re: Steel Dome Oven

Fireplug (aka Kevin),

Insulating concrete is only good in compression, I do not have any numbers but would not expect it to very strong in either shear or tension. As such the insulating layer needs to be supported. For a trailer a properly designed steel support need not be overly heavy. The weight of these ovens is primarily around where the dome itself sits. The weight in the center is actually quite low in comparison.

I would think you should have no problem with a better insulating ratio than 3 to 1. Mine was closer to ten to one of vermiculite to cement and I would think one could easily get similar to that with perlcrete.

When thinking of the weight of a WFO it is good to remember the great majority of the weight is distributed over the actual area of the base of the supporting bricks. While our WFOs are different than the usual WFOs made with bricks they still have a large footprint relative to their total weight. For example: A 4.5 inch brick on it's end of 2.25 inches has a square area of 10.125 inches. So say there were 30 soldiers in the base of an WFO and the dome of that WFO weighed 4000 lbs. That would work out to 30 x 10.125= 303.75 square inches to support 4000 lbs or 13.1687... lbs. per square inch. That's not very much. The total is a bit of weight, but per square inch of area beneath the insulating layer doesn't need much strength to support so little weight. And those figures are high as we left out the grout in the wedge shaped space between the bricks which is increasing the support area and depending upon the insulation of the dome (percrete or vermicrete) some of which is also self-supporting. Now what you are building as well as what I built is different in that we are/didn't use firebricks but the same principle applies just our material is different.

So my suggestion would be to go with a reasonable support upon which you pour a high ratio (9 or 10 to 1) insulating layer. Also I would suggest you design the insulating layer so that it incorporates a split of firebrick upon which the steel sits. This is what I did with my WFO and my reasoning was this:
The steel will expand and contract as it heats and cools. This will cause movement and I didn't want the steel working/worrying its way into the softer insulating layer. By having it set upon a the harder firebrick this movement should have little or no detrimental effect.

Hope this helps,
Wiley

[dmun]

Re: Steel Dome Oven

There's no reason to go denser than the 5:1 ratio recommended in the instructions. It will have plenty of compressive strength after it cures, but the more portland you add, the less insulation properties the concrete will have.

Builders have gone as high a 12:1 by power mixing the portland with water to create a slurry, and then mixing in the vermiculite or perlite.

[firebug]

Re: Steel Dome Oven

Hey Wiley,

Quick question ,I'm making my insulating slab this week that over 200 fire bricks will be laying on.what do you think is a good ratio for perilite and Portland cement.3:1,is what i was thinking for strength something that can help support 200 firebricks, what do you you think?


thanks kevin.

[Wiley]

Re: Steel Dome Oven

Firebug,
It was almost two years ago that I purchased the Fondu that I used and so I would think that the price has probably gone up (hasn't everything?) but if memory serves it was something like 50 cents a pound for a 50 lb bag, The stuff is put up in smaller bags than the usual 94 lb (one cuft) bag of normal portland cement.

As for the recipes in The Bread Builders book, I read that book almost two years ago when I was thinking on building my WFO. It was a borrowed book from a friend and so I don't have it at hand for reference. Since your WFO (like mine) has a steel liner, the problem of spalling or cracking as detrimental to the functioning of the WFO is lessened. The exact limits of what would or would or not work in our application is open to a great deal of conjecture. I used Fondu (calcium aluminate cement) but perhaps portland cement might work. I don't know, and like I said alot of what we are designing and building is "breaking trail" some ideas will work and some not and all gradations between.

That's why I have gone to some effort to document what I have done and give reasons for what I did. No need for anyone following me to make the same mis-steps I made... no need to "reinvent the wheel" so to speak. I would hope that those following and interested in the idea of a steel dome WFO would explore the medium/design a bit; tweak the idea, advance it and see what works. My WFO works pretty well, there is still more smoke out the front than I would like during fireup. However, I have seen more traditional designs with more soot staining and smoke out the front than I have. And I have yet to encounter or learn of any other WFO that "pants" on fireup. So in that little way so far my WFO seems to be unique. That little anomaly was unexpected; it's alive and it breathes!

Bests,
Wiley

[firebug]

Re: Steel Dome Oven

Hey Wile

I would like to know how much fondu cost (ballpark) also in the bread builders book they give some recipes for cladding,is this the same thing?

I look forward to your response..thanks again

[Wiley]

Re: Steel Dome Oven

Firebug, you're certainly welcome for any help I have provided.

I don't have a tested formula for using "Home Brew" refractory mix as the cement. However, one thing I really liked about using the 1/4 minus was that it seemed to have the fines and larger aggregate in a good proportions such that (if memory serves) I simply measured out five containers full of the 1/4 minus and then dry stirred in one measure of the Fondu. I then added the water but Fondu is very sensitive to amount of water so that quanity is less relevant here. The result was a good plastic workable mix. Since the Fondu goes off in a hurry I was using a 1 quart ex-yogurt container and mixing in a wheel barrow and so was doing small batches. I've not worked with the Home Brew so do not know how quickly it goes off.

