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Old 08-08-2013, 08:26 AM
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Default Refactory Cast Oven – New oven build with high density/thermal conductivity material

I want to want to make a smaller oven, inspired by the comments from david s, somewhere between 55 and 60cm (21-24”) and have a couple of questions that someone with more experience may be able to provide. Most of the other issues I was having were answered in the forums already.

Have just picked up 150kg of a refractory material (Density 2.6 g/cm2 ; Thermal Conductivity 2.8 W/mK) which we use at work. As it’s trowelable material with higher density (about 20% more) and higher thermal conductivity (2x as much) than standard firebricks, this provides some opportunities and challenges when making the oven. It’s spec’d for 1000C / 1800F so temperature isn’t an issue.

Using a material with higher thermal conductivity and density – does anyone have any experience with denser materials with higher thermal conductivity. Theoretically I can have twice the mass that will heat up in the same time as bricks. Or I could have the same mass of the bricks that heats up 2x as fast. If you had the option, which would you choose?

Different Dome and Wall Thickness – would it make sense to make the top part of the dome thicker than the sides? Say 60mm for the top and 45-50mm further down on the sides? This would give more thermal mass on top?

Floor Material - Bricks vs Cast Refractory – My gut feeling is that it is best to have the floor and the dome from the same material, particularly if they have different thermal conductivity, but is this really necessary. Would like to use firebrick in the floor but worry if the floor has a thermal conductivity of 1/2 of the dome then the pizza won’t cook evenly? Am I overthinking this?

Dome Shape –I can trowel this material into any shape, so I am thinking of having slightly tapering walls with a completely flat dome with a height of 25-26cm with a small door opening of 40x16cm, similar to what I have read about David S’s oven. As I am not constrained structurally to a dome, then the flat roof could be a benefit over the dome as per some discussions I have seen here.

Ideas to try - anyone have anything they would have liked to try when making their oven but couldn't. One idea I am thinking about is making a small pocket/indentation on the right hand side of the wall to push some of the hot coals into in order to make more floor space available, yet be able to have some wood burning when I need it. I can even fabricate a small tray to put them into that would fit into the pocket.

Any feedback would be greatly appreciated.

Thanks.
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Old 08-08-2013, 04:46 PM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

Mikku and iron pony did some really nice cast refractory. Do a search on their threads.
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Old 08-08-2013, 05:10 PM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

I would think that a pocket in the side would be counter productive for pizza, unless it had a nice smooth transition to the dome, you do want flames to lick across the dome to provide radiant heat to the top of the pizza.

I think if you want an area for burning wood an egg shape might provide what you are looking for, the narrow pointed end could be where you maintained the fire and the wider rounded end would be where you cooked. This is only theory and I have no real basis for this than logical extension of thermodynamics.

As far as the flat roof goes, I have no opinion.

If you are concerned about the floor not being hot enough use high density bricks for the floor, they should be reasonably similar to your castable.
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Last edited by mrchipster; 08-08-2013 at 05:13 PM.
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Old 08-09-2013, 12:11 AM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

Quote:
Originally Posted by killie View Post
I want to want to make a smaller oven, inspired by the comments from david s, somewhere between 55 and 60cm (21-24”) and have a couple of questions that someone with more experience may be able to provide. Most of the other issues I was having were answered in the forums already.

Have just picked up 150kg of a refractory material (Density 2.6 g/cm2 ; Thermal Conductivity 2.8 W/mK) which we use at work. As it’s trowelable material with higher density (about 20% more) and higher thermal conductivity (2x as much) than standard firebricks, this provides some opportunities and challenges when making the oven. It’s spec’d for 1000C / 1800F so temperature isn’t an issue.

Using a material with higher thermal conductivity and density – does anyone have any experience with denser materials with higher thermal conductivity. Theoretically I can have twice the mass that will heat up in the same time as bricks. Or I could have the same mass of the bricks that heats up 2x as fast. If you had the option, which would you choose?

Different Dome and Wall Thickness – would it make sense to make the top part of the dome thicker than the sides? Say 60mm for the top and 45-50mm further down on the sides? This would give more thermal mass on top?

Floor Material - Bricks vs Cast Refractory – My gut feeling is that it is best to have the floor and the dome from the same material, particularly if they have different thermal conductivity, but is this really necessary. Would like to use firebrick in the floor but worry if the floor has a thermal conductivity of 1/2 of the dome then the pizza won’t cook evenly? Am I overthinking this?

