Concrete blocks in my barrel oven build

[mrjl]

I've ordered the 30mm vermiculite board, which arrives tomorrow. I've also heard back from the supplier about the ratio; they said 3 perlite to 1 grog and 1 ciment, by volume. So I'm going to go with that.

[downunderdave]

One inch is not sufficient thickness to insulate properly. You really need 4”. Also the more cement you add the stronger the resulting mix will be, but also the less insulating it becomes. Same goes for adding sand. The recommended mix is 5 parts vermiculite or perlite, one part Portland cement, 3 parts water, by volume. Made stronger produces a slab that doesn’t insulate well. Made weaker is not strong enough to support oven well.

[mrjl]

May 22, 2018 19:28:30 GMT downunderdave said:

One inch is not sufficient thickness to insulate properly. You really need 4”. Also the more cement you add the stronger the resulting mix will be, but also the less insulating it becomes. Same goes for adding sand. The recommended mix is 5 parts vermiculite or perlite, one part Portland cement, 3 parts water, by volume. Made stronger produces a slab that doesn’t insulate well. Made weaker is not strong enough to support oven well.

Thanks for the info but perhaps you misunderstand? I'm talking about making concrete to make the tunnel/barrel, using the steel barrel as a form.

The slab is set and the 30mm board is on it's way. I should probably stress that I'm enormously grateful for the input of forum members and lucky to receive their wisdom for free, but at the end of the day I'm not trying to make the perfect over that will enter the history books. I just want something that will work well enough as a first effort and get me rolling as an WFO cook for me a my family and mates. The oven's deficiencies can always be improved upon next time. In short, I'm trying to get this thing built for under £500 whilst avoiding paralysis by analysis.

[devontiger]

mrjl, for that kind of money I would go for a ready built oven.
Outdoor wood fired Pizza oven 100cm white Deluxe model black door
Brand new
£420.00

[downunderdave]

May 22, 2018 20:17:17 GMT mrjl said:

May 22, 2018 19:28:30 GMT downunderdave said:

One inch is not sufficient thickness to insulate properly. You really need 4”. Also the more cement you add the stronger the resulting mix will be, but also the less insulating it becomes. Same goes for adding sand. The recommended mix is 5 parts vermiculite or perlite, one part Portland cement, 3 parts water, by volume. Made stronger produces a slab that doesn’t insulate well. Made weaker is not strong enough to support oven well.

Thanks for the info but perhaps you misunderstand? I'm talking about making concrete to make the tunnel/barrel, using the steel barrel as a form.

The slab is set and the 30mm board is on it's way. I should probably stress that I'm enormously grateful for the input of forum members and lucky to receive their wisdom for free, but at the end of the day I'm not trying to make the perfect over that will enter the history books. I just want something that will work well enough as a first effort and get me rolling as an WFO cook for me a my family and mates. The oven's deficiencies can always be improved upon next time. In short, I'm trying to get this thing built for under £500 whilst avoiding paralysis by analysis.

Ok, I misunderstood. For a mix over the drum you really need a dense mix to add thermal mass otherwise the highly conductive steel will lose heat just as fast as it heats up. If you use a perlite or vermiculite mix it acts as an insulator rather than providing thermal mass. A good dense mix can be made cheaply from 3 parts sand, 1 part cement 1, part powdered clay, 1 part hydrated lime, by volume (the same as the mortar mix). A couple of inches thick will provide good thermal mass, but this then needs to be insulated to hold in the stored heat. This can be done by covering that layer with a 10:1 vermicrete mix min 3” thick.

The mix you referred to is an insulating mix and is slower to heat up, being less conductive, but also slower to cool down. Having little thermal mass it is unable to store much heat. In addition the perlite or vermiculite in the mix reduces its strength and if used as an inner face makes it subject to abrasion damage. I’m sure it kind of works, but have doubts about its efficiency and longevity. In the end you’re the boss

[mrjl]

May 22, 2018 21:04:48 GMT devontiger said:

mrjl, for that kind of money I would go for a ready built oven.
Outdoor wood fired Pizza oven 100cm white Deluxe model black door
Brand new
£420.00

Thanks for the info but not an option I'm afraid. The build has cost me £200 to get it to where it is now. Plus, I want to build it, not get something ugly and off the shelf delivered.

[mrjl]

Ok, I misunderstood. For a mix over the drum you really need a dense mix to add thermal mass otherwise the highly conductive steel will lose heat just as fast as it heats up. If you use a perlite or vermiculite mix it acts as an insulator rather than providing thermal mass. A good dense mix can be made cheaply from 3 parts sand, 1 part cement 1, part powdered clay, 1 part hydrated lime, by volume (the same as the mortar mix). A couple of inches thick will provide good thermal mass, but this then needs to be insulated to hold in the stored heat. This can be done by covering that layer with a 10:1 vermicrete mix min 3” thick.