However, I would think a good place to start tests on samples would be mixing together the cement part of the Home Brew dry and then trying a five parts quarter minus to one cement mix and see how long the working time is and the final strength (alright not final strength but strength after say five days). Originally I used five to one because a typical concrete mix is 3-2-1 with the cement being the one. So the aggregate and sand together added up to the five.

Hope this helps, and please keep us posted on the results of your tests,
Wiley

[firebug]

Re: Steel Dome Oven

Hey Wiley,

Everything you said makes perfect sense and pretty much was what i planned on doing,I was going to use 3 inch pieces of rebar instead of bolts.bolts make more sense. do you know of a good recipe for basalt mix, or know where i could find one. what percentage of basalt would be used in mix.then on top of heat sink i assume, i would put kaowool 2 inches thick then 6 inches of perilite or equal. i guess the bolts welded on would also keep things uniform.

Wiley Thanks for all your support, seriously you really are a great help.

[Wiley]

Re: Steel Dome Oven

Firebug, Last I heard you thinking of using 1/2 inch steel plate, is this what you ended up with? What I visualize from your description is a shape more in keeping with a barrel dome WFO. Almost All Scott design shape. Regardless of shape the usual ratio of height of the interior to the height of the door is usually stated to be 63% for optimum gas flow. So I would think if you designed your door in keeping with that you would have a good chance of success. I have not seen any ratios relative to length of WFO to width relative to height. One would think there would be some point at which the WFO would be too long (length) to draw intake gasses across the hearth and flow them out across the top of the oven smoothly, However, I do not know if anyone has conducted empirical tests to determine if such a point exists and if so the ratios between the length and width and height of door. Same of course would be true for an WFO too wide, at some point it wouldn't work, but just where that point is I haven't a clue.

As for a thermal mass to be added to the exterior of your structure: My personal experience was using crushed 1/4 inch minus basalt with calcium aluminate cement. This is nice stuff to work with, handles easily and packs well. This so far has worked well for me or at least without problems. The cement is fairly expensive and the size of your WFO is large. So in the interests of keeping expenses to a minimum I would think that you might consider using something more on the lines of the "Home Brew" refractory mortar mix save that I would use crushed 1/4 minus basalt in lieu of the sand in the mix. From your signature you are located in Portland here in the Pacific Northwest. As you are near "The Gorge" you should be able to find somebody who produces crushed basalt in your area. Have you called local quarries? It's a common material here that is used to cover paths and infill between flagstones. It is inexpensive. A cubic yard in my trailer cost all of $13.00(home built trailer with steel deck and designed to carry a solid yard of rock or sand or whatever). If you cannot find that see if you can find somone who carries firebrick and see if you can cut a deal for broken seconds and crush them. (A couple of hours with a sledge hammer will probably end up with your re-vitalizing a quarry search for ready crushed rock...I know it would for me :-)

Regardless of what you end up using some accommodation for the expansion and contraction will need to be included in the design. If it were mine I think I would consider welding a large quanity of bolts on the outside of the steel (threaded end toward WFO and head away from WFO). These would be spaced out such that they provided a gross mechanical attachment between the refractory heatsink and the steel liner. Like I did with my WFO I would cover the steel with a slip surface of aluminum foil. I would then design my sections such that each contained at least two welded bolts (two so that no piece can rotate and "jam") and the joins between the sections having a piece of foil as a predetermined separation/break point.

As to the size of the bolts and the thickness of the heatsink, if you are using the 1/2 inch plate you probably would be fine with a heatsink of around 2 inches perhaps 3 or more if you are thinking multiple bakes on one firing for bread. The bolts themselves would be something inexpensive think along the lines of 1/4 inch x 1 1/2 inch if going for 2 inch refractory thickness and 2 inch bolts if three etc. You are breaking new ground here in your design so what the proper thickness for your intended need is will be determined after you build the WFO (by the results of your WFO!). And get "black bolts" they're cheaper and don't have any zinc wash to cause nasty fumes when welding them on. Not rocket science welding them on, mark positions with soapstone, a touch with a grinder at each spot to get thru the mill scale, hold bolt in position with left hand, stinger in right, bzzz and on to the next bolt.. Mix the basalt concrete so that it is "packable" without slumping, you may want some simple forms when doing the vertical sides and ends but when you start across the curved sections I would think forms would not be needed.

So to sum up" pattern layout for refractory sections, weld on bolts, apply Al foil, apply refractory, insulate, cook pizzas. You can be cooking pizzas before insulating but slow cure the basalt concrete refractory...ie slow fires to drive out the moisture. This will also allow you to observe the expansion and contraction of the ovens surface.

A long post and I hope it has helped,
Wiley