Dome Shape –I can trowel this material into any shape, so I am thinking of having slightly tapering walls with a completely flat dome with a height of 25-26cm with a small door opening of 40x16cm, similar to what I have read about David S’s oven. As I am not constrained structurally to a dome, then the flat roof could be a benefit over the dome as per some discussions I have seen here.

Ideas to try - anyone have anything they would have liked to try when making their oven but couldn't. One idea I am thinking about is making a small pocket/indentation on the right hand side of the wall to push some of the hot coals into in order to make more floor space available, yet be able to have some wood burning when I need it. I can even fabricate a small tray to put them into that would fit into the pocket.

Any feedback would be greatly appreciated.

Thanks.

Hi Killie,
I think that is very dense castable you have. I use a similar product all the time, but I can't recall its density or thermal conductivity. I have it here somewhere and shall search a bit more. I thought mine was about exactly the same as firebrick, although every manufacturer has a number of different castables and their recipes are all secret. Mine is rated to 1450 C with max aggregate around 4mm. I would presume that the density rating is for castable fired to its max operating temp, (shrinkage around 10%) which will be much higher than you will get, so your density will be lower because of the lower amount of shrinkage you'll get.
Regarding the thermal conductivity, remember that a more conductive material will heat faster but also lose that heat faster, so it's not necessarily a good thing, although I expect it probably makes little difference. What you gain on the swing, you lose on the roundabout.
I like your idea about a thicker refractory on the top and have been mulling over this idea myself for a while too. Because the crown of the dome gets hotter faster and the base of the dome much slower, it is logical to increase the thickness (mass) at the crown. This should have a tendency to even the heating of the dome somewhat and reduce expansion stress on the dome. The downside is that this would be contrary to normal dome building where you need to make the base thicker and stronger than the crown to support the mass. I would like to hear others views about this.I make my dome castings in pieces to relieve this pressure as fast heat up times we use for WFO's are tough on refractory.
Regarding your pocket idea, my approach is a little similar see attached pics. The cradle holds the fire in place neatly while the little cut out triangles allow air into the bottom of the fire.
Attached Thumbnails
Refactory Cast Oven – New oven build with high density/thermal conductivity material-p1010224.jpg   Refactory Cast Oven – New oven build with high density/thermal conductivity material-p1010225.jpg  

Last edited by david s; 08-09-2013 at 07:13 AM.
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Old 08-09-2013, 01:07 AM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

I don't know how the density is measured. Whether it is dry density, wet mixed density, dry casting density or fired to max operating temp density.They will all give you different numbers. I just weighed one of my dry castings and its density is the same as my fired dense firebricks @ 2.0 Kg/Litre.
(You said your density was 2.6 gm/cm2, I presume you meant cm3 , same as kg/Litre)

Last edited by david s; 08-09-2013 at 03:12 AM.
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Old 08-09-2013, 03:48 AM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

UtahBeehiver – thanks for the heads up on Mikku and iron pony. Have been lurking here for years and didn’t see these threads.

Pocket and Cradle - definitely plan on not making it boxed shaped, will make a small “depression” in the side and taper it into the dome. I like the idea of the cradle from david s and would shape it to fit maybe 50% of the width of the cradle and tapered into the dome. Just trying to gain more floor real estate without adding much to the diameter.

With a weight of 2.0 kg/litre your looking at an approximate density of 2.0 g/cm3. From the specs I’ve seen the Firebricks are apx 2.16, the FornoBravo refractory material is about 2.27 according to their brochure and I’m looking to use 2.6. Your material is very close to the Firebrick. We normally measure density after mixing, casting and prefiring to 850C.

Based on what I have seen, I am leaning towards the thicker dome with more mass to see if the heat up times can be balanced. Will probably start with just an extra cm as I can always add more material if I need to. Just can’t take any away. Will throw in some thermocouples to play around with. One about 1cm in the inside of the dome, one on the top of the dome, another on the outside wall and then under the floor. I’m justifying that by telling myself it is a good way to teach the kids thermodynamics!

Structurally don’t see any problem with an extra couple of cm’s on the dome. With diameters as small as I am looking for, it will be able to hold. This stuff is extremely strong.

Hadn’t thought about the expansion stress during normal operation, but don’t think it will be a big problem as I will have a linear thermal expansion of .5% up to 850C, it really doesn’t move at all. I've seen temps of 875C on the front side and 125C on the back side (over 40cm or so thickness) with no problems. This though takes some good engineering to make work so that the stresses go where you want them.... for pizza ovens casting in sections should eliminate this regardless of the type of material and structurally going thicker on the top can be solved over these smaller dimensions. I like the idea.