The mix you referred to is an insulating mix and is slower to heat up, being less conductive, but also slower to cool down. Having little thermal mass it is unable to store much heat. In addition the perlite or vermiculite in the mix reduces its strength and if used as an inner face makes it subject to abrasion damage. I’m sure it kind of works, but have doubts about its efficiency and longevity. In the end you’re the boss

This is great news! I have all of those things bar the powdered clay and I can easily put a 50mm layer of that around the barrel. I can then perhaps do another 50mm layer of perlite concrete, plus a layer of 25mm ceramic blanket insulation which I have here already. Then some fire proof render. That should do it, right?

Would you still advise making the barrel removable? I think I've figured a way to do it. It'll be fiddly and annoying but possible.

[downunderdave]

Powdered clay you can get cheaply as “Bricklayers clay” failing that go to pottery suppliers. Regarding the removal of the drum you could cut it along its length at the top and temporarily join it back together with some sheet metal tabs and screws. Some wedges under the bottom where it meets the oven floor will also allow it to drop.To also make its removal easier cover it in sheet plastic so the castable mix won’t bond to it.
The blanket should sit against the dense layer with the vermicrete mix on top of that. The vermicrete mix over the top should be as lean as is practical to apply because the more cement you put in it the less insulating it becomes. I fins a 10:1 mix by volume works well. Ie 10 parts vermiculite or perlite to 1 part Portland cement to 3 parts water. To make the mix more workable I add a handful of powdered clay for every litre of cement added (wear rubber gloves to apply) Because the oven will be then well insulated the final render over the top can be a normal cement render. It doesn’nt have to be fireproof. You will probably need to build an arch at the front, internal height 63% that of the internal height of the barrel arch. Hope this helps.

[mrjl]

May 23, 2018 9:00:04 GMT downunderdave said:

Powdered clay you can get cheaply as “Bricklayers clay” failing that go to pottery suppliers. Regarding the removal of the drum you could cut it along its length at the top and temporarily join it back together with some sheet metal tabs and screws. Some wedges under the bottom where it meets the oven floor will also allow it to drop.To also make its removal easier cover it in sheet plastic so the castable mix won’t bond to it.
The blanket should sit against the dense layer with the vermicrete mix on top of that. The vermicrete mix over the top should be as lean as is practical to apply because the more cement you put in it the less insulating it becomes. I fins a 10:1 mix by volume works well. Ie 10 parts vermiculite or perlite to 1 part Portland cement to 3 parts water. To make the mix more workable I add a handful of powdered clay for every litre of cement added (wear rubber gloves to apply) Because the oven will be then well insulated the final render over the top can be a normal cement render. It doesn’nt have to be fireproof. You will probably need to build an arch at the front, internal height 63% that of the internal height of the barrel arch. Hope this helps.

Thanks again, Dave! Really helpful information. I have got an archway door form made up that's exactly 63% the internal height. I just need to rig the chimney up to that before I make the concrete layer.

[mrjl]

Another question for the hive mind. How should I fix the insulated board to the slab? And The bricks to the board? I'm assuming fire cement, of which I have a tub. Is that ok or do I need something else?

[downunderdave]

Usually the floor insulation and the floor bricks on top just sit there. If you have to replace a cracked floor brick down the track it’s a bitch if it’s mortared down. The gaps just fill with wood ash.

[simonh]

board goes on the slab dry - I was building up around mine so I knew that brickwork would keep it in place. Floor bricks were laid dry on top of that, that way I can walys replace a brick if I need to later on in the ovens life.

[mrjl]

Great, thanks guys. That makes life easier!

[mrjl]

I've been asking myself if I need an oven this big? I want to do roasts in there in addition to pizza so definitely don't want a small oven. But before I commit to the concrete I'm wondering if an oven this big is going to be more than I need. If I'm going to cut up the drum to make it removable after the concrete has set I'm wondering if now is the time to make it a little shorter to save on wood in the long run. Or, is it not really going to make much difference if the chamber is something like 100mm shorter?

[downunderdave]

May 26, 2018 7:12:53 GMT mrjl said:

I've been asking myself if I need an oven this big? I want to do roasts in there in addition to pizza so definitely don't want a small oven. But before I commit to the concrete I'm wondering if an oven this big is going to be more than I need. If I'm going to cut up the drum to make it removable after the concrete has set I'm wondering if now is the time to make it a little shorter to save on wood in the long run. Or, is it not really going to make much difference if the chamber is something like 100mm shorter?

Yes, a shorter barrel will work better. You will probably find that a fire in the back will work best with this form so you still need room for the fire, but a square or circle works better than a rectangular plan for efficient combustion. That's why the hemisphere is so good.