That said, cracking and spalling when starting out is always a challenge and will probably trowel in rows and let it set up for 45-60 minutes before going on to the next row. That will build in stress cracks naturally. Don’t plan on using any steel fibers. Will throw in some PP fibers as I first fire it up through 350-375C range to avoid any explosion issues during curing, particular as I don’t know how easy it is will be to control the temps. Normally we hold at 275C and 350C for 2 hours/cm thickness to allow the water to work its way out. Don’t think I will be able to control the temps that closely. Don’t want to blow the roof of my back patio so I need to think this portion through before starting. Maybe trowel and preheat in 2.5cm increments just to make sure.

The thermal conductivity shouldn’t be to much of a problem If I have it properly insulated. Once the oven is heat soaked and in balance, the only way for the heat to get out should be through the door. I see people talking about using soapstone which is in the “7’s” so this shouldn’t be a deal killer.

thanks for the feedback, this has gotten me thinking and will check out the Mikko and ironpony threads.
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Old 08-09-2013, 04:55 AM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

Sieve a sample of the castable to see if it contains fibres. Most of them do, but it wouldn't hurt to add some more. Don't forget that your castable will not be sintered at the lower temps we fire to so it will not be as hard as a fire brick because the castable is not truly fired..This means that it is in and out of the danger zone 500-650C where lots of chemical and physical changes take place. Reducing the size of sections is a good way to reduce cracks but the stainless steel needles also help. They are a bit of a nuisance to handle in the wet mix but worth adding IMO. I think working in layers may be asking for trouble. I cast mine in one go at 2" thick with 2% by weight stainless needles.

An added thought, if you work with this stuff then maybe you have access to firing it also. If cast in sections they may fit in your work kilns. If not read through the curing thread under "Firing your oven", to see how others have done their water elimination.

Last edited by david s; 08-09-2013 at 05:14 AM.
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Old 08-11-2013, 03:30 AM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

"Hadn’t thought about the expansion stress during normal operation, but don’t think it will be a big problem as I will have a linear thermal expansion of .5% up to 850C, it really doesn’t move at all"

Don't ignore thermal expansion. Maybe you only have .5%, but clay does most of its thermal expansion up to 500 C and then begins to shrink as the temp rises further. Even a small amount of expansion can cause problems. A dome cast in one piece will usually develop a vertical crack at its base.(Not that it compromises the structure, it won't fall down). That's one reason why casting in sections is preferable.

Last edited by david s; 08-26-2013 at 03:06 AM.
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Old 08-14-2013, 12:11 PM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

We don’t normally use any PP fibers in the trowelable material, but if you were to take the castable version and pre-fire it in the oven you would definitely want to add the PP fibers. I’ve seen ovens completely blown out from people trying to speed up the firing curve. Danger areas are in both the 275C and 350C. Not sure if it is the same for standard refractories. This material is so dense that the space between the particles is the same size or smaller than the water molecules themselves, so it takes time to get out. But of course that higher density give you 20% more thermal mass and with the higher thermal conductivity really curious to see if I can make a workable pizza oven out of it!

I’m very curious to see how the thermal expansion works here on this small scale. Right now I am planning on trowelling the sides in a continuous piece, let it set for a few hours and do the flat roof in a single section. Will be using a wire mesh for support while trowelling (standard operating procedure for this material) so if it cracks, no harm done. Normally we use around 6% water, but will back off to 5.5% as that will should make is slightly easier to work with as I am no professional on the installation side.

Have purchased a huge piece of Styrofoam today and will start making the mold over the weekend. Plan so far is to make a flat roof for the dome at 25-26cm, door opening apx 16-17cm with a width of 40cm. Will start with 4cm walls and 5cm roof. If I need more mass then can add it later. Still debating the diameter - will either be smaller at 55-56cm or 62cm. My feeling is to go smaller and if that isn’t big enough then I can always make second one and have two smaller ovens, rather than one large oven. I am also looking to put in some sort of pocket, indentation on the left side for coals/logs. I think I just need to start cutting and assembling and see where it all ends up!
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Old 08-14-2013, 01:06 PM
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Default Re: Refactory Cast Oven – New oven build with high density/thermal conductivity mater

God luck, keep us posted on your progress. Try to add some photos.
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